Abstract: The objective of this
study was to analyse changes in stream flow patterns
with reference to dynamics in land cover/use in a typical watershed, the Chemoga, in northwestern highland Ethiopia. The results show that,
between 1960 and 1999, total annual stream flow decreased at a rate of 1(.)7 mm
year(-1), whereas the annual rainfall decreased
only at a rate of 0(.)29 mm year(-1). The decrease in
the stream flow was more pronounced during the dry season (October to May), for
which a statistically significant decline (0(.)6 mm year(-1))
was observed while the corresponding rainfall showed no discernible
trend. The wet season (June to September) rainfall and stream flow did
not show any trends. Extreme low flows analysed at
monthly and daily time steps reconfirmed that low flows declined with time, the
changes being highly significant statistically. Between 1960 and 1999, the
monthly rainfall and stream flow amounts of February (month of lowest
long-term mean flow) declined by 55% and 94% respectively. Similarly, minimum
daily flows recorded during the three driest months (December to February)
showed statistically highly significant declines over the same period. It
declined from 0(.)6 m(3) s(-1) to 0(.)2 m(3) s(-1) in December, from 0(.)4 m(3)
s(-1) to 0(.)1 m(3) s(-1) in January and from 0(.)4 m(3) s(-1) to 0(.)02 m(3) s(-1) in
February (1.0 m(3) s(-1) = 0.24 mm day(-1) in the Chemoga
watershed). In contrast, extreme high flows analysed
at monthly (for August) and daily (July to September) time steps did not reveal
discernible trends. The observed adverse changes in the stream flow have partly
resulted from changes in land cover/use and/or degradation of the watershed
that involved destruction of natural vegetative covers, expansion of croplands,
overgrazing and increased area under eucalypt plantations. The other
contributory factor has been the increased dry-season water abstraction to be
expected from the increased human and livestock populations in the area. Given
the significance of the stream flow as the only source of water to the local
people, a set of measures aimed at reducing magnitudes of surface runoff
generation and increasing groundwater recharge are required to sustain the
water resource and maintain a balanced dry-season flow in the watershed.
Generally, an integrated watershed management approach, whereby the whole of
the watershed can be holistically viewed and managed, would be desirable. Copyright (C) 2004 John Wiley Sons, Ltd.
Author
Keywords: land
cover change; water yield; low flows; watershed management; Ethiopia
Abstract: The Upper BlueNile river basin is the largest
in Ethiopia in terms of volume of
discharge, second largest in terms of area, and contributes over 50 per cent of
the longterm river flow of the Main Nile. This paper provides a
review of the nature and variability of the climate and hydrology in the source
region of the BlueNile - the central Ethiopian Highlands. Annual rainfall over the basin
decreases from the south-west (>2000 mm) to the north-east (around 1000 mm),
with about 70 per cent occurring between June and September. A basin-wide time
series of annual rainfall constructed from 11 gauges for the period 1900 to
1998 has a mean of 1421 millimetres, minimum in 1913
(1148 mm) and maximum in 1903 (1757 mm). Rainfall over the basin showed a
marked decrease between the mid-1960s and the late 1980s and dry years show a degree of association with low values of the
Southern Oscillation Index (SOI). The October to February dry season in 1997/98
was the wettest on record and responsible for widespread flooding across Ethiopia and also parts of Somalia and Kenya. Available
river flow records, which are sparse and of limited duration, are presented for
the BlueNile and its
tributaries upstream of the border with Sudan. Runoff over the basin
amounts to 45.9 cubic kilometres (equivalent to 1456 m(3)s(-1)) discharge, or 261 millimetre
depth (1961-1990), a runoff ratio of 18 per cent. Between 1900 and 1997 annual
river flow has ranged from 20.6 cubic kilometres
(1913) to 79.0 cubic kilometres (1909), and the
lowest decade-mean flow was 37.9 cubic kilometres
from 1978 to 1987. Annual river flow, like rainfall, shows a strong association
with the SOI.
Author
Keywords:Ethiopia; BlueNile; climate; hydrology; water
resources
KeyWordsPlus: INDIAN-OCEAN; RAINFALL;
PRECIPITATION; ETHIOPIAN; AFRICA
Abstract: Substantial fluctuations in precipitation and
runoff have occurred over the NileBasin in recent decades. Ten-year mean flows of the BlueNile (Khartoum gauge) during the 20th century have ranged from
42.2 to 56.7 km(3) and for the White Nile
(Malakal gauge) from 25.5 to 36.9 km(3). These
fluctuations have been responsible for changes in decade-mean Main Nile discharge of up to +/-20% which
have had important consequences for water resource management in both Egypt and Sudan.
This paper provides a review
of the NileBasin hydrology incorporating, for the first time, analyses
of the relationships between precipitation and runoff fluctuations in the
instrumental period for each of the eight major sub-basins within the NileBasin. These sub-basins possess very different physical,
climatic and hydrological characteristics. Over 90% of the Main Nile discharge originates from only
four of the sub-basins; Lake
Victoria, BlueNile, Atbara, and the Sobat. Interbasin correlations of 40 y (1945-84) precipitation and
runoff annual time series identify two broadly homogeneous regions; the
Ethiopian highlands (BlueNile and Atbara) and the Lake Victoria and EquatorialLakes (White
Nile). These regions posses contrasting
precipitation regimes whose interannual variations
are uncorrelated in time and which are therefore associated with different
atmospheric circulation anomalies. The observed relationships between catchment precipitation and runoff, however, are not
straightforward and the sensitivity of runoff to precipitation fluctuations
varies from basin to basin. Some of the water resource management implications of
these fluctuations for Egypt are discussed. With water demand in Egypt alone set to increase 17% by the year 2000, it is
critical that the role of future climate change in Nile water management is thoroughly assessed based on a
correct modelling of the diverse hydrological
characteristics of the various Nile sub-basins.
ABSTRACT: In Tigray
(Northern
Ethiopia, significant achievements were made, mainly from 1994 to 2002, on the
development of agriculture through irrigation by employing seasonally harvested
runoff using earth dams. However, most of the implemented schemes are not
serving the intended purpose well because of constraints associated with both
pre- and post-implementation.Sediment
deposition in reservoirs is a serious off-site consequence of soil erosion in
the region. However, the extent of the problem is not well studied. Moreover,
there are no sufficient and reliable sediment-yield data for Northern Ethiopia, which are important for
designing new reservoirs and for implementing soil conservation practices. This
study addresses those problems by undertaking: (1) a survey and evaluation of
the general characteristics and problems of 54 recently built reservoirs and
the characteristics of their respective catchments; and (2) a detailed sediment
survey in ten reservoirs and related catchment
characteristics. A field survey in 2002–2003, a review of technical reports and
interviews were the bases for this study. Most of the reservoirs are under risk
of insufficient inflow, excessive seepage and sediment deposition. These
problems are mainly attributed to the use of a poor database on hydrology and
sediment yield, and the lack of adaptable methodologies for assessing
controlling factors at the planning stage.
The reservoir survey, which is the first of its
kind in Ethiopia, also indicates that specific sediment yield (SSY) varies significantly
between catchments: i.e. from 237 to 1817 t km_2 y_1 with an average of 909
(_500) t km_2 y_1. The high spatial variability is mainly associated with differences
in lithology, cover, extent of gully network and
human activities. Therefore, adapting an average SSY value for the whole region
is not recommended for future planning purposes. The sediment deposition
problem is significant: i.e. 70 per cent of the study reservoirs have important
siltation problems so that they will end their useful
life well before the dam design period. Hence, sediment management in
reservoirs could be an effective approach towards maintaining the existing
storage capacity.
For the realization of sustainable land and water
development in Tigray, sufficient and reliable
database building, development and standardization of appropriate methodologies
for predicting sediment yield and capacity building of designers needs to be
given top priority. Moreover, awareness creation among policy makers, donors
and beneficiaries is also important for action. Copyright #
2005 John Wiley & Sons, Ltd.
Keywords: catchment; dam
design period; northern Ethiopia; reservoir; sediment survey; sediment deposition;
specific sediment yield
ABSTRACT: The Nile is a vital resource of
north-eastern Africa.After a description of its
basin, and its hydrology, the different phases in the process of man gaining
control on the river are discussed.Presently, the Nile is used for irrigation, the generation of hydroelectric
power, navigation, and, of particular importance to limnologists,
fisheries.Future developments in the
management of the Nile are summarized.
Keywords:Sudan, Nile, hydrology, control,
utilization
M C Todd, E
C Barrett, M J Beaumont,and T J Bellerby
Abstract:A
continuously calibrated infrared (IR) geostationary satellite rainfall
estimation technique (CCB4) is introduced, in the context of the Nile River
Forecast System, an operational system for hydrological modelling
and forecasting. The CCB4 incorporates near-real-time rain gauge data to
continuously calibrate optimum IR rain/no-rain thresholds and daily rain rates
on a daily time step. The ability of the CCB4 and two comparative techniques to
estimate daily rainfall at the regional and pixel scales is assessed, using Meteosat IR imagery and gauge data from six wet season
months covering three years.The CCB4
shows improved skill in identifying rain days and estimating daily rain amounts
at a range of spatial scales, from regional to pixel scales. At the pixel
scale, however, improved root mean square errors remain relatively high,
ranging between 66% and 84% of the mean unconditional rain rate.
van Griensven A., Schuol, J.,
Diez Herrera, M., Bauwens, W.
Abstract: From Lake Victoria to the Mediterranean Sea, the length of the Nile is ca. 5600 km and the basin has an area of more ca. 3350000 km2. The
ten countries that make up the Nile river basin all contribute differently to
the basin and have different needs for the water resources. While the river Nile has been providing life to the
basin for thousands of years, many fear that population growth and the looming
water scarcity may result in international conflict in the coming decades.
