Catchment and Lake Research
LARS 2007
Water Balance Modelling in the Southern Ethiopian Rift Valley:
The Example of the Bilate River Catchment
Ingrid Althoff and Gerd Förch
University of Siegen, Research Institute for Water and Environment
Abstract
A variety of standardised hydrological models are available in order to estimate the spatial and
temporal distribution of water resources in natural systems. However, their applicability is
generally limited to regions where data availability, temporal and spatial, is sufficient, i.e. in
developed countries. For most developing countries, the lack of data currently limits the use of
verified hydrological models. The aim of this study is to investigate whether a water balance
analysis with a standardised hydrological model in regions of lower temporal and spatial data
availability is achievable. The precipitation runoff model “NASIM” was used for this study.
The area of investigation, the catchment area of the Bilate River (approximately 5500 km2), is
part of the Abaya Chamo Basin in the southeast of the Ethiopian Rift Valley. The Bilate River
covers a difference in altitude of over 2000 m over its length of ca. 250 km. The lack of
sufficient data at an adequate scale, i.e. hydrometeorological and land use data, for the catchment
makes modelling relatively difficult. For instance, only five gauging stations are available in the
catchment that can be used for model calibration; three of these stations are located in the upper
catchment, while only one gauging station is in the lower catchment of the Bilate. Moreover, the
operation of some of these stations is outdated.
The Precipitation Runoff Model NASIM takes into account the necessary water balance
parameters. The modelling utilises spatially distributed data with a temporal resolution of one
day. NASIM emulates the precipitation runoff process relatively well in relation to the
background of the available data. Certainly, the simulated discharges should be examined
closely, as, for the discharge peaks; they do not really represent the reality of the situation. The
greatest influence of the results is seen for the meteorological data. For this it is noted that the
strong relief and the great variability of the precipitation, as well as the influence of evaporation,
are not represented adequately by the number of climatic stations. In relation to the gauging
stations it was noticed that the calibration of the model could only lead to a good description of
the natural conditions in the Precipitation Runoff Model through the use of qualitatively good
discharge measurements at several gauges. In addition, in NASIM, the meteorological data were
considered as station data and not as interpolated regional data that implies that NASIM does not
work on a raster basis at present. Thus, improvements towards a raster basis are recommended.
A further point is the non-satisfactory use of GIS for the visualisation of the simulation results,
which is due to the not existing raster basis. Overall, it can be stated that the modelling with
NASIM was possible, but it demands a very close examination of the input parameters and their
emphasis for the model. It is recommended that more research is required to modify the model
and conduct more field work.
Keywords: Bilate River Catchment, Precipitation Runoff Model NASIM
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