EVALUATING THE PERFORMANCE OF A CROP WATER BALANCE MODEL IN ESTIMATING REGIONAL CROP PRODUCTION

The comparison between a spatially distributed crop index and reported yield was evaluated. The crop index was generated by a crop water balance model that simulates the reduction of crop yield (as a percentage of the potential) due to water deficit. This model is currently operational as a monitoring and forecasting tool for region-wide food security analyses in drought prone countries in Sub-Saharan Africa. The most important input parameters of the model are Africa-wide satellite-based rainfall estimate images and spatially distributed potential evapotranspiration images calculated from 6-hourly numerical weather model data. District level crop yield data from Ethiopia were used to evaluate the performance of the model. Historical sorghum yield data from 1996-1999 were used to evaluate the performance of a seasonal water requirement satisfaction index (WRSI) for sorghum. WRSI values and reported district yield data were significantly correlated. The WRSI model was particularly successful in capturing the response of the crop during a relatively dry year. In districts that never experienced water deficit during the study period, it was possible to infer the magnitude of yield variability that was caused by factors other than water supply