Crop water relations under different CO2 and irrigation:: testing of ecosys with the free air CO2 enrichment (FACE) experiment

Increases in crop growth under elevated atmospheric CO2 concentration (C-A) have frequently been observed to be greater under water-limited versus non-limited conditions. Crop simulation models used in climate change studies should be capable of reproducing such changes in growth response to C-A with changes in environmental conditions. We propose that changes with soil water status in crop growth response to C-A can be simulated if stomatal resistance is considered to vary directly with air-leaf C-A gradient, inversely with leaf carboxylation rate, and exponentially with leaf turgor. Resistance simulated in this way increases with C-A relatively less, and CO2 fixation increases with C-A relatively more, under water-limited versus non-limited conditions. As part of the ecosystem model ecosys, this simulation technique caused changes in leaf conductance and CO2 fixation, and in canopy water potential, temperature and energy balance in a modelling experiment that were consistent with changes measured under 355 versus 550 mu mol mol(-1) C-A and low versus high irrigation in a free air CO2 enrichment (FACE) experiment on wheat. Changes with C-A in simulated crop water relations allowed the model to reproduce under 550 mu mol mol(-1) C-A and low versus high irrigation a measured increase of 20 versus 10% in seasonal wheat biomass, and a measured decrease of 2 versus 5% in seasonal evapotranspiration, The basic nature of the processes simulated in this model is intended to enable its use under a wide range of soil, management and climate conditions. (C) 1999 Elsevier Science B.V. All rights reserved.