How changing forest structure and species composition can influence the hydrologic impacts of climate change

Trees transpire a substantial proportion of incoming precipitation, often more than 70 percent in semi-arid regions. As temperatures warm and droughts become more frequent and severe, forests are changing. Some these changes are incremental shifts in water use with greater atmospheric water demand. Climate driven changes in forests, however, also include changes in growth and more dramatically altered disturbance patterns such changes in fire regimes or drought-mortality. Accounting for the impact of incremental growth and disturbance related changes in forest structure and composition in hydrologic models is active area for model development. We offer some examples of how we have adapted an eco-hydrolgic model, RHESSys, to better account for climate-related changes in forest structure by strategic model-data fusion. Results from model experiments demonstrate the hydrologic importance of fine-scale within stand features, including subsurface storage capacity, species composition, and forest gaps. For semi-arid case studies in California, Spain and Israel, accounting for these climate-driven changes in forest structure can significantly alter hydrologic estimates. Importantly, forest management is often directed at altering forest structure and composition. Thus accurately accounting for these forest dynamics is particularly relevant when assessing the consequences of forest management actions for water resources in a changing climate.