However, initiatives such as the Nile Basin Initiative try to avert this
danger. Improved understanding of the integrated aspects of the water resources
management at the scale of the entire river basin is a key issue in order to
avoid future conflicts.For this
purpose, the Nile river basin hydroinformatics system is
developed, aiming at providing decision support for the management of the water
resources in the Nile river basin. The system consists of data-bases, linked modelling tools, GIS and webbased
features that are linked to public available data and public domain open-source
software such as Soil and Water Assessment Tool (SWAT).The benefit of such a system is to: (1) Centralise all data relevant to public domain data on the
water resources of the river Nile (2) Synthesise the
data and construct reports (3) Analyse causes and
impacts in the system (4) Design alternative management scenarios (5) Apply
models to evaluate management scenarios (6) Propagate small scale management
scenarios to larger scales (7) Communicate potential policies and effects to
stakeholders and the public (8) Support the design monitoring programmes
Vorosmarty CJ, Meybeck M, Fekete B, Sharma K, Green P, Syvitski
JPM
Abstract: In this paper, we
develop and apply a framework for estimating the potential global-scale impact
of reservoir construction on riverinesediment
transport to the ocean. Using this framework, we discern a large, global-scale,
and growing impact from anthropogenic impoundment. Our study links information
on 633 of the world's largest reservoirs (LRs)
(greater than or equal to 0.5 km(3) maximum storage capacity) to the geography
of continental discharge and uses statistical inferences to assess the
potential impact of the remaining >44,000 smaller reservoirs (SRs). Information on the LRs was
linked to a digitized river network at 30' (latitude x longitude) spatial
resolution. A residence time change (Deltatau(R)) for
otherwise free-flowing river water is determined locally for each reservoir and
used with a sediment retention function to predict the proportion of
incident sediment flux trapped within each impoundment. The
discharge-weighted mean Deltatau(R) for individual
impoundments distributed across the globe is 0.21 years for LRs
and 0.011 years for SRs. More than 40% of global
river discharge is intercepted locally by the LRs
analyzed here, and a significant proportion ( approximate
to 70%) of this discharge maintains a theoretical sediment trapping
efficiency in excess of 50%. Half of all discharge entering LRs
shows a local sediment trapping efficiency of 80% or more. Analysis of
the recent history of river impoundment reveals that between 1950 and 1968,
there was tripling from 5% to 15% in global LR sediment trapping,
another doubling to 30% by 1985, and stabilization thereafter. Several large
basins such as the Colorado and Nile show nearly complete
trapping due to large reservoir construction and flow diversion. From the
standpoint of sediment retention rates, the most heavily regulated
drainage basins reside in Europe. North America, Africa, and Australia/Oceania are
also strongly affected by LRs. Globally,
greater than 50% of basin-scale sediment flux in regulated basins is
potentially trapped in artificial impoundments, with a discharge-weighted sediment
trapping due to LRs of 30%, and an additional
contribution of 23% from SRs. If we consider both
regulated and unregulated basins, the interception of global sediment
flux by all registered reservoirs (n approximate to 45,000) is conservatively
placed at 4-5 Gtyear(-1) or
25-30% of the total. There is an additional but unknown impact due to still
smaller unregistered impoundments (n approximate to 800,000). Our results
demonstrate that river impoundment should now be considered explicitly in
global elemental flux studies, such as for water, sediment, carbon, and
nutrients. From a global change perspective, the long-term impact of such
hydraulic engineering works on the world's coastal zone appears to be
significant but has yet to be fully elucidated. (C) 2003 Elsevier Science B.V.
All rights reserved.
Abstract: The Blue Nile basin is severely affected
by slope failures, and the characteristics of its deep gorges and rugged valley
walls called for a study on the relationships between topography and the
process of landsliding and rock falling. Work was
commenced with the conception of nine types of landforms on the basis of a
one-to-one combination of lateral and vertical slope profiles and thence the
determination of the effect of these landforms on the occurrence of slope
failures. Observations showed that topographic surfaces with concave lateral
profiles shelter mudflows and some retrogressive rotational slumps while slopes
characterized by planar lateral profiles are sites mainly for translational
slides. Landslides are rare in convex-shaped slopes but when they occur,
they are big and deep-seated. As an effort to understand the significant
contributions of landslides and rock falls to landscape development,
direct and indirect methods are employed. Direct methods are based on
quantitative relationships between the volume of material that had been removed
from the area and the amount that could, in principle, be taken away based on
available erosion rates. Indirect methods used the nature of river incision and
the effect of the present-day landslides on the landscape. In general,
discrepancy in calculated figures in the first, and the overall drop and form
of the AbayRiver gorge coupled with the
observed landslide-caused landform changes in the second, led us to deduce that
slope failures were part of the mega-forces that shaped the entire Blue Nile basin, and in fact, played
the dominant role in landscape evolution. (C) 2003 Elsevier Science B.V. All
rights reserved.
Author
Keywords: slope
failure; landslide; rock fall; landscape; Ethiopia; Blue Nile
Abstract: Geochemical and
environmental isotope data were used to gain the first regional picture of groundwater
recharge, circulation and its hydrochemical evolution
in the upper Blue Nile River basin of Ethiopia. Q-mode statistical cluster
analysis (HCA) was used to classify water into objective groups and to conduct
inverse geochemical modeling among the groups. Two major structurally deformed
regions with distinct groundwater circulation and evolution history were
identified. These are the LakeTanaGraben
(LTG) and the YererTulluWellel Volcanic Lineament Zone (YTVL). Silicate hydrolysis
accompanied by CO2 influx from deeper sources plays a major role in groundwater
chemical evolution of the high TDS Na-HCO3 type thermal groundwaters
of these two regions. In the basaltic plateau outside these two zones, groundwater
recharge takes place rapidly through fractured basalts, groundwater flow
paths are short and they are characterized by low TDS and are Ca-Mg-HCO3 type
waters. Despite the high altitude (mean altitude similar to 2500 masl) and the relatively low mean annual air temperature
(18 degrees C) of the region compared to Sahelian
Africa, there is no commensurate depletion in 6180 compositions of groundwaters of the Ethiopian Plateau. Generally the
highland areas north and east of the basin are characterized by relatively
depleted 6180 groundwaters. Altitudinal depletion of
6180 is 0.1 parts per thousand/100 m. The meteoric waters of the Blue Nile
River basin have higher d-excess compared to the meteoric waters of the
Ethiopian Rift and that of its White Nile sister basin which emerges from the
equatorial lakes region. The geo-chemically evolved groundwaters
of the YTVL and LTG are relatively isotopically
depleted when compared to the present day meteoric waters reflecting recharge
under colder climate and their high altitude. (c) 2005 Elsevier Ltd. All rights
reserved.
Abstract: One of the best places
to investigate the role of a mantle plume in creating topography at the onset
of continental breakup is the Ethiopian volcanic province since it is the
youngest and best preserved case of a large igneous province dissected by a
system of rifts. In the center of the volcanic province, the northwestern
Ethiopian plateau which surrounds the Afar depression displays the highest
topography and experienced more than I km of surface uplift. Because hydrology
and physiography are genetically linked,
understanding the long-term evolution of these upper basins has implications
regarding the morphotectonic evolution of the plateau
as well as the paleo-hydrological evolution of the
whole NileRiver. We report the results of a
combined thermochronological and morphological study
aimed at understanding the long-term stability of the upper Blue Nile drainage network. Apatite
and titanite He ages have been determined for
crystalline basement samples collected below the lava pile, in various key
sites of the drainage network. Titanite He ages,
which range from 213 to 520 Ma, are in good agreement with published K-feldspar
Ar-Ar data and most likely reflect post-Pan-African
cooling of the Ethiopian basement. Apatite He ages, which range from 45 to 107
Ma, display a trend of decreasing age with increasing crustal
depth in the Blue Nile gorge, which is interpreted as partial resetting of
pre-existing ages due to basement burial because of the thick pile of flood
lavas erupted 30 Ma ago. Simulations of apatite He age partial resetting
suggest that erosion initiated in the Blue Nile canyon as early as 25-29 Ma
ago, whereas erosion would have started after 11 Ma along the present retreat
scarp. The early onset of erosion in the Blue Nile canyon suggests that: (i) volcanic and uplift-related tectonic divides represent
long-lived (20-30 Myr) pre-breakup divides, and (ii)
the elevated plateau physiography, which controls
most of the present-day Nile hydrology, has existed since the Oligocene. We
propose that the plateau represents the preserved part of a large uplifted
dome, related to Afar plume impingement, and/or to
massive underplating triggered by Oligocene
continental flood basalt differentiation. In this case, the topographic
development of the western Afar margin, which is related to extension and drift
of the Arabian plate, would be mainly the result of the collapse of the Afar
area from an initially elevated region. (C) 2003 Elsevier B.V. All rights
reserved.
Author
Keywords:thermochronology; (U-Th)/He; Nile basin; Ethiopian plateau;
surface uplift; Afar
Abstract: A rainfall-runoff model
based on an artificial neural network (ANN) is presented for the Blue Nilecatchment.
The best geometry of the ANN rainfall-runoff model in terms of number of hidden
layers and nodes is identified through a sensitivity analysis. The Blue Nilecatchment
(about 300 000 km(2)) in the Nile basin is selected here as a
case study. The catchment is classified into seven subcatchments, and the mean areal
precipitation over those subcatchments is computed as
a main input to the ANN model. The available daily data (1992-99) are divided
into two sets for model calibration (1992-96) and for validation (1997-99). The
results of the ANN model are compared with one of physical distributed
rainfall-runoff models that apply hydraulic and hydrologic fundamental
equations in a grid base. The results over the case study area and the
comparative analysis with the physically based distributed model show that the
ANN technique has great potential in simulating the rainfall-runoff process
adequately. Because the available record used in the calibration of the ANN
model is too short, the ANN model is biased compared with the distributed
model, especially for high flows. Copyright (c) 2005 John Wiley
& Sons, Ltd.
