The importance of the plant physiological response to rising CO2 in projections of future water availability

Reliable and useful future projections of water scarcity are vital for incorporating into climate policy and national adaptation plans for building climate resilience. However, projections of water scarcity are often based on hydrology models which do not include an important climate feedback affecting the water cycle: the response of plant physiology to rising atmospheric CO₂, or “physiological forcing”. With higher atmospheric CO₂, plant physiology can affect the water cycle in two contradictory ways. Plant stomata do not open as widely in higher CO₂, and therefore transpiration rates are lower, leaving relatively more water in the ground increasing runoff and soil moisture. However, faster rates of photosynthesis with higher CO₂ also encourages greater leaf area, and thus higher overall canopy transpiration (even though transpiration of an individual stomata still decreases). The influence of physiological forcing on physical quantities within the water cycle such as transpiration and runoff have been well studied; however, there is a requirement to quantify how this translates to human impacts and more policy-relevant metrics on water resources, such as the water scarcity index. I will present findings from experiments using the Joint UK Land Environmental Simulator (JULES) forced with four earth system models which quantify and highlight the importance of including the plant physiological response in water-related impact studies.