HYDROLOGIC INVENTORY OF MEREB GASH RIVER BASIN IN ERITREA

Abstract

Prediction of runoff hydrograph is essential for the assessment of water availability, design of various hydraulic structures and watershed development and management. Different methods have been in practice in surface water hydrology for quite some time back. Derivation of unit hydrograph using rainfall and runoff data [Sherman] as well as synthetic unit hydrograph (SUH) approaches and conceptual models for gauged and ungauged catchments are the efforts in this direction. Nonetheless, most of them have limitations for one or the other reasons. Thus, more recently, use of geomorphologic instantaneous unit hydrograph (GIUH) coupled with other conceptual models has been proved to be the most successful approach for flood prediction from ungauged catchments. In this respect, the Geographic Information System (GIS) and remote sensing image processing tools have been found to be helpful for the determination of geomorphologic characteristics on which entire GIUH development relies upon. In this study, the applicability of GIUH based Nash model is tested on the Mereb-Gash basin in Eritrea. Due to the uncertainties of the recorded rainfall data at the upper Mereb-Gash, direct surface runoffs are not computed with any of the approaches referred to above. Rather, the GIUH based Nash model unit hydrographs (UH) are developed and compared with the UH [Sherman] for the Debarwa catchment and SUH for other ungauged catchments. However, results indicate that SUH method is applicable for small catchment areas to the likes of Debarwa, Adi-Ghergera, Ethio1 and Ethio2 whereas divergence between the GIUH based Nash model and SUH is clearly observed for the bigger catchments (Tsorona, Tokombia, Adi-Chigono and Tessenei).