A COMPARISON OF THE MODELS AFRCWHEAT2, CERES-WHEAT, SIRIUS, SUCROS2 AND SWHEAT WITH MEASUREMENTS FROM WHEAT GROWN UNDER DROUGHT

The predictions of five simulation models were compared with data from a winter sown wheat experiment performed in a mobile automatic rainsh elter at Lincoln, New Zealand in 1991/1992, where observed grain yield s ranged from 3.6 to 9.9 t ha(-1). Four of the five models predicted t he yield of the fully irrigated treatment to within 10%, and SWHEAT un derestimated by more than 20%. The same four models also predicted the grain yield response to varying water supply with reasonable accuracy , but SWHEAT again underestimated the yield reduction with increasing drought. However, the performance of all the models in predicting both the time course and final amount of aboveground biomass, of leaf area index (LAI) and evapotranspiration, varied substantially. These varia tions were associated with their diverging assumptions about the effec ts of root distribution and soil dryness on the ability of the crops t o extract water, the value of the ratio of water supply to water deman d at which stress begins to reduce leaf area development, and photosyn thetic, or light-use efficiency (LUE). All the models predicted, to va rying degrees, that reductions in photosynthetic efficiency or LUE was an important contributor to reductions in the rate of biomass accumul ation. In contrast, analysis of the experimental data indicated that t his factor was a minor contributor to the reduction, and variation in light interception, associated with changes in LAI, was the major caus e. (C) 1998 Elsevier Science B.V.