Abstract
The application of the Soil and Water Assessment Tool (SWAT) to the Olifants Basin in South Africa was the focus of our study with emphasis on calibration, validation and uncertainty analysis. The Basin was discretized into 23 sub-basins and 226 Hydrologic Response Units (HRUs) using 3 arc second (90 m × 90 m) pixel resolution SRTM DEM with stream gauge B7H015 as the Basin outlet. Observed stream flow data at B7H015 were used for model calibration (1988-2001) and validation (2002-2013) using the split sample approach. Relative global sensitivity analysis using SUFI-2 algorithm was used to determine sensitive parameters to stream flow for calibration of the model. Performance efficiency of the Olifants SWAT model was assessed using Nash-Sutcliffe (NSE), coefficient of determination (R2), Percent Bias (PBIAS) and Root Mean Square Error-Observation Standard deviation Ratio (RSR). Sensitivity analysis revealed in decreasing order of significance, runoff curve number (CN2), alpha bank factor (ALPHA_BNK), soil evaporation compensation factor (ESCO), soil available water capacity (SOIL_AWC, mm H2O/mm soil), groundwater delay (GW_ DELAY, days) and groundwater “revap” coefficient (GW_REVAP) to be the most sensitive parameters to stream flow. Analysis of the model during the calibration period gave the following statistics; NSE = 0.88; R2 = 0.89; PBIAS = -11.49%; RSR = 0.34. On the other hand, statistics during the validation period were NSE = 0.67; R2 = 0.79; PBIAS = -20.69%; RSR = 0.57. The observed statistics indicate the applicability of the SWAT model in simulating the hydrology of the Olifants Basin and therefore can be used as a Decision Support Tool (DST) by water managers and other relevant decisions making bodies to influence policy directions on the management of watershed processes especially water resources.
Highlights
Like many other basins in the world [1]-[4], the Olifants Basin in South Africa is faced with challenging watershed issues ranging from water quantity, quality and the influence of anthropogenic activities on available water resources [5]
We present in this work an outlook on the application of the Soil and Water Assessment Tool (SWAT) a semi distributed hydrologic model to simulate the stream flow of the Olifants Basin with emphasis on 1) identification of sensitive parameters to stream flow; 2) calibration and validation of the model for stream flow for the study basin; and 3) an assessment of model performance
Discretization of the Olifants watershed resulted in a total of 23 sub basins with 226 Hydrologic Response Units (HRUs) using land use, soil and slope thresholds of 10%, 10% and 10% respectively
Summary
Like many other basins in the world [1]-[4], the Olifants Basin in South Africa is faced with challenging watershed issues ranging from water quantity, quality and the influence of anthropogenic activities on available water resources [5]. These watershed problems have over the years increasingly gained global attention owing to the fact that water resources are professed to be the pivot for which developmental agenda and improvement in livelihoods can be achieved [6]. In line with meeting the water supply needs of man and the ecosystem, a new paradigm shift for water resources planning has been suggested [13] [14]
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