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 crops grown in the subtropics. The high rainfall variability and limited planting opportunities in these regions make crop production risky. A robust crop simulation model can assist farmer decision-making via simulation analyses to quantify production risks. Accordingly, we developed 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 area, phenology and soil water balance. The model uses a daily time-step and readily available weather and soil information and assumes no nutrient limitation. The model was tested on numerous data (n = 38) from experiments spanning a broad range of environments in the semi-arid tropics and subtropics. Potential limitations in the model were identified and examined in a novel testing procedure by using combinations of predicted 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 outcome for biomass and yield was caused by limitations in predicting harvest index. The concepts involved, and the limitations encountered, in developing a crop model to be simple but consistent with the biophysical rigour required for application to such a diverse range of environments, are discussed.