ASSESSING CLIMATIC RISK TO SORGHUM PRODUCTION IN WATER-LIMITED SUBTROPICAL ENVIRONMENTS .1. DEVELOPMENT AND TESTING OF A SIMULATION-MODEL

Sorghum (Sorghum bicolor (L.) Moench.) is one of the major summer crop s grown in the subtropics. The high rainfall variability and limited p lanting opportunities in these regions make crop production risky. A r obust crop simulation model can assist farmer decision-making via simu lation analyses to quantify production risks. Accordingly, we develope d a simple, yet mechanistic crop simulation model for sorghum for use in assessing climatic risk to production in water-limited environments . The model simulates grain yield, biomass accumulation, crop leaf are a, phenology and soil water balance. The model uses a daily time-step and readily available weather and soil information and assumes no nutr ient limitation. The model was tested on numerous data (n = 38) from e xperiments spanning a broad range of environments in the semi-arid tro pics and subtropics. Potential limitations in the model were identifie d and examined in a novel testing procedure by using combinations of p redicted and observed data in various modules of the model. The model performed satisfactorily, accounting for 94% and 64% of the variation in total biomass and grain yield, respectively. The difference in outc ome for biomass and yield was caused by limitations in predicting harv est index. The concepts involved, and the limitations encountered, in developing a crop model to be simple but consistent with the biophysic al rigour required for application to such a diverse range of environm ents, are discussed.