Author
Keywords: rainfall;
runoff; neural networks; distributed model
KeyWordsPlus: SATELLITE
CEEPA (Centre
for Environmental Economics and Policy in Africa) (R. Hassan)
Web Information: All the reports produced under this GEF/WB/CEEPA
funded project, Regional Climate, Water and Agriculture: Impacts on and
Adaptation of Agro-ecological Systems in Africa, are found on CEEPA
e-Library at its website link (http://www.ceepa.co.za/discussionp2006.html)
and can also be accessed directly through the project link (http://www.ceepa.co.za/Climate_Change/project.html)
ABSTRACTThis paper describes the application of hydrologic
models of the Blue
Nile and Lake Victoria sub-basins to assess the magnitude of potential
impacts of climate change on Main Nile discharge. The models are calibrated to
simulate historical observed runoff and then driven with the temperature and
precipitation changes from three general circulation model (GCM) climate
scenarios. The differences in the resulting magnitude and direction of changes
in runoff highlight the inter-model differences in future climate change
scenarios. A `wet’ case, `dry’ case and composite case produced +15
(+12), -9 (-9)
and +1 (+7) per cent changes in mean annual Blue Nile (Lake
Victoria) runoff for 2025, respectively. These ® gures
are used to estimate changes in the availability of Nile water in Egypt by making assumptions about the runoff response in
the other Nile sub-basins and the continued use of the Nile
Waters Agreement. Comparison of these availability scenarios
with demand projections for Egypt show a slight surplus of water in 2025 with and without
climate change. If, however, water demand for desert reclamation is taken into
account then water de® cits occur for the present-day
situation and also 2025 with (`dry’ case GCM only) and without climate change.
A revision of Egypt’s allocation of Nile water based on the recent low-¯ ow decade-mean ¯ ows of the Nile (1981± 90) shows that during this period Egypt’s water use actually exceeded availability. The
magnitude of `natural’ fluctuations in discharge therefore
has very important consequences for water resource management regardless
of future climate change.
BULLETIN
OF THE AMERICAN METEOROLOGICAL SOCIETY 75 (1): 83 - 94JAN 1994
Abstract:Long-term
rainfall forecasting is used in making economic and agricultural decisions in
many countries. It may also be a tool in minimizing the devastation resulting
from recurrent droughts. To be able to forecast the total annual rainfall or
the levels of seasonal floods, a class of models has first been chosen. The
model parameters have then been estimated with an appropriate parameter
estimation algorithm. Finally, diagnostic tests have been performed to verify
the adequacy of the model. These are the general principles of system
identification, which is the most crucial part of the forecasting procedure. In
this paper several sets of data have been studied using different statistical
procedures. The examined data include a historical 835-year record representing
the levels of the seasonal Nile floods in Cairo,
Egypt, during the period A.D. 622-1457. These readings were
originally carried out by the Arabs to a great degree of accuracy in order to
be used in estimating yearly taxes or Zacat (Islamic
duties). The observations also comprise recent total annual rainfall data over
Addis Ababa (Ethiopia) (1907-1984), the total annual discharges of Ethiopian
rivers (including the river Sobat discharges at HilletDoleib, Blue Nile
discharge at Roseris, river Dinder,
river Rahar, and river Atbara),
equatorial lake plateau supply as contributed at Aswan during the period
1912-1982, and the total annual discharges at Aswan during the period
1871-1982. Periodograms have been used to uncover
possible periodicities. Trends of rainfall and discharges of some rivers of
east and central Africa have been also estimated.
Using the first half of
the available record, two autoregressive integrated moving average (ARIMA) time
series models have been identified, one for the levels of the seasonal Nile floods in Cairo, the second to model the
annual rainfall over Ethiopia. The time series models
have been applied in I-year-ahead forecasting to the other half of the
available record and give fairly promising results, thus indicating the
adequacy of the fitted models.
Abstract: A simple water balance model was used
to generate monthly water surplus for 0.5 degrees grid cells of the Upper Blue Nile basin. An application of a
spatially distributed unit hydrograph method is presented to route generated
runoff from the grid cells. The work differs from previous modelling
work on the basin in that it accounts the spatial variability of the flow
parameters. The study illustrates that velocity is a more significant parameter
in determining the response of the basin. The key inference from the simulation
with spatially variable parameters is that, if variable velocity zones exist in
a basin, then it is important to consider the effect of non-uniform dispersion
coefficients as well. The study also demonstrated the successful simulation of
the hydrological cycle for a local/national data-poor basin using data sets
mainly from global archives.
Author Keywords:Blue Nile basin; flow routing model;
distributed model; unit hydrograph; water balance
Technical papers / American
Society for Photogrammetry and Remote Sensing (ASPRS) and the American Congress
on Surveying and Mapping (ACSM) Annual Convention: 133-1401991
Abstract: Three main sub-watersheds were
considered in this research.These
sub-watersheds are: the Rahad, the Dinder, and the rest of the Blue Nile Coarse, started from Lake Tana and
ended by Sennar Station.Different set of water discharge observations
were used as well as the regression sediment-yield model for the outlet station
of the Blue Nile, which
was established by Moussa (1987). The minimum norm
solution was carried out in order to determine the sediment discharges at each
outlet station.The relationship between
water discharges and sediment discharges was determined for each sub-watershed.This research will help us to construct the
Geographic Information System (GIS) for monitoring the sediment-yield from the Blue
Nile watershed.
ASPRS technical papers: 1989
ASPRS-ACSM Fall Convention, Cleveland, Ohio,
September, 17-21.From
compass to computer:379-3861989
Abstract: The Past drought in Ethiopia had resulted in a massive loss of vegetation within
the watersheds of the Blue Nile and AtbaraRivers.Due to the
enormous size of the watersheds and their inaccessibility, remote sensing
techniques are especially appropriate for assessing the effects of the drought
on soil and vegetation cover.The
objective of this research is to develop satellite data based regression
sediment-yield models for the Blue Nile and the AtbaraRiver watersheds during the storm period (August-October)
of the drought years in order to determine the watershed sediment-yield during
drought.These watersheds are ungaged and because there were no observations for the
suspended sediment, SS, concentration measured at or near the outlet stations
of the two watersheds, a hydraulic regression sediment-yield model was
established to determine the relationship between the water discharge and the
SS discharge at each outlet station. A regression sediment-yield model for each
watershed was developed to find the relationship between the Global Vegetation
Index (GVI), which determined from the Normalized Difference Vegetation Index
(NDVI) using the Advanced Very High Resolution Radiometer (AVHRR) sensors, and
the suspended sediment discharge during the drought period.
Abstract: The feasibility of
using a bi-spectral frequency analysis method to estimate daily mean areal precipitation (MAP) from 3-hourly METEOSAT visible
(VIS) and infrared (IR) images over the BlueNile river catchment (about 35-40 degrees East Longitude and 8-12
degrees North Latitude) in support of hydrologic studies is explored. The data
record used spans the period 14 July 14 August
1995.
At first, the study addresses the spatiotemporal variability of the satellite
images, the determination of the relationship between topography and satellite
data and the inference of MAP from satellite images using the bi-spectral
method and a multivariate regression. Validation of the estimated MAPs is carried out with data from a sparse raingauge network in the region. These estimates are next
used in a sensitivity study to determine the dependence of the BlueNile region hydrologic response
on the type of precipitation forcing (raingauge-based
vs, satellite-based estimates). A conceptual
semi-distributed hydrologic model is used to simulate hydrologic processes
pertaining to soil water and channel routing with a 1 X 1 degrees resolution.
Principal conclusions of this initial sensitivity study are: (a) use of the
bi-spectral method, complemented with an appropriate multivariate regression
formulation, improves MAP estimates during the aforementioned time interval
substantially, for daily rainfall rates greater than about 7 mm day(-1); (b)
aggregate hydrologic response of the BlueNile region is very
sensitive to the type of precipitation forcing used; and (c) substantial
spatial variability of the sensitivity of hydrologic response to the type of
precipitation forcing exists in the region. The use of satellite-derived MAP
estimates is recommended together with recalibration of hydrologic models
using spatially variable parameter values, and analysis of uncertainty
propagation through model components and for Various
sub-catchments. (C) 1999 Elsevier Science B.V. All rights reserved.
Author
Keywords: daily
mean areal precipitation; spatio-temporal
variability; bi-spectral method
Abstract: This paper describes the development and
validation of a water balance model of the Upper BlueNile in Ethiopia. A major requirement of any modelling attempt is the availability of climatic and
hydrological data. However, for the Upper BlueNile, only a limited number of
observation sites are available over a very large area. As a result, the model
described here is a grid-based water balance model which requires limited data
inputs, few parameters and runs on a monthly time-step. Climate is dominated by
the influence of elevation in the river basin. Estimates of potential evapotranspiration (PE) and rainfall are predicted for
10-minute resolution grid cells for input to the model. These estimates are
based on multiple regression models using latitude, longitude and elevation. In
the basin, annual mean PE and rainfall range, with increasing elevation, from
1800 mm to 1200 mm and 924 mm to 1845 mm, respectively. In the model,
vegetation cover is not explicitly treated and soil characteristics are
spatially invariant. The model is calibrated to reproduce mean monthly runoff
over a 37-year period (1953-1987), and validated by its ability to simulate
sub-catchment runoff and historical variations in BlueNile runoff. The key factor that
determines model performance is the quality of the rainfall inputs, with the best
results obtained with a time series comprised of long, good quality station
data. Over a 76-year period the correlation between observed and simulated
annual flows was 0.74 and the mean error was 14%, although fairly large errors
occurred in individual years. Considering the paucity of data for the basin,
these results are encouraging. The model is used to investigate spatial
variability in the sensitivity of runoff to changes in rainfall and PE. The
sensitivity is greatly affected by the runoff ratio of the model grid cells and
it increases as grid cell runoff ratio decreases. The sensitivity is also
affected by the seasonal distribution of rainfall. The paper ends with a
discussion of the model's performance and its potential for future development.
JOURNAL OF IRRIGATION AND DRAINAGE ENGINEERING-ASCE,
120 (3): 573-590
MAY-JUN 1994
Abstract: The Blue NileRiver in Ethiopia has a drainage area of
approximately 324,530 km2 and supplies nearly 84% of the water to the NileRiver during highflow
season, making it the main source of water for Ethiopia, Sudan, and Egypt. Relatively few analyses
have been conducted on the Blue NileRiver basin largely due to the lack of
hydrologic data. The objective of this paper is to present existing stream-flow
data for the Blue NileRiver and its tributaries within
the Blue NileRiver basin in Ethiopia and to use that data to
provide insight into the river's hydrology in Ethiopia. A monthly water-balance
model was developed for this purpose. Comparisons of the predicted and observed
monthly hydrographs are provided for selected subwatersheds
within the Blue Nile basin. Spatial distribution of the calibrated coefficients is also
discussed. Results will be useful in forecasting flows along the Blue Nile and NileRivers and in determining the
effect of global climatic changes on continental hydrology.
