Precipitation-Runoff Modelling in the Ziway-Shala Basin, Ethiopian Rift Valley
by
Dagnachew Legesse
CEREGE-CNRS/UMR 6635, B.P. 80, 13545 Aix-en-Provence Cedex 04, France
Coauthors: Vallet-Coulomb,C. (CEREGE), Gasse, F. (CEREGE)

This study aims to identify and assess effects of human impact and climate variability on catchment runoff in the Ziway-Shala basin, Ethiopian Rift Valley, using catchment-scale hydrological model. The final goal is to simulate past climatic conditions which may explain multi-proxy signals observed in lake sediment cores from this basin. The Ziway-Shala basin is characterised by a semi-arid to sub-humid type of climate with mean annual precipitation and mean annual temperature varying from 650 mm and 25°C close to the lakes, to 1200 mm and 15°C on the humid plateaux and escarpments, respectively. The basin is very sensitive to short-term as well as millennial-scale climatic variations. Modelling of watershed response to normal and extreme climatic conditions or to changes in the physical conditions of a watershed requires the simulation of a variety of complex hydrological processes and process interactions. In this study the PRMS (Precipitation-Runoff Modelling System) implemented under the MMS (Modular Modelling System) which was developed by the USGS (Leavesley et al., 1983) is used at a daily time step. The PRMS model is a modular design, distributed-parameter, physical-process watershed model that was developed to evaluate the effects of various combinations of precipitation, climate and land use on watershed response (Leavesley, 1983). Distributed-parameter capabilities are provided by partitioning a watershed into units (HRUs), using characteristics such as slope, aspect, elevation, vegetation type, soil type, and precipitation distribution (Leavesley et al., 1983, 1992). A water balance and an energy balance are computed daily for each HRU. The sum of the response of all HRUs, weighted on unit area basis, produces the daily watershed response.

A tentative vegetation map of the basin is prepared from Landsat-TM 7 images. Digital terrain model, slope and aspect were derived from topographic maps of 1:250 000 and 1:50 000 scales digitised and analysed under a Geographic Information System (GIS; ILWIS and ArcView). The distribution of soil in the basin is only known in outline. A soil map based on a reconnaissance survey by Makin et al. (1975) was used. The basin was divided into several sub-catchments based on topography and drainage patterns and HRUs were identified for each sub-basin. Model parameter calibration was performed using the Rosenbrock optimisation technique implemented under PRMS. The model simulates the runoff reasonably well. It particularly simulates well the dry-wet season transition as well as some of the extreme picks. Although absolute error values remain high and predication errors tend to vary proportionally to the magnitude of the prediction (or observations), PRMS seems to be a very good model to do sensitivity analysis in the Ziway- Shala basin at different time scales. The first results on sensitivity analysis based on variations on land use, precipitation and temperature are presented.

Date received: March 23, 2001

Copyright © 2001 by the author(s). The author(s) of this work and the organizers of the conference have granted their consent to include this abstract in Topology Atlas. Document # cagc-23.