The predictions of five simulation models were compared with data from a winter sown wheat experiment performed in a mobile automatic rainshelter at Lincoln, New Zealand in 1991/1992, where observed grain yields ranged from 3.6 to 9.9 t ha−1. Four of the five models predicted the yield of the fully irrigated treatment to within 10%, and SWHEAT underestimated 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 variations were associated with their diverging assumptions about the effects of root distribution and soil dryness on the ability of the crops to extract water, the value of the ratio of water supply to water demand at which stress begins to reduce leaf area development, and photosynthetic, or light-use efficiency (LUE). All the models predicted, to varying degrees, that reductions in photosynthetic efficiency or LUE was an important contributor to reductions in the rate of biomass accumulation. In contrast, analysis of the experimental data indicated that this factor was a minor contributor to the reduction, and variation in light interception, associated with changes in LAI, was the major cause.
- Simulation models