Evaluating Water Balance Variables under Land Use and Climate Projections in Heavily Forested Watershed in a Humid Region

Abstract

The hydrological processes in Upper Choctawhatchee River Watershed were modeled using the Soil and Water Assessment Tool (SWAT) to investigate the impacts of climate and land use change. We integrated land use projection based in the Shared Socioeconomic Pathways with future climate data to study the combined effects on Hydrological response of the watershed. Future rainfall and temperature, for two time periods, were obtained using General Climate Models to provide SWAT with the climatic forcing in order to project water balance variables. The simulation was carried out under two radiative forcing pathways of RCP4.5 and RCP6.0. Model calibration metrics of NSE, R2, and p-factor were 0.88, 0.89, and 0.68, respectively. Results revealed increases as high as 2.55 and 2.4 for average annual minimum and maximum temperature, respectively, especially during Summer and Winter. Average annual precipitation was estimated to increase up to 11% under both emission scenarios; however, under severe emissions of RCP6.0, it was projected for two decades earlier. Land use change focused on urbanization dominated the climate changes. Impacts on water balance variables differed seasonally. Results showed surface runoff experienced major changes under both emission scenarios in some months up to 5 times increase. Among the water balance variables, ET as the least dominant pathways for water loss, showed the modest changes with the largest decrease during Fall and Summer. Projection indicated more frequent extreme behavior regarding water balance during midcentury. Discharge was estimated to increase through the year and the highest changes were projected during Summer and Fall with 186.3% increase in November under RCP6.0. Relying on rainfall for farming along with reduced agricultural landuse (11.8%) and increased urban area (47%) and population growth, would likely make the water use efficiency critical. The model demonstrated satisfactory performance, capturing the hydrologic parameters. It thus can be used for further modelling of water quality to determine the sustainable conservation practices, extreme weather events such as hurricane and tropical storms.