Modeling the Impacts of Historic Climate Change and Extreme Droughts on Water Yield and Productivity of National Forests over the Conterminous U.S

Quantifying the impacts of droughts on the U.S National Forests (NFs) is necessary to develop sound forest management plans to mitigate and adapt to climate change. This study applied a water balance model (WaSSI) to 170 National Forests (NFs) over the conterminous U.S to examine how long-term climatic change and extreme climate events impacted forest water yield and productivity. Our model predicted that mean water yield decreased by 5% while mean productivity increased by 10% between 1961-2012 across the NFs. Overall 32% of NFs showed a significant increasing trend in forest gross ecosystem productivity (GEP), while 5% of the NFs had a significant decreasing trend. This study also suggested that the extent and severity of drought events occurring in the NFs had an increasing trend during the past 50 years. Taking the 170 NFs as a whole, the top-five droughts were characterized by a 261 mm/yr (or 30%) reduction in precipitation, that resulted in reductions in evapotranspiration by 55 mm/yr (or 10%), water yield by 154 mm/yr (or 49%) and GEP by 121 gC/m2/yr (or 10%). However, distribution of these changes varied spatially due to differences in vegetation types, weather, and geography. Overall, this study provided an assessment of historical impacts of droughts on forest watershed hydrology and productivity across diverse geographic regions using a consistent database. The study also identified forest watersheds that were severely influenced by historical drought, and provided a reference to develop appropriate adaptation strategies for potential future extreme droughts on the forest ecosystem services of NFs.