Assessing the Impacts of Land Use Change from Cotton (Gossypium hirsutum L.) to Cellulosic Bioenergy Crops on Watershed Hydrology and Water Quality in the Texas High Plains

The semi-arid Texas High Plains (THP) is one of the intensively managed agricultural regions in the United States (US) where cotton (Gossypium hirsutum L.) is a major crop. The THP region produces about a quarter of the US cotton. About 97% of the groundwater from the underlying Ogallala Aquifer is used for irrigating row crops including cotton in this semi-arid region. However, groundwater levels/quality in this region are experiencing a continuous decline/deterioration. This region also experiences recurring droughts and climate change studies predict warmer and drier summers in the future. These challenges may induce change in land use in the THP from high-water-demanding crops such as cotton to high water-and nitrogen-use-efficient cellulosic bioenergy crops such as perennial grasses and biomass sorghum [Sorghum bicolor (L.) Moench]. The region also holds enormous potential for the biofuel production according to the United States Department of Agriculture (USDA). The overall goal of this study is to assess the impacts of biofuel-induced land use change and climate change on hydrology, water quality and biomass production in the Double Mountain Fork Brazos watershed in the THP using the Soil and Water Assessment Tool (SWAT), Agricultural Policy/Environmental eXtender (APEX) and an integrated APEX-SWAT models. Switchgrass (Panicum virgatum L.) and Miscanthus × giganteus were found to be ideal bioenergy crops to replace cotton under the irrigated and dryland conditions, respectively. About 18 and 19 Mg ha^-1 yr^-1 of biomass could potentially be produced under the irrigated switchgrass and dryland Miscanthus scenarios. The land use change from cotton to perennial grasses decreased average annual (1994-2009) surface runoff, total nitrogen (TN) load through surface runoff and NO3-N leaching to groundwater by 88%, 86% and 100%, respectively and increased percolation by 28%. The climate change analysis indicated that the simulated annual irrigation water use and TN load under the future perennial grass land uses reduced by 60% and 30%, respectively, when compared to future cotton land use. However, under future climate scenarios, irrigated switchgrass yields were projected to reduce by 16-28% and dryland Miscanthus yields were simulated to increase by 32-38% when compared to the historic yields