Improving hydrological modeling to close the gap between elevated CO concentration and crop response: Implications for water resources.

Rising atmospheric carbon dioxide concentrations ([CO]) affect crop growth and the associated hydrological cycle through physiological forcing, which is mainly regulated by reducing stomatal conductance (g) and increasing leaf area index (LAI). However, reduced g and increased LAI can affect crop water consumption, and the overall effects need to be quantified under elevated [CO]. Here we develop a SWAT-g-LAI model by incorporating a nonlinear g-CO equation and a missing LAI-CO relationship to investigate the responses of water consumption of grain maize, maize yield, and losses of water and soil to elevated [CO] in the Upper Mississippi River Basin (UMRB; 492,000 km). Results exhibited enhanced maize yield with decreased water consumption for increases in [CO] from 495 ppm to 825 ppm during the historical period (1985-2014). Elevated [CO] promoted surface runoff but suppressed sediment loss as the predominant impact of LAI-CO leading to enhanced surface cover. A comprehensive analysis of future climate change showed increased maize water consumption in comparison to the historical period, driven by the more pronounced effects of overall climate change rather than solely elevated [CO]. Generally, future climate change promoted maize yield in most regions of the UMRB for three Shared Socioeconomic Pathway (SSP) scenarios. Surface runoff was shown to increase generally in the future with sediment loss increasing by an average of 0.39, 0.42, and 0.66 ton ha for SSP1-2.6, SSP2-4.5, and SSP5-8.5, respectively. This was due to negative climatic change effects largely surpassing the positive effect of elevated [CO], particularly in zones near the middle and lower stream. Our results underscore the crucial role of employing a physically-based model to represent crop physiological processes under elevated [CO] conditions, improving the reliability of predictions related to crop growth and the hydrological cycle.