IMPROVED SWAT MODEL PERFORMANCE WITH TIME-DYNAMIC VORONOI TESSELLATION OF CLIMATIC INPUT DATA IN SOUTHERN AFRICA 1 Jafet C.M. Andersson, Alexander J.B. Zehnder, Bernhard Wehrli, and Hong Yang 2 ABSTRACT: In this study, we compared two approaches to obtain climatic time series for the Soil and Water Assessment Tool (SWAT), namely the conventional centroid method and time-dynamic Voronoi tessellation, and assessed the performance of SWAT in simulating discharge and smallholder maize yields in Southern Africa. Climatic time series were estimated with each method. The Voronoi method utilized all available precipitation and temperature data, but the centroid method used only 14.5 and 82.5%, respectively. After centroid process- ing, sub-basin time series were on average 42 and 63% incomplete, respectively. After Voronoi processing, all time series were complete. SWAT was fed with each climate dataset. Each model setup was independently cali- brated and validated against discharge and maize yield. Similar model performance was obtained with both methods for yield. The root mean squared error during calibration was 0.26 and 0.27 t ha )1 for the centroid and Voronoi methods, respectively (p-value: 0.80). However, daily discharge simulations improved significantly with the Voronoi method. The coefficient of determination increased from 0.24 to 0.39 in the calibration period (p- value: 9.6 · 10 )13 ) and from 0.41 to 0.48 in the validation period (p-value: 3.1 · 10 )3 ). The Voronoi method improved the simulation of the river flow regime. The largest improvements were obtained in data scarce situa- tions, at high spatial and temporal resolution, and where the centroid method performed the worst. (KEY TERMS: geospatial analysis; streamflow; time-dynamic Voronoi tessellation; centroid method; data use efficiency; precipitation; temperature; crop yield; SWAT.) Andersson, Jafet C.M., Alexander J.B. Zehnder, Bernhard Wehrli, and Hong Yang, 2012. Improved SWAT Model Performance with Time-Dynamic Voronoi Tessellation of Climatic Input Data in Southern Africa. Journal of the American Water Resources Association (JAWRA) 48(3): 480-493. DOI: 10.1111 ⁄ j.1752-1688.2011.00627.x INTRODUCTION The need to evaluate suggested measures of improving the global food security and water scarcity conditions fuels a demand for reliable models (Yang et al., 2003). Southern Africa is a key area of concern because of high levels of undernourishment, poverty, and population growth (CAWMA, 2007). A set of strategies aimed at improving the situation through altered agronomic and water management practices in smallholder agriculture have been suggested 1 Paper No. JAWRA-10-0200-P of the Journal of the American Water Resources Association (JAWRA). Received November 18, 2010; accepted October 26, 2011. ª 2012 American Water Resources Association. Discussions are open until six months from print publica- tion. 2 Respectively, Doctoral Student, Eawag, Swiss Federal Institute of Aquatic Science and Technology, 8600 Du ¨ bendorf, Switzerland, and Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Zurich, Switzerland; Scientific Director, Alberta Water Research Institute (AWRI), Edmonton, Alberta, Canada, and Nanyang University of Technology, Singapore; Professor, Institute of Biogeochemistry and Pollu- tant Dynamics, ETH Zurich, Zurich, Switzerland; and Senior Scientist, Eawag, Swiss Federal Institute of Aquatic Science and Technology, Du ¨ bendorf, Switzerland (E-Mail ⁄ Andersson: aqua@jafet.org). JAWRA 480 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION Vol. 48, No. 3 AMERICAN WATER RESOURCES ASSOCIATION June 2012