Abstract: In this study, a
regional flood frequency analysis has been carried out, using the index flood
L-moments approach. Annual maximum stream flood data observed at 14
gauged sites on the NileRiver tributaries (Blue Nile, White Nile, and AtbaraRiver) are investigated. The aim
of the study is to investigate and derive hydrologically
homogeneous region or regions and to identify and establish the regional
statistical distribution. To this end, five distribution functions are used,
namely: generalized pareto,
generalized extreme-value, generalized logistic, generalized normal, and
Pearson type-3 distributions. Analyses have shown that 8 sites form a hydrologically homogeneous region, and this region follows
a generalized logistic (GLO) distribution. Furthermore, the other remaining two
regions (possibly heterogeneous and definitely heterogeneous) are also defined.
Regional dimensionless growth curves for the identified three regions are
derived. Results are assessed on the basis of relative RMSE% and relative BIAS%
through the use of Monte Carlo simulation.
Author
Keywords:flood
frequency; L-moments; probability weighted moments; regionalization
KeyWordsPlus: PROBABILITY-WEIGHTED
MOMENTS; FREQUENCY-ANALYSIS; STATISTICS; MODEL
Abstract: The National Oceanic
and Atmospheric Administration is developing a river
forecast system for the NileRiver in Egypt. The river forecast system
operates on scientific work stations using hydrometeorologicalmodels and software to predict inflows into the high Aswan Dam and
forecast flow hydrographs at selected gaging
locations above the dam.
The
Nile
Forecasting System (NFS) utilizes satellite imagery from the METEOSAT satellite
as the input to the forecast system. Satellite imagery is used to estimate
precipitation over the BlueNIleBasin
using five different techniques. Observed precipitation data and climatic
statistics are used to improve precipitation estimation. Precipitation data for
grid locations are input to a distributed water balance model, a hill slope
muting model, and a channel routing model. A customized Geographic Information
System (GIS) was developed to show political boundaries, rivers, terrain
elevation, and gaging network. The GIS was used to
develop hydrologic parameters for the basin and is used for multiple display
features.
Abstract:Correlation analysis is used to determine the
linear relationship between the NileRiver flows and leading climatic indicators, such as SST
and precipitation, in an effort to establish a basis for quantitative long-term
streamflow prediction. The analysis of the lead–lag
correlations between the Blue Nile River flows during the “flood season’’
[July–August–September–October (JASO)] and SSTs led to the identification of a
number of regions in the oceans that are significantly correlated and suggests
that the SSTs may be useful for predicting the Blue Nile flows. The significant
correlation regions between SST in the Pacific and Blue Nile JASO flows evolve
through time in a manner that is consistent with the ENSO development; that is,
the evolution of the ENSO signal in the Pacific Ocean is reflected in the evolution of the referred
cross-correlation field. In addition, the Blue Nile River JASO flow is
significantly correlated with the previous year August–November Guinea
precipitation, which suggests that the Guinea precipitation is another potential predictor of
the Blue
NileRiver flows with 11 months of lead time. Furthermore,
models based on multiple linear regression (MLR) and principal component
analysis (PCA) are used to forecast the Blue Nile flows based on SST in the three oceans and the previous
year of Guinea precipitation. The models based on PCA showed
significant improvement in forecast accuracy over MLR models that were
developed in terms of the original variables. The predictability is shown to be
the highest for forecasts made in the preceding season and decreases as the
lead time increases. The coefficients of multiple determinationR2 for validation based on PCA models vary in the
range 84%–59% for forecast lead times of 4–16 months. Further analysis using
only SST predictors for the period 1913–89 indicates that the predictability of
the Blue Nile River JASO flows is more affected by the variability of SSTs in
the Pacific
Ocean than by
those of the other oceans. The conclusion is that long-range forecasting of the
Blue
NileRiver flows with lead times over 1 yr is possible with a
high degree of explained variance by using SST in a few regions in the Pacific Ocean and the previous year of Guinea precipitation.
Abstract: Natural variability in
the annual flow of the NileRiver has been the subject of
great interest to the civilizations that have historically occupied the banks
of that river. Here we report results from analysis on two extensive data sets
describing sea surface temperature of the Pacific Ocean, and the flow of water in
the NileRiver. The analysis suggests that 25% of the natural
variability in the annual flow of the Nile is associated with ElNinooscillations, A procedure is developed for
using this observed correlation to improve the predictability of the Nile flood. A simple hypothesis
is presented to explain physically the occurrence of the Hurst phenomenon in the Nile flow.
Abstract: We constructed a
hydrologic model to estimate the groundwater recharge rate for alluvial
aquifers of the EasternDesert from sporadic precipitation
over the Red Sea hills. To estimate initial losses over sub-basins, transmission losses
through channel routing, and downstream runoff, we developed an
integrated model combining spatial rainfall distribution, ail appropriate basin
unit hydrograph. and appropriate infiltration
parameters. Watersheds and stream networks identified from digital terrain
elevation data were verified by comparison with co-registered Landsat thematic mapper scenes
and geologic maps. Records of a November 1994 storm event acquired from rain
gauges along the NileRiver and the Red Sea shore were used to generate
a spatial precipitation distribution for the study area, A 2 hour design
hyetograph was adopted from rain gauge data for the 1994 flood event. The model
was tested against records from the November 1994 flood event at the outlets of
the Tarfa and Hammamat
watersheds. Groundwater recharge rates were estimated for the alluvial aquifers
within the major watersheds of the north EasternDesert. We estimated that during
the 1994 flood event. the ground waterrecharge through
transmission losses ranged from 2 1 to 3 1 (Tarfa:
15.8 X 10(6) m(3): Asyuti: 20 X 10(6) m(3). Qena: 49 X 10(6) m(3), Hammamat: 59 X 10(6) m(3)) of the precipitated volume. The
initial losses ranged from 65 to 77% Only 3-7% of the
precipitation reached the watershed outlets, Archival data show that rainfall
events of the size of the November 1994 storm or larger occur every 40 months
thus. the annual recharge rates for the Tarfa. Asyuti, Qena, and Hammamat alluvial
aquifers are estimated at 4.7 X 10(6) in. 6 X 10(6) m(3)
14.7 x 10(6) m(3) and 17.7 X 10(6) m(3), respectively. Implications for the use
of these renewable ground waters, and similar water
resources in other arid areas of Egypt and in neighboring
countries are clear. (C) 2002 Elsevier Science B.V. All rights reserved.
Author
Keywords: hydrologic
model; sporadic precipitation; alluvial aquifer's; Wadirunoff; groundwater recharge; landsat;
geographical information systems
Abstract: Sediment deposition in
reservoirs is a serious off-site consequence of soil erosion in Tigray (Northern Ethiopia). So far insufficient and
less reliable sediment yield data have been collected for Northern Ethiopia. Nor are there any adaptable
methodologies for sediment yield assessment in the country as a whole, which
could be used when designing new reservoirs. This study addresses those
problems by (1) undertaking reservoir sediment deposition measurements and (2)
by calibrating and adapting the Pacific Southwest Inter Agency Committee
(PSIAC) and the Factorial Scoring Model (FSM) sediment yield assessment models
to Ethiopian conditions. Field rating of catchment
characteristics and the sediment yield data from the reservoir survey were used
for calibration and validation of the models. Our reservoir survey
indicates that specific sediment yield (SSY) varies significantly between
catchments: i.e. 487 t km(-2) year(-1) to 1817 t
km(-2) year(-1) with an average of 1054 ( 446) t km(-2) year(-1). Since the
variability of SSY is high between the studied reservoirs, care should be taken
in the study area to adopt representative SSY values during reservoir and soil
water conservation planning. The PSIAC SSY prediction is found to fit well with
observed SSY without adjustment. While the FSM was found to have, after
modifying the description of factors and incorporating new controlling factors,
a good fit between the predicted and observed SSY. Studies of the relationship
between the known sediment yield rates and the catchment
conditions using semi-quantitative approaches such as PSIAC and FSM can be of
substantial benefit in extrapolating data for areas where no detailed
information is available in a cheap and quick way. However, calibration and
modification of such models may be necessary if they are to be used
beyond the region where they were developed. (c) 2005 Elsevier B.V. All rights
reserved.
Author
Keywords:catchment; Northern Ethiopia; reservoir survey;
sedimentation; specific sediment yield; PSIAC and FSM models
Agriculture,
Ecosystems and Environment 105: 29-402005
Abstract: Severe land degradation
affects the livelihood of many farmers in the highlands of Tigray,
northern Ethiopia. Various soil and water
conservation practices have been proposed to reduce land degradation and to
improve the quality of the natural resource base but quantitative information
on their agro-ecological effects is often lacking. In this study, effects of
three soil and water conservation practices are assessed using a crop growth
simulation model (WOFOST), a nutrient monitoring model (NUTMON) and a
hydrological erosion model (LISEM), which are applied
at field, farm and regional scale, respectively. Evaluated soil and water
conservation practices include (i) bunds along field
contours to improve water availability for crop production, (ii) mulching of
crop residues to improve soil nitrogen stocks and (iii) reforestation to reduce
erosion. Data from the watershed Gobo Deguat in
northeastern Tigray have been used as a case-study.
Bunds slightly increased crop productivity at sowing dates when water-limited
yields were low, while productivity decreased at more favorable sowing dates
due to the reduction in cropped area required for the construction of bunds.
Crop residues used as mulch hardly contributed to lower soil nitrogen depletion
at farm level as the total amount of nitrogen in crop residues was limited,
while part of the crop residue nitrogen returned to the soil anyway in the form
of manure and compost. Reforestation reduced erosion up to 14% but this is
insignificant compared to the sacrifice in cultivated land which needs to be
reforested. Applied tools allow rapid ex-ante evaluation of soil and water
conservation practices and may contribute to improved cost-benefit analysis of
proposed measures, and identification of more appropriate means to combat land
degradation. The analyses show important trade-offs and the effects of the
assessed soil and water conservation practices may partially explain their low
rate of adoption in Tigray.
