The Compensatory CO2 Fertilization and Stomatal Closure Effects on Runoff Projection From 2016-2099 in the Western United States

Water availability in the dry western United States (US) under climate change and increasing water use demand has become a serious concern. Previous studies have projected future runoff changes across the western US but ignored the impacts of ecosystem response to elevated CO2 concentration. Here, we aim to understand the impacts of elevated CO2 on future runoff changes through ecosystem responses to both rising CO2 and associated warming using the Noah-MP model with representations of vegetation dynamics and plant hydraulics. We first validated Noah-MP against observed runoff, leaf area index (LAI), and terrestrial water storage anomaly from 1980 to 2015. We then projected future runoff with Noah-MP under downscaled climates from three climate models under Representative Concentration Pathway 8.5. The projected runoff declines variably from the Pacific Northwest by -11% to the Lower Colorado River basin by -92% from 2016 to 2099. To discern the exact causes, we conducted an attribution analysis of the modeled evapotranspiration from two additional sensitivity experiments: one with constant CO2 and another one with static monthly LAI climatology. Results show that surface greening (due to the CO2 fertilization effect) and the stomatal closure effect are the second largest contributors to future runoff change, following the warming effect. These two counteracting CO2 effects are roughly compensatory, leaving the warming effect to remain the dominant contributor to the projected runoff declines at large river basin scales. This study suggests that both surface greening and stomatal closure effects are important factors and should be considered together in water resource projections. Plain Language Summary Water shortage in the western United States (US) is becoming increasingly serious due to increasing socioeconomic demands and climate change. Although previous studies have projected various degrees of runoff changes, they neglect the impact of rising CO2 on runoff projections. To explore the possible role that CO2 may play in the hydrologic cycle, we conducted three experiments with the newly improved Noah-MP land model including vegetation dynamics and plant hydraulics. Consistent with previous studies, the western US tends to be drier toward the end of the 21st Century. CO2-induced leaf area index increases (surface greening) contribute considerably to the projected widespread transpiration increases and runoff reductions; however, these changes are nearly compensated by the stomatal closure effect of CO2 on transpiration, leaving the warming effect to remain the major cause to these transpiration and runoff changes. Therefore, the dual roles of CO2 in the hydrologic cycle through interactions with vegetation processes need to be considered in water resource projections.

Automated summary

This study has shown that elevated CO2 concentrations impact future water availability in the western United States through ecosystem responses, mainly through surface greening and stomatal closure effects. These two counteracting CO2 effects offset each other, leaving warming as the dominant contributor to projected decreases in runoff at large river basin scales.