Modeling the Effects of Land Use Change from Cotton (Gossypium hirsutum L.) to Perennial Bioenergy Grasses on Watershed Hydrology and Water Quality Under Changing Climate.

Assessing the impacts of climate change and land use conversion on hydrology, water quality and crop yield are critical for appropriate land use selections and crop management practices in the semi-arid Southern Cotton Belt (SCB) region. This study was conducted at the Double Mountain Fork Brazos watershed in the SCB using the Soil and Water Assessment Tool (SWAT) model. The responses of hydrology, water quality and crop yield were simulated based on 19 General Circulation Models (GCMs) projections of the Coupled Model Intercomparison Project Phase 5 (CMIP5). Two emission scenarios of Representative Concentration Pathway (RCP) of RCP4.5 and RCP8.5 during two 30-year periods of middle (2040-2069) and end (2070-2099) of the 21^st century were adopted. Results showed the evapotranspiration (ET) and irrigation water amount decreased substantially based on all 19 GCM projections under the cotton land use of the climate change scenarios compared to the baseline cotton scenario. When comparing the perennial grass land use to the cotton land use under the same climate change scenario, the median values of the change percentage of the annual irrigation water amount reduced by 62%, 69%, 66% and 89% under the 2040-2069 RCP4.5, 2040-2069 RCP8.5, 2070-2099 RCP4.5 and 2070-2099 RCP8.5 scenarios, respectively. In addition, the median values of the change percentage of annual total nitrogen load decreased by approximately 30%. Both the irrigated and dryland cotton yields increased under the climate change scenarios compared to the baseline cotton scenario. There was a decline in the biomass productions of irrigated switchgrass but an increase in the biomass productions of dryland Miscanthus under the climate change scenarios compared to their biomass productions during the historical period (1994-2009). Elevated CO₂ concentration, decreasing water stress days and changing temperature stress days were the major factors for the variations of the crop yields.