Rainfall nunoff modeling and determination of the actual evaporation from the water balance study for Kilombero basin.

A hydrological model is a simplified quantitative (mathematical) description of parts of the hydrological cycle to develop an effective Rainfall-Runoff relationship. The study area of this project is the Kilombero basin with an area of 39,990 Km2. Kilombero basin fall between the longitudes 34° 30 and 37° 00 east and latitudes 8° 10 and 9° 30 south. Five sub-catchments in Kilombero basin are located in this study. The sub-catchments are Kilombero at Ifwema, Mpanga at Mpanga, Ruhudji at Mkasu, Ruhudji at Kifunga and Hagaf catchment. A set of system model and conceptual models were applied to model the rainfall-runoff relations, the performance of the different models were then compared. The criterion used to compare the model performance was R2 for the least square solution (Nash and sutcliffe efficiency criterion, 1970). A spread sheet model was developed to do the Tank Model computations and the Soil Conservation Service model using the Curve Number method, A spread sheet model was developed to calculate the desegregation of the actual evaporation on a monthly and annually time step and compare the computed annual actual evaporation with another method for determining the annual actual evaporation which is the Turck and pike method. The system model used is the seasonal Linear pertubation model (LPM) Nash and Barsi (1983), the efficiencies of the calibration periods for the five catchments vary between (86.91% and 73.73%), while for the verification periods the efficiencies vary between (70.57% and 81.99%). The conceptual models used are Xinanjiang Model (Zehao, et al, 1980) and the Tank model (Sugawara, 1960), the calibration periods for the Xinanjiang Model vary between (48.69% and 65.66%), while for the verification periods the efficiencies vary between (26.03% and 53.37%). The Tank Model efficiencies vary between (67.00% and 74.00%)for the monthly data and between (73.00% - 81.00%) for the annual data. For the determination of the actual evaporation for Kilombero basin at monthly and annually time scales the segregation method is applied which adopts the application of a Linear Transfer Model with the water balance equation for the study area. The actual annual evaporation was also computed by the Turck and Pike method. The ratio of the annual actual evaporation computed by the segregation method to that which is computed by the Turck and Pike method is ranging between (0.64 and 1.40). The thesis compares the predicted surface-runoff for the five catchments computed by the Curve Number method with that surface runoff computed by Tank model. The results of the ratio of the computed with the Tank model is ranging between (0.37 - 1.22). The simple Linear Perturbation Model shows reasonable results, while in the conceptual models the Tank Model shows better results than the Xinanjiang Model in predicting the runoff. On the basis of the results obtained the Linear Perturbation Model and the Tank Model were recommended for simulating runoff from catchments, which have similar hydrological characteristics of Kilombero Basin. The curve number method was observed to predict small volumes of the surface runoff compare to those values computed by the tank model. The segregation method, which used to estimate the actual annual evaporation gave reasonable estimates compared to Turck and Pike method which under estimates the actual evaporation.