Crop water relations under different CO2 and irrigation: testing of ecosyswith 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 wit h changes in environmental conditions. We propose that changes with soil wa ter status in crop growth response to C-A can be simulated if stomatal resi stance is considered to vary directly with air-leaf C-A gradient, inversely with leaf carboxylation rate, and exponentially with leaf turgor. Resistan ce simulated in this way increases with C-A relatively less, and CO2 fixati on increases with C-A relatively more, under water-limited versus non-limit ed conditions. As part of the ecosystem model ecosys, this simulation techn ique caused changes in leaf conductance and CO2 fixation, and in canopy wat er 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) expe riment 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 ir rigation a measured increase of 20 versus 10% in seasonal wheat biomass, an d a measured decrease of 2 versus 5% in seasonal evapotranspiration, The ba sic nature of the processes simulated in this model is intended to enable i ts use under a wide range of soil, management and climate conditions. (C) 1 999 Elsevier Science B.V. All rights reserved.