Evaluating SWAT-3PG simulation of hydrologic and water quality processes in a forested watershed: A case study in the St. Croix River basin

Authors: KARKI, R - University Of Maryland; QI, J - University Of Maryland; Zhang, Xuesong; SRIVASTAVA, P - University Of Maryland

Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/12/2024
Publication Date: 11/29/2024
Citation: Karki, R., Qi, J., Zhang, X., Srivastava, P.K. 2024. Evaluating SWAT-3PG simulation of hydrologic and water quality processes in a forested watershed: A case study in the St. Croix River basin. Journal of Hydrology. 648. https://doi.org/10.1016/j.jhydrol.2024.132393.
DOI: https://doi.org/10.1016/j.jhydrol.2024.132393

Interpretive Summary: Forests are often an important component of a watershed and can have significant influence on terrestrial hydrology and water quality. Here, we incorporated the 3-PG forest model into the widely used Soil and Water Assessment Tool (SWAT) watershed model to better represent forest growth. The SWAT-3PG model accurately simulated net primary productivity, leaf area index, and aboveground biomass of forest ecosystems in the St. Croix Watershed, leading to improved performance for watershed streamflow, sediment, and nutrient simulation. The SWAT-3PG model provides an enhanced watershed tool for studying hydrology and water quality in forest-dominated watersheds.

Technical Abstract: Forests impart significant influence on terrestrial hydrologic and water quality processes impacting infiltration, soil moisture, runoff, surface erosion, nutrient and carbon dynamics, as well as transport of sediment, nutrients, and other pollutants to the aquatic systems. As a result, accurate representation of forests and their influences on watershed processes is critical in hydrologic and water quality models to evaluate the impacts of land use/land cover (LULC) change and climate change-driven natural disturbances such as wildfires, droughts, and intense precipitation events on hydrologic and water quality variables. Inaccurate representation of such processes can be detrimental as they can lead to misinformed conservation practice recommendations and policy decisions. Although the Soil and Water Assessment Tool (SWAT) has been widely applied in hydrologic and water quality simulation, the default forest module in SWAT is limited in that it does not simulate many important forest processes such as biomass partitioning, changes to forest structure, and the direct connection between forest biomass assimilation and LAI, leading to its limited application in forest-dominated watersheds. This study evaluated a newly developed version of the SWAT model that incorporates a new forestry module based on 3-PG (SWAT-3PG) for forest simulation and quantifies their impact on hydrologic and water quality simulaton by performing a case study in the St. Croix River basin of north-central U.S. Evaluation against remotely-sensed Net Primary Productivity (NPP), Leaf Area Index (LAI), and forest aboveground biomass (AGB) snapshots available for the region from different published sources showed that SWAT-3PG can accurately simulate NPP, biomass assimilation, the temporal trend in forest AGB, and seasonal variability in LAI. The model was also able to adequately replicate observed streamflow (RSR < 0.7 and NSE > 0.55), sediment, total nitrogen, and total phosphorus (RSR < 0.78 and NSE > 0.43) downstream of the forest-dominated region in the watershed. Comparison against outputs from a calibrated SWAT model with default forest module for the study watershed showed a reduction in surface runoff, increased soil moisture storage and groundwater recharge as well as reduction in the loss of nitrate, sediment, and phosphorus from forest HRUs with SWAT-3PG, indicating improved representation of forest hydrology and water quality processes with the new model.