ASSESSMENT OF THE USE OF SATELLITE BASED RAINFALL PRODUCTS FOR RUNOFF SIMULATION: THE CASE OF GUMARA WATERSHED, ETHIOPIA

Abstract

Hydrological modeling for runoff simulations requires accurate rainfall data as a model input, but in many developing countries like Ethiopia the rainfall observation network is relatively sparse. In recent years, several techniques have been developed for estimating rainfall using satellite data. The Climate Prediction Center Morphing Method (CMORPH) and the Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis (TMPA) near-real-time product (TRMM3B42RT) are among the satellite rainfall products emerged in recent periods. Hence, the main focus of this study was to compare the performance of Soil and Water Assessment Tool (SWAT) hydrologic model using rainfall data input from satellite estimates (CMORPH and TRMM3B42RT) and in situ measurements for stream flow simulations over Gumara catchment located in Ethiopia. The study period 2003-2014 was used for downloading and extracting the selected satellite rainfall estimates with daily-temporal and 0.25 0 x 0.25 0 spatial resolution. The satellite rainfall estimates were downloaded freely from their sources as a NetCDF file format and extracted for the specific study area using MATLAB R2013a and Panoply NetCDF, HDF and GRIB data viewer version 4.5.1. Sensitivity and uncertainty analysis, calibration and validation of the model was done using SWAT-CUP particularly the Sequential Uncertainty Fitting (SUFI-2) algorism for all rainfall inputs independently. The calibration period was from 2004-2010 leaving one year as a “warm up” period and the validation period was from 2011-2014 for satellite rainfall based simulations as well as in situ based simulations. Based on modeling results SWAT model had showed poor performance and model uncertainties were relatively large when calibrated with CMORPH satellite rainfall estimates compared to TRMM3B42RT satellite rainfall estimates in the study area. Although, satellite rainfall based simulations specifically TRMM3B42RT based simulations capture the shape of observed stream flow hydrograph, there was underestimation of the stream flow volume simulated by satellite rainfall products followed by the reduction of model performance statistics. Bias corrected satellite rainfall based simulations had significantly improved the model performance as well as the volume of stream flow simulated, but overall SWAT model yields good results for in situ rainfall based simulations than satellite rainfall based simulations. In future applying other bias correction schemes and extracting satellite rainfall estimates at finer temporal and spatial resolutions may improve the model performance even better.