Improving forest ecosystem characterization to advance water quality modeling at the watershed scale

Forests often cover large areas of a watershed and play an important role in carbon, nutrient, and water cycling. Intrinsic limitations in the forest module of the widely-used Soil and Water Assessment Tool (SWAT) model have introduced uncertainties to watershed modeling. Here, we developed a new process-based forest module (SWAT-Forest) within the SWAT framework to improve the simulation of forest growth and development. We evaluated SWAT-Forest estimates of evapotranspiration (ET), net primary productivity (NPP), net ecosystem exchange (NEE), and forest biomass at both site and watershed scales, and validated the capability of the module in simulating critical water and carbon fluxes in forest ecosystems in United States. Improvements in forest ecosystem simulations also enhanced SWAT estimates of streamflow, nutrient, and sediment loading at the watershed scale, because of the impacts of forests on water yield, nutrient uptake, and soil erosion. Results of this study highlight the important role of forest ecosystems in hydrological and biogeochemical cycling. The new module lays a solid foundation for investigating water quantity and quality changes associated with forest disturbances. The newly developed algorithms will complement existing strengths of SWAT in modeling agroecosystems and will contribute to advancing knowledge of watershed hydrology and biogeochemistry in the context of environmental changes, and enable the use of models such as SWAT for management and stewardship of the critically important forest watersheds globally.