Abstract: Despite its local and regional
importance, hydrometeorological data on the Sudd (one of Africa's largest wetlands) is very scanty. This is
due to the physical and political situation of this area of Sudan. The areal
size of the wetland, the evaporation rate, and the influence on the micro and meso climate are still unresolved questions of the Sudd hydrology.
The evaporation flux
from the Sudd wetland has been estimated using
thermal infrared remote sensing data and a parameterization of the surface
energy balance (SEBAL model). It is concluded that the actual spatially
averaged evaporation from the Sudd wetland over 3
years of different hydrometeorological
characteristics varies between 1460 and 1935 mm/yr. This is substantially less
than open water evaporation. The wetland area appears to be 70% larger than
previously assumed when the Sudd was considered as an
open water body. The temporal analysis of the Sudd
evaporation demonstrated that the variation of the atmospheric demand in
combination with the inter-annual fluctuation of the aroundwater
table results into a quasi-constant evaporation rate in the Sudd,
while open water evaporation depicts a clear seasonal variability. The
groundwater table characterizes a distinct seasonality, confirming that
substantial parts of the Sudd are seasonal swamps.
The
new set of spatially distributed evaporation parameters from remote sensing
form an important dataset for calibrating a regional climate model enclosing
the NileBasin. The Regional Atmospheric Climate Model
(RACMO) provides an insight not only into the temporal evolution of the hydro-climatological parameters, but also into the land surface
climate interactions and embedded feedbacks. The impact of the flooding of the Sudd on the Nilehydroclimatology has been analysed
by simulating two land surface scenarios (with and without the Sudd wetland). The paper presents some of the model results
addressing the Sudd's influence on rainfall,
evaporation and runoff of the river Nile,
as well as the influence on the microclimate.
The paper presents a
case study that confirms the feasibility of using remote sensing data (with
good spatial and poor temporal coverage) in conjunction with a regional climate
model. The combined model provides good temporal and spatial representation in
a region characterized by extremely scarce ground data.
Abstract: This paper presents the
result of the regional coupled climatic and hydrologic model of the NileBasin. For the first time the
interaction between the climatic processes and the hydrological processes on
the land surface have been fully coupled. The hydrological model is driven by
the rainfall and the energy available for evaporation generated in the climate
model, and the runoff generated in the catchment is
again routed over the wetlands of the Nile to supply moisture for
atmospheric feedback. The results obtained are quite satisfactory given the
extremely low runoff coefficients in the catchment.
The
paper presents the validation results over the subbasins:
Blue Nile,
White Nile,
Atbarariver, the Sudd swamps, and
the Main Nile for the period 1995 to 2000. Observational datasets were used to
evaluate the model results including radiation, precipitation, runoff and evaporation data. The evaporation data were derived
from satellite images over a major part of the Upper
Nile. Limitations in both the observational data and
the model are discussed. It is concluded that the model provides a sound
representation of the regional water cycle over the Nile.
The sources of atmospheric moisture to the basin, and location of
convergence/divergence fields could be accurately illustrated. The model is
used to describe the regional water cycle in the Nile
basin in terms of atmospheric fluxes, land surface fluxes and land surface-climate
feedbacks. The monthly moisture recycling ratio (i.e. locally generated/total
precipitation) over the Nile
varies between 8 and 14%, with an annual mean of 11%, which implies that 89% of
the Nile water resources
originates from outside the basin physical boundaries. The monthly
precipitation efficiency varies between 12 and 53%, and the annual mean is 28%.
The mean annual result of the Nile
regional water cycle is compared to that of the Amazon and the Mississippi
basins.
Abstract: The water balance of
the upper Nile swamps, in particular, the Sudd, has been the
topic of debate for many years. Surface Energy Balance Algorithm for Land
(SEBAL) is a parameterization scheme of surface heat fluxes based on spectral
satellite measurements. The SEBAL scheme has been applied to derive the energy
balance components from National Oceanic Atmospheric Administration-Advanced
Very High Resolution Radiometer (NOAA-AVHRR) images over the extensive swamps
in the upper Nile. The study area covers the swamps of the Sudd,
Bahr
el Ghazal and the Sobat
sub-basins. The actual evaporation and soil moisture for an area between
2-12degreesN and 26-36degreesE (approximately 1000 km x 1000 km) have been
investigated. Monthly (actual) evaporation and soil moisture maps for the year
2000 have been generated. The evaporation results were verified against longer-term
averaged rainfall and flow data of the three sub-basins. A close resemblance
was obtained for the Sudd (1.8% error) and the SobatBasin (5.7% error), while the
balance lacks closure for the Ghazal basin (27%) due
to un-gauged or inadequately gauged inflow from the upper catchments. It is
concluded that soil moisture availability controls the monthly evaporation
rates for all areas that are not saturated with water, and that a significant
dry-down in the winter period occurs. It is concluded that the evaporation for
the Sudd wetlands is 20% less and the average area
occupied by the wetlands is 74% larger than assumed in earlier hydrological
studies (e.g. Jonglei canal studies). The derived
results can serve as a sound basis to support the widely debated evaporation
losses from the Sudd, as well as form an input to
regional scale climate models for studying atmospheric circulation patterns
over Africa and the NileBasin. (C) 2003 Elsevier B.V. All
rights reserved.
Author
Keywords: evaporation;
water balance; remote sensing; surface energy balance algorithm for land; Nile; Sudd
swamps
Abstract: Ongoing land
degradation in Tigray (Ethiopia) requires urgent action at
different levels of society. Soil and water conservation activities are now
widespread, integrating local knowledge, farmers' initiatives and introduced
technologies. Hengsdijk et al. [Hengsdijk, H., Meijerink, G., Mosugu, M., 2005.Modelling
the effect of three soil and water conservation practices in Tigray, Ethiopia. Agric. Ecosyst.
Environ. 105, 29-40.] evaluate this through the
application of a suite of models, the results of which show that after
stone bund building, productivity would decrease. Furthermore, their
simulations suggest that crop residues used as mulch would hardly contribute to
lower soil nitrogen depletion at farm level. Nearly complete forestation of the
catchment would reduce erosion by only 14%,
which is deemed insignificant compared to the sacrifice in cultivated land that
needs to be forested. Overall, the results of the model simulations lead Hengsdijk et al. to suggest that conservation efforts in Tigray are inefficient and absorbing a disproportionate
amount of resources, which could have been spent differently and more
efficiently if model simulations would have been used as an ex-ante evaluation.
Here, we compare the results of the model simulations by Hengsdijk
et al. with field data that we collected over the last decade in the Tigray area. Based on the results of this comparison, we
question the validity of the conclusions by Hengsdijk
et al. regarding the efficiency of soil conservation measures in Tigray. We believe this discussion illustrates, at a more
general level, the difficulties in transposing environmental models from
one region to another. Extensive fieldwork remains necessary for site-specific
calibration and validation. Neglecting to do so may result in improper
understanding of the issues at hand and consequently in ill-targeted and costly
remediation schemes. (c) 2005 Elsevier B.V. All rights reserved.
Author
Keywords: soil
and water conservation; model calibration and validation; sediment yield; stone
bund; exclosure; crop yield
KeyWordsPlus:EROSION;
RUNOFF; VEGETATION; IMPACT; CHINA
Nyssen, J., Poesen, J., Moeyersons, J., Deckersd, J., Haile, M., Lange, A.
Abstract: This review
analyses the environmental evolution of the Ethiopian highlands in the late
Quaternary. The late Pleistocene (20,000–12,000 14C years BP) was
cold and dry, with (1) low lake levels in the Rift Valley, (2) large debris
fans on the flanks of Lake Abhe´ basin, and (3) the
Blue Nile transporting coarse bedload. Then, a period
with abundant and less seasonal rains existed between 11,500 and 4800 14C years BP, as
suggested by increased arboreal pollen, high river and lake levels, low river
turbidities and soil formation. Around 5000–4800 14C years BP,
there was a shift to more arid conditions and more soil erosion.Many phenomena that were previously
interpreted as climate-driven might, however, well be of anthropic
origin. Thick sediment deposits on pediments as well as an increase of
secondary forest, scrub and ruderal species in pollen
diagrams are witnesses of this human impact.One important aspect of the late Quaternary palaeoenvironment
is unclear: changes in Nile flow discharges
and Rift Valley lake levels have been linked to changes in precipitation depth.
Most authors do not take into account changes in land use in the highlands, nor changes in the seasonality of rain, both of which can
lead to a change in runoff coefficients. Tufa and speleothem deposition around 14,000 years ago tend to
indicate that at the end of the Last Glacial Maximum (LGM), conditions might
have been wetter than generally accepted.The most important present-day geomorphic processes are sheet and rill
erosion throughout the country, gullying in the highlands,
and wind erosion in the Rift Valley and the peripheral lowlands. Based on
existing sediment yield data for catchments draining the central and northern
Ethiopian highlands, an equation was developed allowing to
assess area-specific sediment yields:
SY = 2595A-0.29(n = 20; r2 = 0.59)
where SY=
area-specific sediment yield (t km_ 2 year_ 1), and A= drainage area (km2).
With respect to
recent environmental changes, temporal rain patterns, apart from the
catastrophic impact of dry years on the degraded environment, cannot explain
the current desertification in the driest parts of the country and the
accompanying land degradation elsewhere. Causes are changing land use and land
cover, which are expressions of human impact on the environment. Deforestation
over the last 2000–3000 years was probably not a linear process in Ethiopia. Studies on
land use and land cover change show, however, a tendency over the last decades
of increasing removal of remnant vegetation, which is slowed down or reversed
in northern Ethiopia by a set-aside
policy.Ongoing land degradation
requires urgent action at different levels of society. Soil and water
conservation (SWC) structures are now widely implemented. Local knowledge and
farmer’s initiatives are integrated with introduced SWC techniques at various
degrees. Impact assessments show clear benefits of the soil conservation
measures in controlling runoff and soil erosion.In high rain areas, runoff management
requires greater emphasis during the design of soil conservation structures. In
such areas, investment in SWC might not be profitable at farm level, although
benefits for society are positive. This pleads in favour
of public support.The present land
degradation in the Ethiopian highlands has a particular origin, which includes
poverty and lack of agricultural intensification. Causes of these are to be
found in the nature of past and present regional social relations as well as in
international unequal development. This review strengthens our belief that,
under improved socio-economic conditions, land husbandry can be made
sustainable, leading to a reversal of the present desertification and land
degradation of the Ethiopian highlands.D 2003 Elsevier
B.V. All rights reserved.
