YIELD BUILDUP IN WINTER-WHEAT UNDER SOIL- WATER DEFICIT .2. DEVELOPMENT AND TESTING OF A SIMULATION-MODEL FOR WHEAT UNDER VARIOUS WATER ANDNITROGEN REGIMES - EPICPHASE-WHEAT

The EPIC model (Williams et al, 1989) is commonly used to simulate the wheal crop in a crop rotation, though it overestimates grain yield in dry conditions. A new version of the model (EPICPHASE) has been devel oped by INRA in Toulouse to improve the model performance in drought-p rone environments. The refinements introduced were: 1) a better descri ption of the soil water extraction pattern, 2) a response of harvest i ndex to water and nitrogen stress as a function of phenological phase. This study describes this new Version and proposes the corresponding calibration for winter soft wheat Literature and various experiments ( lysimeters, pluri-annual field testing) were used to determine the opt imal set of parameters. Functions describing wheat growth without limi ting factors under water constraint were calibrated tie, biomass accum ulation and partitioning, leaf area and roof depth, water uptake). The model was tested in Auzeville (Haute-Garonne, SW France) during two s uccessive dry years (1989 and 1990) differing in their drought pattern , and on a range of fields differing in water and nitrogen supply. Ref erring to the extreme variability of the test situations, the predicti ons of the model were satisfactory for water budget, biomass, yield an d nitrogen uptake, with mean relative errors of 8-18%, depending on va riable and year. The sowing-heading duration was predicted with a root mean squared error of 59 degree days. The EPICPHASE-Wheat model shoul d be quite valuable for assessing the impact of different management s chemes in rainfed or irrigated wheat crops and for defining optimum st rategies.