Book Description

Changes in water balance variables such as runoff and evapotranspiration (ET) are essential in planning and management of land and water resources. Two major factors affecting these variables are climate and land use change. There is a need to investigate the combined effects of land use and climate change at local scales. Towards that end, the hydrological processes were modeled using the Soil and Water Assessment Tool (SWAT) to investigate the impacts of climate and land use change in Southeast US (Makhtoumi, Li, Ibeanusi, and Chen, 2020). We integrated land use based on the Shared Socioeconomic Pathways (SSPs) with future climate data (CMIP5) to study the combined effects on hydrological response of Upper Choctawhatchee Watershed (UCW.) Future rainfall and air temperature, for two time periods (2040-2069 and 2070-2099), were obtained using Global 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 Representative Concentration Pathways (RCP4.5 and RCP6.0.) Our results indicate that increased imperviousness resulted from urbanization has more impact on runoff than that of projected changes in climate. Impacts on water balance variables (runoff, ET, discharge) differed seasonally. Results showed peak surface runoff experienced changes under both emission scenarios in June up to five 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. Projections indicated more frequent extreme behavior regarding precipitation, peak surface runoff, water yield (WY) and ET, during midcentury. Discharge was estimated to increase through the year and the highest changes were expected during summer and fall with 186.3% increase in November under RCP6.0. Relying on rainfall for farming along with reduced agricultural land use (11.8%) and increased urban area (47%) and population growth, would likely make the water use efficiency critical. In our second study, we focused on the combined impact of land use and climate change on soil erosion at local scales. Topsoil loss is a widespread environmental concern causing adverse impacts on natural and human systems. Severe weather accompanied with human activities can exacerbate this issue degrading soil health and consequently accelerating global and regional food insecurity and injustice. Erosion impairs soil physical and chemical properties such as infiltration rate, water holding capacity, loss of nutrients including soil carbon and nitrogen. Although, temporal properties of a rainfall event have meaningful implications for soil erosion, spatial heterogeneity of a rainfall contributes substantially and cannot be overlooked. Therefore, in the third chapter we investigated soil loss using SWAT in Northern Mississippi. First, we built a hydrological model and calibrated it for both flow and sediment discharge. Then we developed land use and climate scenarios. The land use scenarios include farming (soybean and corn) and grazing practices. The climate scenarios comprise of four different precipitation time series, S0 which no concentration is forced, while S1, S2, and S3 have 3%, 6%, and 9% concentration in top four rainy days, respectively. We coupled the land use and climate scenarios and evaluated a small watershed (Hickahala Creek Watershed) in response. We classified the subbasins into different classes of soil loss severity and then determined the hotspots for soil loss at subbasin scale. Our result suggests that the resolution of rainfall data is crucial in studying the soil loss. We found that pasture management by itself can manifold soil loss, and if accompanied with extreme rainfalls, soil loss accelerates impacting different subbasins each time. We found that spatial heterogeneity of extreme rainfalls (ERs) can be more substantial than land use in individual extreme rainfalls; however, over a year, soil moisture and type of the management practices (grazing and farming) could contribute more to soil loss. Soil loss can go as high as 350 (ton/ha/yr) under the ERs. Adding only the management practices can increase erosion 3600%. Under S1 parts of watershed yield more than 150 ton/ha/yr (extremely severe). Under S2 and S3 more soil loss hotspots emerge yielding approximately 200 ton/ha/yr. We found that in the hotspots, up to 10% increase in CI can increase annual soil loss up to 75%. Single ER can generate up to 35% of annual soil loss. Under one ER event hotspot subbasins can lose up to 160 ton/ha/day (subbasin 15). The results reveal that adding grazing and farming (S0) under one ER event can increase soil loss by 95%. 32% and 80% increase in rainfall amount in one ER event can increase soil loss by 94% and 285% respectively. Our results suggested the importance of site-specific managements to mitigate soil loss and all the consequences. It is essential to consider the varying sensitivity of subbasins for the sustainability of agricultural landscapes.