Keywords: Deforestation; Desertification; Ethiopia; Human
impact; Land degradation; Late Quaternary; Sediment deposition; Soil erosion
Sene KJ,
Tate EL, Farquharson FAK Sensitivity
studies of the impacts of climate change on White Nile flows
CLIMATIC CHANGE 50 (1-2): 177-208 2001
Abstract: Several exploratory
studies are presented on the sensitivity of the water balance of the White Nile to climate change, using
both observed and stochastic time series to drive the models. Example
results are presented using various assumed climate change scenarios and
results from a General Circulation Model (GCM). The relative merits and
shortcomings of each modelling approach are also
discussed. A simple analytical model for Lake Victoria is also used to illustrate
some of the overall features of the lake's likely response. Particular
difficulties with the White Nile system are that, due to the huge area
of open water in the basin, transient responses to short-lived events can occur
over timescales comparable with those for which long term climate change
impacts are being studied, and predicted changes in flows are extremely
sensitive to estimates for the rainfall and evaporation at lake and swamp
surfaces. Of the modelling approaches considered, the
network simulation approach with stochastic inputs is recommended as a way of
smoothing out these transient effects, and assessing the uncertainty in the
results due to inaccuracies in the data, the model parameters and the climate change
predictions. The paper concludes with a brief discussion of some other areas of
uncertainty in the hydrological modelling of White Nile flows and possible
alternative external forcing mechanisms for flows in the next few decades.
KeyWords Plus: WATER-BALANCE MODEL; LAKE VICTORIA; EASTERN
AFRICA; INDIAN-OCEAN; SHORT RAINS; BLUE-NILE; RAINFALL; ETHIOPIA;
BASIN; SIMULATION
Abstract: In many developing
countries, sustainable land management and water resources development are
threatened by soil erosion and sediment-related problems. In response to
such threats, there is an urgent need for improved catchment-based
erosion control and sediment management strategies. The design and
implementation of such strategies require data on erosion rates and
understanding of the factors that control the delivery of sediment through the catchment system. In this study, reservoir sedimentation
and corresponding catchment attribute data were used
to investigate the major factors controlling sediment yield variability in a
mountainous dryland region of nor-them Ethiopia. Sediment yield data were
acquired for representative 11 catchments above reservoirs. Geomorphological
and anthropogenic catchment attributes were extracted
for each reservoir from different sources including digital elevation models,
satellite images and field surveys. Different statistical analyses such as
Pearson's correlation, principal components and multiple regression
were implemented to analyze the relationship between sediment yield and catchment characteristics and to determine the major
factors controlling the variability of sediment yield. The results show that
terrain form, gully erosion, surface lithology,
and land cover explain most of the variability in sediment yield among the
catchments. The implications of the results, for relevant management
intervention targeted at ameliorating the major causative factors of erosion,
are also outlined. (c) 2005 Elsevier B.V. All rights reserved.
Abstract: Using the macrophysical climate model of Bryson [Bryson, R.A., 1992. A macrophysical model of the Holocene intertropical convergence and jetstream
positions and rainfall for the Saharan region.Meteorol.Atmos. Phys., 47, pp. 247-258], it is possible to calculate
the monthly latitude of the jetstream and the
latitude of the subtropical anticyclones. From these and modem climatic data,
it is possible to model the two-century mean latitude of the intertropical convergence (ITC) month by month and estimate
the monthly monsoon rainfall using the ITC-Rainfall model of Ilesanmi [Ilesanmi, O.O., 1971. An empirical formulation of an ITD rainfall model for the tropics -
a case study of Nigeria.J. Appl. Meteorol., 10, pp. 882-891]
and similar relationships. Input to this model is only calculated
radiation and atmospheric optical depth estimated from a database of global volcanicity. Recent work has shown that it is possible to
extend these estimates to both precipitation and temperature at specific sites,
even in mountainous terrain.
Testing
of the model against archaeological records and climatic proxies is now
underway, as well as refining the fundamental model. Preliminary indications
are that the timing of fluctuations in the local climate is very well modeled.
Especially well matched are the modeled Nile
flood based on calculated rainfall on the Blue and White
Nile watersheds and the level of LakeMoeris
[Hassan, F., 1985. Holocene lakes
and prehistoric settlements of the Western Faiyum, Egypt. J. Archaeol.
Res., 13, pp. 483-501].
Modeled
precipitation histories for specific sites in China,
Thailand,
the Arabian Peninsula,
and North Africa
will be presented and contrasted with the simulated rainfall history of Mesopotamia.
(C) 2000 Elsevier Science B.V. All rights reserved.
Abstract: A global hydrological routing algorithm
(HYDRA) that simulates seasonal river discharge and changes in surface water
level on a spatial resolution of 5' long x 5' lat is presented. The model is
based an previous work by M. T. Coe and incorporates major improvements from
that work including 1) the ability to simulate monthly and seasonal variations
in discharge and lake and wetland level, and 2) direct representation of
man-made dams and reservoirs. HYDRA requires as input daily or monthly mean
averages of runoff, precipitation, and evaporation either from GCM output or
observations.
As an example of the
utility of HYDRA in evaluating GCM simulations, the model is forced with
monthly mean estimates of runoff from the National Centers for Environmental
Prediction (NCEP) reanalysis dataset. The simulated river discharge clearly
shows that although the NCEP runoff captures the large-scale features of the
observed terrestrial hydrology, there are numerous differences in detail from
observations. The simulated mean annual discharge is within +/-20% at only 13
of 90 fluvial gauging stations compared. In general, the discharge is
overestimated for most of the northern high latitudes, midcontinentalNorth America,
eastern Europe. central and
eastern Asia,
India,
and northern Africa.
Only in western Europe and eastern North
America is the discharge consistently
underestimated. Although there appears to be a need for improved simulation of
land surface physics in the NCEP product and parameterization of how velocities
within HYDRA, the timing of the monthly mean discharge is in fair agreement with
the observations.
Including lakes within
HYDRA reduces the amplitude of the seasonal cycle of discharge and the
magnitude of the annual mean discharge of the St.
Lawrence River system, in qualitative agreement with
the observations. In addition, including the wetlands of the Sudd reduces the magnitude of the simulated annual
discharge of the NileRiver
to values in better agreement with observations.
Finally, the impact of
man-made dams and their reservoirs on the magnitude of monthly mean discharge
can be explicitly included within HYDRA. As an example, including darns and
reservoirs on the Parana
River improves the agreement of the simulated mean
monthly discharge with observations by reducing the amplitude of the seasonal
cycle to values in good agreement with the observations.
The results of this
study show that, although improvements can be obtained through better
representations of flow velocities and more accurate digital elevation models.
HYDRA can be a powerful tool for diagnosing simulated terrestrial hydrology and
investigations of global climate change.
Abstract: Hydrological impacts of climate change
are frequently assessed by off-line forcing of a hydrological model with
climatic scenarios from either Global Circulation Models (GCMs)
or simpler analogue models. Most hydrological models require a daily time step
or smaller while observed climatology and GCM and analogue model output is
generally available on a monthly time step. This study investigates and
improves a rainfall disaggregation model currently
used to convert monthly rainfall totals down to the daily time step. The
performance of the model is evaluated using daily data from a network of raingauges covering the Nile basin and contrasted with
data from a relatively dense raingauge network from
the BlackwaterCatchment,
in the Southeast of the UK. Whilst the model preserves
the mean properties of rainfall occurrence and depth, there is significant
overestimation of rainfall variability. Regional calibration and better
formulation of the generator improve simulation of variability as well as other
aspects of rainfall properties. Hence the parameters required by the weather
generator model cannot be regarded as universal. Proportional correction of
daily amounts is applied to insure that monthly totals are preserved,
allowing retention of interannual variability, and
this was shown to have little effect on the distribution of wet day amounts.
The calibration of parameter estimation equations has investigated spatial
dependence of climate variables and parameters and found that (as expected)
rainfall properties exhibit scale-dependence, which may be utilized to transfer
data from one spatial scale to another. In order to complete the framework, a
model is developed to estimate the wet fraction from monthly total when the
former is not available. (c) 2005 Elsevier B.V. All rights reserved.
Abstract: A nonparametric kernel
estimation model is proposed and developed for estimating low flow quantiles. Based on annual minimum low flow data and Monte Carlo simulation tests, the
descriptive and predictive ability of the proposed model is compared with that
of Weibullmodels. The results indicate that
it has small bias and root mean square error in low flow quantile
estimates. Application of the model to data from the BlueNile has shown that the
nonparametric approach is a viable alternative to the Weibullmodels.
Abstract: The majority of data
sets in the geosciences are obtained from observations and measurements of
natural systems, rather than in the laboratory. These data sets are often full
of gaps, due to to the conditions under which the
measurements are made. Missing data give rise to various problems, for example
in spectral estimation or in specifying boundary conditions for numerical
models. Here we use Singular Spectrum Analysis (SSA) to fill the gaps in
several types of data sets. For a univariate record,
our procedure uses only temporal correlations in the data to fill in the
missing points. For a multivariate record, multi-channel SSA (M-SSA) takes
advantage of both spatial and temporal correlations. We iteratively produce
estimates of missing data points, which are then used to compute a
self-consistent lag-covariance matrix; cross-validation allows us to optimize
the window width and number of dominant SSA or M-SSA modes to fill the gaps.
The optimal parameters of our procedure depend on the distribution in time (and
space) of the missing data, as well as on the variance distribution between
oscillatory modes and noise. The algorithm is demonstrated on synthetic
examples, as well as on data sets from oceanography, hydrology, atmospheric
sciences, and space physics: global sea-surface temperature, flood-water
records of the NileRiver, the Southern Oscillation
Index (SOI), and satellite observations of relativistic electrons.
