Remote Sensing Based Assessment of Water Resource Potential for Lake Tana Basin

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Addis Ababa University


1. Remote sensing techniques have been applied to augment estimation of hydrologic variables. Thus rainfall, evaporation, flooded zone delineation and runoff estimation has been carried out. TRMM monthly rainfall products were employed in rainfall gap filling techniques, the satellite estimates were first validated against observed ground station records and results indicated good correlation, average of 81%. Combination equation (Energy balance-mass transfer) is used to determine gridded potential evapotranspiration from the sub-basin by making use of MODIS 16 days Albedo products. Spatial extent and amount of water on flood plains during wet seasons of the year has also been determined by making use of spot NDVI temporal images through calibrating with high resolution SAR and optical images. Furthermore, land use/land cover information is extracted from Landsat imagery and existing data of land cover has been updated. Using the update land cover information spatial Curve Number (CN) map was developed. Runoff estimation, using SCS method and a soil water balance simulation using WatBal model, was then carried out in order to determine the runoff from un-gauged catchments and the results from the two models were compared. Results indicated that the soil water balance simulation model gave better correlation with the observed than the SCS method. Therefore, runoff estimation from un-gauged catchments was carried out using the soil water balance simulation techniques. Lake water balance simulation was finally carried out using continuity equation within simulation time span of 1998 to 2003 considering the variables, i.e. inflow estimation form the contributing catchments, the improved evaporation estimation; the rainfall data obtained incorporating TRMM images and water lost in the flood plains due to evaporation. Simulation results were compared with the Lake Tana Storage computed using the recorded Lake water surface elevation. The six year average results indicated that out of the total of 7,688 MCM estimated runoff from the catchments 3.59% is lost in the flood plains due to evaporation. Similarly, out of the 10,574 MCM inflow to the lake (net runoff plus rainfall over lake area) 5,110 MCM is lost through evaporation from the lake surface, 5,012 MCM is outflow from the lake to the head of main Abbay river. Average reduction in reservoir storage was computed to be 330 MCM. Therefore, the remaining 1414 MCM is assumed to be seepage loss and other unaccounted losses from the system



Tana ;basin