Crop water relations under different CO 2 and irrigation: testing of ecosys with the free air CO 2 enrichment (FACE) experiment

Abstract

Increases in crop growth under elevated atmospheric CO 2 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 CO 2 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 CO 2 fixation, and in canopy water potential, temperature and energy balance in a modelling experiment that were consistent with changes measured under 355 versus 550 μmol mol −1 C A and low versus high irrigation in a free air CO 2 enrichment (FACE) experiment on wheat. Changes with C A in simulated crop water relations allowed the model to reproduce under 550 μmol 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.