HYDROLOGIC ANALYSIS OF MALEWA WATERSHED AS A BASIS FOR IMPLEMENTING PAYMENT FOR ENVIRONMENTAL SERVICES (PES)

Abstract

This paper investigates the hydrological effects of specific land use changes in Malewa catchment through the application of the Soil and Water Assessment Tool (SWAT) as a basis for implementing PES on a daily time step. The model's calibration efficiency is verified by comparing the simulated and observed discharge time series at the outlet of the watershed, where long series of hydrometrical data exist. The model is used to simulate the main components of the hydrologic cycle, in order to study the effects of land use changes. The model was calibrated and validated for the prediction of flow. Extensive continuous flow data over 10-year period from three locations within the basin were used for model calibration and validation. Sensitive model parameters were adjusted within their feasible ranges during calibration to minimize model prediction errors for daily and monthly flows. Water quality parameters (sediment, nitrogen and phosphorous loadings) were not available hence were not calibrated but the model default values were used after calibrating the flow data. At the main gauging station 2GB01; monthly calibration resulted in model prediction average flow within 19% of the measured average flow while the monthly Nash-Sutcliffe (ENS) measure was 0.58. Monthly validation results for 2GC05 and 2GB07 showed the model predicted average flow within 20% of the measured average flow with ENS of 0.58 and 0.61 respectively. Once the model was calibrated for flow, it was used to run scenario analyses for the selected target areas for PES implementation. A criterion was developed based on several parameters to select the target areas for PES implementation. Some of these parameters included annual rainfall, water yield, population density, water conflicts, and pressures on vegetation and water bodies. Based on the mentioned parameters, two areas were identified to be suitable for PES implementation. The two areas are within the upper catchment near GETA and Wanjohi areas. Four land-use scenarios were simulated in the selected headwater sub-basin areas to assess the impact of landuse change on Malewa hydrological regime. The deforestation scenario was the one that resulted in the greatest modification of total monthly runoff.