Evapotranspiration and Water Balance Mapping for Forest Management in California's Sierra Nevada

Abstract:

We examined the annual spatial water balance response of densely forested watersheds in northern California to disturbance caused by wildfires and climate variability. The forests in the Yuba and American River watersheds examined here exhibit little apparent annual water limitation. We assessed changes in annual evapotranspiration (ET), and what ET patterns imply for the availability of subsurface water storage for drought resistance using a statistical model, correlating measured annual evapotranspiration from eddy-covariance towers across California with NDVI (Normalized Difference Vegetation Index) measured by Landsat satellite, and with annual precipitation. Wildfires in the 1985-2013 period resulted in substantial post-fire ET reductions for at least 5 years, and in some cases for more than 20 years. The relative levels of biomass removed in medium-intensity fires (25-75% basal area loss), similar to magnitudes expected from forest treatments for fuels reduction and forest health, reduced evapotranspiration by as much 150-200 mm yr^-1 for the first 5 years. Post-fire evapotranspiration recovery rates confirm that continued forest treatments at intervals of 5-20 years are needed to sustain lower evapotranspiration, depending on local landscape attributes and interannual climate. Interannual cumulative precipitation minus evapotranspiration (P-ET) deficits during multi-year dry periods generally did not exceed 300 mm yr^-1, suggesting little forest moisture stress during the 1985-2018 period of our analysis, owing to relatively small reliance on interannual subsurface water storage to meet dry-year evapotranspiration needs of vegetation. Finally, we examined the potential impacts of a warmer climate and more-severe drought periods on forest ET with implications for future drought stress and forest mortality with a goal of informing forest management objectives.