Collaborative Research: Water partitioning between trees, soils, and streams following forest disturbance

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Over half of the freshwater supply in the contiguous United States originates in forested watersheds. It is thus critical that forested headwater catchments are properly managed to ensure clean and ample water supplies for aquatic ecosystems and downstream communities. Research on forested experimental watersheds has established that reducing forest cover generally increases water yield and that afforestation decreases water yield. However, there is still not agreement on the magnitude and persistence of changes in streamflow, particularly during periods of the highest and lowest streamflow. This is because the connection between forest harvesting, or other forest disturbances, and streamflow is essentially a "black box", whereby the mechanisms influencing streamflow are not understood. This research will broaden our process-based understanding of forest hydrology by investigating how contemporary forest management affects water movement and storage in a well-studied coastal California watershed. Results of this research will provide evidence to motivate new forest management in coastal watersheds and lead to better foundational knowledge of the linkages between vegetation and water supply. These results will be generated in conjunction with the USDA Forest Service and the California Department of Forestry and Fire Protection, which will use the results to update California's Forest Practice Rules. It is increasingly critical to improve our quantitative understanding of how forest disturbances affect the storage and release of water through catchments. However, it is not yet understood how disturbances impact water compartmentalization at time scales other than the annual water balance. This is because the mechanisms by which streamflow increases can only be inferred in paired watershed studies, which have been the most popular tool for quantifying hydrological effects of forest disturbance. A combined mass balance and dual water isotope approach offers the potential to critically improve our understanding of the effects of disturbances on the processes and mechanisms that drive water storage and release. Here, it is hypothesized that the ecohydrological response of a forested watershed is proportional to the severity of disturbance. Using different intensities of forest harvesting (0%, 35%, 55%, and 75% removal of pre-treatment vegetation) as a mechanism for studying forest disturbances, the following research questions will be addressed: How do increasing levels of disturbance affect the water budget at different hillslope positions? How does forest disturbance affect the separation of water between soils, groundwater, streams, and trees? To answer these questions, this study takes a novel approach to connect the processes by which forest disturbance affects water budgets, subsurface water movement, and plant water use by combining a mass balance analysis and dual water isotopes. All components of the water budget (i.e., streamflow, soil moisture, groundwater, evapotranspiration, and fog) will be monitored and coupled with dual stable isotopes (2H and 18O). This study also addresses critical calls for field-based hydrologic research to inform hydrologic models to answer key questions about disturbance impacts on water resources.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.