Regional Watershed Sustainability I: Hydrologic Alteration and Evaluating Spatiotemporal Impacts at the Wildland Urban Interface

Abstract:

Hydrologic models are key tools needed by land managers and urban planners to synthesize varying spatiotemporal datasets into a framework for simulating alterations to the hydrologic cycle. This research develops a suite of hydrologic models for watersheds across southern California (United States), which can be used to inform a comprehensive planning approach that connects wildlands and communities. This work meets state objectives by supporting water sustainability, biodiversity, and wildfire risk for urban, tribal, and rural communities. We utilize a physically based hydrologic model, the Loading Simulation Program C++, to assess changes to the hydrologic cycle induced by land use/land cover change and wildfire scenarios throughout southern California. We synthesize an array of land-surface and subsurface characteristics to develop Hydrologic Response Units (HRUs) across the region to predict baseline stormwater, sediment, metals, and nutrient fluxes as they flow overland and are routed through streams and channels. Wildfire, fire recovery, and future land use scenarios are simulated and changes in fractional cover, soil scour, soil detachment rate, gully erosion, and infiltration capacity are compared to pre-disturbance conditions. The models provide a systematic platform to assess the impacts of wildfire incidence and land management practices on water supply and quality for wildland areas and human communities at local and regional scales. We assess the hydrologic impacts on upstream wildland-dominated watersheds and downstream built systems. Local and regional precipitation and evapotranspiration data is used to interpolate rainfall depths along 1/8th degree grid cells. Model calibration is performed using an array of national and municipal streamflow and water quality data throughout the southern California region over the previous 20 years, as well as post-fire samples in the Los Angeles River and San Diego River basins. Initial baseline model results were calibrated to data from water year 2019. Ultimately, this research will provide an integrated decision-making framework that supports multi-benefit landscape-scale planning and facilitates science-informed climate adaptation and water sustainability strategies across the region.