ANALYZING THE LIMITATIONS SET BY CLIMATIC FACTORS, GENOTYPE, WATER AND NITROGEN AVAILABILITY ON PRODUCTIVITY OF WHEAT .1. THE MODEL DESCRIPTION, PARAMETRIZATION AND VALIDATION

A mechanistic crop growth simulation model, WTGROWS, is developed for use in analyzing effects of climatic variables and crop management on productivity of wheat in tropical and sub-tropical wheat regions of In dia. The model, written in CSMP and FSE, simulates daily dry matter pr oduction as a function of radiation and temperature, and water and nit rogen availability. Crop aspects of the model are arranged in submodel s covering development, photosynthesis, respiration, carbohydrate part itioning, dry matter production, leaf area, grain growth and transpira tion. A soil water balance model is attached to simulate water uptake and to determine water stress. Another submodel determines nitrogen up take, distribution and N stress. Water and nitrogen stresses, dependin g upon their severity, affect various physiological processes. The mod el requires inputs relating to site, daily weather, soil physical char acteristics and crop management. Switches allow water and/or nitrogen stresses to be terminated to establish climatically determined potenti al grain yield. Various aspects of the model were validated using a la rge number of independent experiments. Comparison of simulated and mea sured quantities indicated satisfactory performance of the model in re ference to water and nitrogen uptake, dry matter growth and grain yiel d in potential as well as Water- and N-limited environments. The model appears useful as a tool for optimizing use of water and nitrogen.