Abstract: With the objective of
studying conceptual rainfall-runoffmodelling
performance in different climates, the HBV-96 model was applied on four
catchments located in Europe, Africa and South America. Manual, automatic and Monte
Carlo techniques were used for model calibration and parameter analyses. It was
found that the magnitude of the water balance components had a significant
influence on model performance. Performance decreased and demands of
calibration period length increased with increased catchment
dryness primarily because of a neater water balance and higher climatic
variability in drier areas. A large degree of equifinality
was discovered in all catchments where different calibration methods yielded
equally good results but with different parameter combinations. Thus, it may be
impossible to know if an optimum parameter set exists and to relate parameter
values to physical properties of the catchment. On
the other hand the validation results indicated that it might not matter if
parameter values were not unique when studying runoff solely, provided
the model application is within the same regime of flows. (C) 2000 Elsevier
Science B.V. AU rights reserved.
Author
Keywords: conceptual
rainfall-runoffmodelling; climate;
calibration; Monte Carlo simulation
Abstract: The hydrologic cycle is a major part of the global
climate system. There is an atmospheric flux of water from the ocean surface to
the continents. The cycle is closed by return flow in rivers. In this paper a
river routing model is developed to use with grid box climate models for the
whole earth. The routing model needs an algorithm for the river mass flow and a
river direction file, which has been compiled for 4-degrees X 5-degrees and
2-degrees X 2.5-degrees resolutions. River basins are defined by the direction
files. The river flow leaving each grid box depends on river and lake mass,
downstream distance, and an effective flow speed that depends on topography. As
input the routing model uses monthly land source runoff from a 5-yr simulation
of the NASA/GISS atmospheric climate model (Hansen et al.). The land source
runoff from the 4-degrees X 5-degrees resolution model is quartered onto a 2-degrees x 2.5-degrees grid. and
the effect of grid resolution is examined. Monthly flow at the mouth of the
world's major rivers is compared with observations, and a global error function
for river flow is used to evaluate the routing model and its sensitivity to
physical parameters. Three basinwide parameters are
introduced: the river length weighted by source runoff, the turnover rate, and
the basinwide speed. Although the values of these
parameters depend on the resolution at which the rivers are defined, the values
should converge as the grid resolution becomes finer. When the routing scheme
described here is coupled with a climate model's source runoff, it provides the
basis for closing the hydrologic cycle in coupled atmosphere-ocean models by
realistically allowing water to return to the ocean at the correct location and
with the proper magnitude and timing.
Abstract: The food security problem in Africa is tied to the small farmer
whose subsistence farming relies heavily on rain-fed agriculture. A dry spell
lasting two to three weeks can cause a significant yield reduction. A
small-scale irrigation scheme from small-capacity ponds can alleviate this
problem. This solution would require a water harvest mechanism at a farm level.
In this study, we looked at the feasibility of implementing such a water
harvest mechanism in drought prone parts of Africa. A water balance study was
conducted at different watershed levels. Runoff (watershed yield) was estimated
using the SCS curve number technique and satellite derived rainfall estimates
(RFE). Watersheds were delineated from the Africa-wide HYDRO-1K digital
elevation model (DEM) data set in a GIS environment. Annual runoff volumes that
call potentially be stored in a pond during storm events were estimated as the
product of the watershed area and runoff excess estimated from the SCS Curve
Number method. Estimates were made for seepage and net evaporation losses. A
series of water harvest index maps were developed based on a combination of
factors that took into account the availability of runoff, evaporation losses. population density, and the required watershed size needed
to fill a small storage reservoir that con be used to alleviate, water stress
during a crop growing season. This study presents Africa-wide water-harvest
index maps that could be used for conducting feasibility, studies at a regional
scale in assessing the relative differences in runoff potential between regions
for the possibility of using ponds as a water management tool.
Abstract: An annual water balance
model of Lake Victoria is derived for the period 1925-2000. Regression
techniques are used to derive annual inputs to the water balance,
based on lake rainfall data, measured and derived inflows and estimated
evaporation during the historical period. This approach acknowledges that runoff
is a nonlinear function of lake rainfall. A longer inflow series is
produced here which is representative of the whole inflow to the lake, rather
than just from individual tributaries. The results show a good simulation of
annual lake levels and outflows and capture the high lake level in 1997-1998.
Climate change scenarios, from a recent global climate model experiment, are
applied to the lake rainfall inflow series and evaporation data to
estimate future water balances of the lake. The scenarios produce a potential
fall in lake levels by the 2030s horizon, and a rise by the 2080s horizon. A
discussion of the application of climate change data to this complex
hydrological system is presented.
Author
Keywords: water
balance; climate change; rainfall-runoff model; regression; Lake Victoria
KeyWordsPlus:UPPER WHITE NILE; EAST-AFRICA; FLOWS; RAINFALL
Abstract: Water resource
management in WestAfrica is often a complicated process due to inadequate resources, climatic
extremes, and insufficient hydrological information. Insufficient data hinder
sustainable watershed management practices, one of the top priorities in the VoltaRiver Basin. This research properly
fills in missing data by modeling the hydrological distribution in the VoltaRiver Basin. On average, discharge gages
across the basin are missing 20 percent of their monthly data over 20 years.
Two methods were used to supplement missing data: a statistically linear model
and a conceptual hydrological model. A linear equation, developed from the
regression of precipitation and runoff, was used to evaluate the quality of
existing data. The hydrological model separates the system into root and
groundwater zones. Measured values were used to calibrate the hydrological
model and to validate the statistical model. The quality of existing data was
analyzed and organized for usability. Accuracy of the hydrological model was
also evaluated for its effectiveness using R-2 and standard error. It was found
that the hydrological model was an improvement from the linear model on a
monthly basis; R-2 values improved by as much as 0.5 and monthly error
decreased. Monthly predictions of the hydrological model were used to fill gaps
of measured data sets.
Author
Keywords: water
balance; modeling; hydrology; statistics; VoltaRiver Basin; WestAfrica
JOURNAL OF WATER RESOURCES PLANNING AND MANAGEMENT,
132 (6): 488 - 502NOV2006
Abstract: In the Rio Grande Basin of North America, water is overappropriated
and demand for water grows while supplies are constrained by drought and
climate change. The Basin is currently in its seventh year of drought, and
reservoirs are at historically low levels. Thus agricultural and municipal
river diversions have been sharply curtailed, and low flows threaten endangered
species. A central policy challenge is the design and implementation of plans
that efficiently and fairly allocate the Basin’s water supplies. Such plans are
complicated by the demands of existing water users, potential new users, three
state governments, and two sovereign nations. To address these issues, an
integrated basinwide nonlinear programming model was
designed and constructed for the purpose of optimizing water allocations and
use levels for the Basin. The model tests whether institutional adjustments can
limit damages caused by drought and identifies changes in water uses and
allocations that result from those adjustments. Compared to existing rules
governing the river system’s water use, future drought damages could be reduced
by one-fifth to one-third per year from intrastate and interstate water
markets, respectively, that permit water transfers across jurisdictions.
Results show hydrologic and economic trade-offs among water uses, regions, and
drought control programs.
Abstract: Variations of water stocks in the upper Zambezi river basin have been
determined by 2 different hydrological modelling
approaches. The purpose was to provide preliminary terrestrial storage
estimates in the upper Zambezi, which will be compared with estimates
derived from the Gravity Recovery And Climate
Experiment (GRACE) in a future study. The first modelling
approach is GIS-based, distributed and conceptual (STREAM). The second approach
uses Lumped Elementary Watersheds identified and modelled
conceptually (LEW). The STREAM model structure has been assessed using GLUE
(Generalized Likelihood Uncertainty Estimation) a posteriori to determine
parameter identifiability. The LEW approach could, in
addition, be tested for model structure, because computational efforts of LEW
are low.
Both models are
threshold models, where the non-linear behaviour of
the Zambezi
river basin is explained by a combination of thresholds and linear reservoirs.
The models were forced
by time series of gauged and interpolated rainfall. Where available, runoff
station data was used to calibrate the models. Ungauged
watersheds were generally given the same parameter sets as their neighbouring calibrated watersheds.
It appeared that the
LEW model structure could be improved by applying GLUE iteratively. Eventually,
it led to better identifiability of parameters and
consequently a better model structure than the STREAM model. Hence, the final
model structure obtained better represents the true hydrology.
After calibration,
both models show a comparable efficiency in representing discharge. However the
LEW model shows a far greater storage amplitude than
the STREAM model. This emphasizes the storage uncertainty related to
hydrological modelling in data-scarce environments
such as the Zambezi
river basin. It underlines the need and potential for independent observations
of terrestrial storage to enhance our understanding and modelling
capacity of the hydrological processes. GRACE could provide orthogonal
information that can help to constrain and further enhance our models. In the
near future, other remotely sensed data sources will be used to force modelling efforts of the Zambezi
(e.g. satellite rainfall estimates) and to identify individual storage
components in the GRACE observations (e.g. altimeter lake levels and microwave
soil moisture). Ultimately, this will create possibilities for state updating
of regional hydrological models using GRACE.
Abstract: The water balance of the closed freshwater LakeAwassa was estimated using a
spreadsheet hydrological model based on long-term monthly hydrometeorological
data. The model uses monthly evaporation, river discharge and precipitation
data as input. The net groundwater flux is obtained from model simulation as a
residual of other water balance components. The result revealed that
evaporation, precipitation, and runoff constitute 131, 106 and 83 x 10(6) m(3) of the annual water balance of the lake, respectively.
The annual net groundwater outflow from the lake to adjacent basins is 58 x
10(6) m(3). The simulated and recorded lake levels fit
well for much of the simulation period (1981-1999). However, for recent years,
the simulated and recorded levels do not fit well. This may be explained in
terms of the combined effects of land-use change and neotectonism,
which have affected the long-term average water balance. With detailed
long-term hydrogeological and meteorological data,
investigation of the subsurface hydrodynamics, and
including the effect of land-use change and tectonism
on surface water and groundwater fluxes, the water balance model can be used
efficiently for water management practice. The result of this study is expected
to play a positive role in future sustainable use of water resources in the catchment.
Author Keywords:Awassa;
Ethiopian Rift; lake hydrology; modelling; water balance; water management
Abstract: A methodology for the
analysis of land surface control on evapotranspiration
and heat fluxes is presented, based on the assimilation of remotely sensed
data. The method is suitable for application in areas with limited data availability
since only standard micro-meteorological measurements and land surface
temperature maps are required, while no a priori information about landcover
is required. The landcover effect on the heat exchange is
inferred from the assimilation in terms of heat transfer coefficients. The
methodology has been applied in the Ethiopian Rift Valley where desertification
processes are progressing at a high rate in association with remarkable
temperatures recorded in the last decades. Measurements of radiation,
temperature and wind profiles were taken in a field campaign. The land
surface temperature maps were obtained from NOAA-AVHRR. The results show
reasonable estimates of the retrieved surface fluxes and spatial patterns of
heat transfer coefficients consistent with those of landcover
and vegetation.
Abstract: The annual soil erosion on the
Ethiopian highlands is estimated at 1.5 billion tons. The poor soil management
and land use practices are the causes of the high soil erosion rate.
Assessment of soil erosion is the basis for planning soil conservation
work. There are currently erosionmodels ranging from empirical
ones like the Universal Soil Loss Equation (USLE) which provide long-term
average annual soil loss estimates to process-based models such as the
agricultural non-point source pollution model (AGNPS), which provide the
spatial and temporal variations of soil erosion. This study was
conducted to test and evaluate the AGNPS model on Auguchocatchment. The AGNPS model was calibrated and
validated for the study area with observed data of 8-10 years. Data year 1988
was used for validation while data year 1990 was used for calibration of the
model. The model was evaluated at 100 and 200 m grid cells. GIS was employed to
derive some of the parameters in addition to the primary and secondary data
collection techniques. Correlation coefficients, coefficient of efficiency and
homogeneity test of the correlation coefficients were used to evaluate the two
grid cells and the overall model performance. The validation result indicated
that the correlation coefficients were 0.59 and 0.58 for runoff, 0.96 and 0.95
for peak runoff rate, and 0.97 and 0.97 for the 100 and 200 m grid cells,
respectively. Only the runoff event was non-significant. The coefficients for
sediment yield and peak runoff rate were highly significant (p greater than or
equal to 0.01) and the pair of correlation coefficients for the same event for
the two grid cells were homogeneous. The coefficients
of efficiency were -1.0286 and -1.006 for runoff, 0.75 and 0.74 for peak runoff
rate, and 0.656 and 0.654 for sediment yield for the 100 and 200 m grid cells,
respectively. With the exception of the runoff event, the peak runoff rate and
sediment yield are well predicted. There was, however, no significant difference
in the output between the 100 and 200 m grid runs. For the average year of 1991
and for the 100 m grid cell run, the model estimated an average soil loss of 22
t ha(-1) per year, which is much greater than the rate
of soil formation (1 t ha(-1) per year). This showed that appropriate soil erosion
control measures should be taken in the catchment.
There is a potential for the model to be used for planning and management of
agricultural watersheds under the Ethiopian highland conditions. (C) 2002 Elsevier
B.V. All rights reserved.
Abstract: A hydrological modelling
at a catchment scale has been used to investigate the
impact of climatic and land use change on water resources in data scarce
Tropical Africa using a distributed precipitation-runoff modelling
system. The model divides a catchment into
homogeneous hydrological response units, providing the ability to impose
changes in climate or land use spatially. Model parameters were either
estimated from different existing data or by calibration against measured
discharge data available over 11 years (1985-1995). The model simulation-period
was divided into calibration (1986-1990) and validation (1991-1995) periods.
The model provided relatively good fits between measured and simulated
discharge both at a daily and monthly scales. Based on sensitivity analyses, a
10% decrease in rainfall produced a 30% reduction on the simulated hydrologic
response of the catchment, while a 1.5degreesC
increase in air temperature would result in a decrease in the simulated
discharge of about 15%. Converting the present day dominantly
cultivated/grazing land in the studied river basin by woodland would decrease
the discharge at the outlet by about 8%. In order to use the results of this
kind of model for decision making and water resources management, the model
should be tested under different environment and different scenario conditions.
Rainfall measurement and stage-discharge rating curves should be given priority
to improve model performance. (C) 2003 Elsevier Science B.V. All rights
reserved.
Author Keywords: hydrological modelling; Tropical Africa; model calibration; sensitivity
analysis; climate change; land use change
Abstract: Estimates of evaporation from an open
shallow lake in tropical Africa (LakeZiway, Main Ethiopian Rift) are
made by using monthly hydrometeorological data
available for the past three decades. On the one hand, annual average estimates
are inferred from three climatic approaches, which can be applied in areas with
limited meteorological data. The lake energy balance yields an
evaporation rate of 1780 mm yr(-1), assuming a Bowen
ratio of 0.15 (that of Lake Victoria). The Penman method gives an annual
evaporation rate of 1870 mm. The complementary relationship lake
evaporation model (CRLE) applied on monthly averaged values of air temperature,
air humidity and sunshine duration gives 1730 mm yr-l. The sensitivity of each
method to changes in input variables is analyzed in order to test the stability
of the resulting estimates. This helps discuss uncertainties and possible
inter-annual variations of the evaporation rate. On the other hand, the monthly
lake level records together with precipitation and river discharge data
between 1969 and 1990, allow us to estimate the water balance, providing an
annual rate of 1937 mm for the combined evaporation and groundwater losses. The
chloride budget is used to discriminate the groundwater from the evaporation
loss. It gives us an annual evaporation rate of 1740 mm and a corresponding
groundwater loss of 200 mm yr(-1). The groundwater
loss estimate is of the same order of magnitude as the surface outflow, but the
associated error in the former is significant because the result is sensitive
to the poorly known chloride content of river inflows. Our results can be used
to forecast the impact of increased water consumption in the basin. (C) 2001
Elsevier Science B.V. All rights reserved.
Author Keywords: evaporation; lake; energy budget; water
balance; Ethiopian rift
Abstract: Differentially eroded regions selected
over the Ethiopian Plateau, Northeast Africa, were statistically analyzed
using satellite images of various electromagnetic spectrum regions and
resolutions (Landsat TM and ERS-1). The power
spectrum exponent beta values for the Landsat TM2
(visible) and ERS-1 images over the same surface are associated to the
intrinsic properties of the different sensor type and postprocessing
of the data. Differences in the beta values were observed between eroded area (BlueNileCanyon, BNC) and relatively noneroded area (plateau, PLA) for all data sets. These
differences are associated to the mechanical erosion of the plateau. The
remotely sensed data fields show scaling from 35 in to 15 km, with no break at
1.5 km, and are highly multifractal. Analyses of the Landsat TM bands over each area demonstrated something
particular: beta values for bands in the shortwave infrared (SWIR) range
differed from beta values for bands in the visible spectrum range in the
plateau area by about 0,48, while in the drainage
area, this difference is around 0.13. Landsat TM SWIR
bands are sensitive to spectral signature of clay minerals, while data in the
visible spectrum range mostly depict topography gradients. Two concurrent
processes are highlighted, mechanical erosion and chemical erosion/deposition,
which interact to produce the observed differences. In the drainage area, where
cliffs and steep slopes are present and mechanical erosion intensively occurs,
the alteration minerals are remobilized quickly, whereas in the plateau area,
mechanical erosion is low, and alteration mineral deposition is less disturbed.
Such new statistical highlights of topographic versus chemical surfaces will
have to be taken into account in landformingmodels.
(C) 2002 Elsevier Science Inc. All rights reserved.
Abstract: Daily runoff depths from 28 plots (5 m x
2 m) recorded during a 2-year period in the semi-arid to subhumid
highlands of Tigray were analyzed to study the effect
of vegetation restoration in exclosures and to
identify other factors influencing runoff production. Plots are distributed
over three study sites and located in different land use types and on different
combinations of soil type, vegetation cover and slope gradient. Runoff was
found to be significantly reduced when a degraded area is allowed to
rehabilitate after closure. Runoff depth is significantly correlated with event
variables such as rain depth, rainfall intensity, storm
duration and soil moisture content. Total vegetation cover is the most
important plot variable explaining about 80% of the variation in runoff
coefficients through an exponential decay function. Also the runoff generating
rainfall threshold has a positive correlation with total vegetation cover.
Runoff was found to be negligible when the vegetation cover exceeds 65%. Other
important variables affecting runoff production in the study sites are soil
organic matter, soil bulk density, litter cover and slope gradient. (c) 2006
Elsevier B.V. All rights reserved.
Author Keywords:exclosure;
rangeland; grazing; eucalyptus; East Africa; surface hydrology
Abstract: The effect of grazing pressure on
infiltration, runoff, and soil loss was studied on a natural pasture during the
rainy season of 1995 in the Ethiopian highlands. The study was conducted at two
sites with 0-4% and 4-8% slopes at the International Livestock Research
Institute (ILRI) DebreZeit
research station, 50 km south of Addis Ababa. The grazing regimes were:
light grazing stocked at 0.6 animal-unit-months (AUM)/ha; moderate grazing
stocked at 1.8 AUM/ha; heavy grazing stocked at 3.0 AUM/ha; very heavy grazing
stocked at 4.2 AUM/ ha; very heavy grazing on ploughed soil stocked at 4.2
AUM/ha; and a control with no grazing. Heavy to very heavy grazing pressure
significantly reduced biomass amounts, ground vegetative cover, increased
surface runoff and soil loss, and reduced infiltrability
of the soil. Reduction in infiltration rates was greater on soils which had
been ploughed and exposed to very heavy trampling. It was observed that, for
the same % vegetative cover, more soil loss occurred from plots on steep than
gentle slopes, and that gentle slopes could withstand more grazing pressure
without seriously affecting the ground biomass regeneration compared to steeper
slopes. Thus, there is a need for developing 'slope-specific' grazing
management schedules particularly in the highland ecozones
rather than making blanket recommendations for all slopes. More research is needed
to quantify annual biophysical changes in order to assess cumulative long-term
effects of grazing and trampling on vegetation, soil, and hydrology of
grazing lands. Modelling such effects is essential
for land use planning in this fragile highland environment.