Quantifying the Impacts of Land Use, Management and Climate Change on Water Resources in Missouri River Basin

A location-specific evaluation of hydrological landscape responses concerning past and projected climate and land use land cover (LULC) changes can provide a powerful intellectual basis for developing efficient and profitable agroecosystems, and overcoming uncertain and detrimental consequences of LULC and climate shifts. This dissertation assessed the impacts of land use, management, and climate change on water resources in the Missouri River Basin (MRB) through four specific studies that included: (i) to study the responses of leached nutrient concentrations and soil health to winter rye cover crop (CC) under no-till corn (Zea mays L.)-soybean [Glycine max (L.) Merr.] rotation, (ii) to simulate hydrological responses of integrated crop-livestock (ICL) system under projected climate changes in an agricultural watershed, (iii) to evaluate the hydrological landscape responses in relation to past (1986-2018) LULC and climate shifts across South Dakota (SD), and (iv) to evaluate the hydrological landscape responses in relation to past (1986-2018) LULC and climate shifts across MRB. Cover cropping has been promoted for the ecological agricultural intensification, however, the vulnerability of CC establishment and expected soil health and water quality benefits under short and cold growing periods for CC are of concerns among producers in the northern Great Plains (NGP) region. Thus, a field experiment from 2017 to 2020 was conducted to assess the impacts of winter rye (Secale cereale L.) CC on soil health and water quality parameters under a no-till corn-soybean rotation at Southeast Research Farm (SERF), Beresford, SD. Interestingly, the study site faced one dry (2020) and two abnormally wet (2018 and 2019) years which received 31% lower (2020), and 31% (2018) and 23% (2019) higher precipitation, respectively, than the annual average (1953-2019). Data showed that biomass of the rye CC was 251 kg ha-1 in 2018, 1213 kg ha-1 in 2019, and 147 kg ha-1 in 2020, coinciding with contrasting growing degree days i.e., 1458, 2042, 794, respectively, as a consequence of variable weather conditions. Cover cropping did not impact water quality for the majority of the study period. However, a significant reduction in leached nitrate (~19-20%) and total nitrogen (TN) (~8.5-16%) concentrations were found only in 2019, pertaining to sequestered 18.8 kg N ha-1. Rye CC showed 13 and 11% significantly higher microbially active carbon and water-extractable organic nitrogen, respectively, than the control (No CC) treatment. The non-significant impacts on soil health indicators due to winter rye showed that study duration (3 years) may not be sufficient to see the beneficial impacts of cover crop on soils. However, significant reductions in leached nitrate and TN concentrations for one (2019) out of three study years suggest that well-established rye CC (biomass = 1213 kg ha-1; which was 4.8 and 8.3 times higher than that in 2018 and 2020) has the potential of reducing nutrient leaching and enhancing soil health for the study region. The ICL systems, when well managed properly, have beneficial impacts on soils and water yield, however, very limited studies are available due to the complexity of these integrated systems. Thus, a simulation study was conducted to assess the hydrological impacts of long-term implementation of ICL systems at watershed scale with the projected climate scenarios on water yield using the Soil and Water Assessment Tool (SWAT) model over two time periods [i.e. Near Future (2021-2050) and Far Future (2070-2099)]. This study was conducted in three phases over Skunk Creek Watershed (SCW), SD, USA. In phase I, the impact of long-term ICL system implementation (1976- 2005; 30 years) on soil hydrology was evaluated. Phase II and phase III evaluated the impacts of projected climate changes under existing land cover and ICL system, respectively. Outcomes of phase I showed a significant decrease in water yield and surface runoff. Phase II showed the susceptibility of SCW to extreme events such as floods and waterlogging during spring, and droughts during summers under the projected climate changes. Phase III showed the reduction in water yield and surface runoff due to the ICL system and minimizing the induced detrimental impacts only due to climate change. Evapotranspiration (ET) plays a significant role in crop growth and development, therefore, an accurate estimation of ET is very important for water use and availability. The past hydrological landscape responses were studied using well-validated (r2 = 0.91, PBIAS= -4%, and %RMSE = 11.8%) actual evapotranspiration (ETa) time-series (1986- 2018) estimations. The developed ETa products were further used to understand the crop water-use (CWU) characteristics and existing historic mono-directional (increasing or decreasing) trends across the SD and MRB regions. Spatial variability of the Operational Simplified Surface Energy Balance (SSEBop) model- and Landsat-based ETa estimations showed strong correspondence with land cover and climate across the basin. The drier foothills in northwestern MRB, dominated by grassland/shrubland, showed lower ETa (\u3c 400 mm/year), whereas, cropland dominated regions in lower semi-humid MRB and forested headwater exhibited higher ETa (\u3e 500 mm/year). For the SD region, Mann Kendall trend analysis revealed an absence of a significant trend in annual CWU at a regional scale due to the combined impact of varying weather conditions, and the presence of both increasing (12%) and decreasing (9%) CWU trends over a substantial portion at the pixel-scale. Whereas, for the MRB, summer season CWU trend analysis revealed a significant increasing trend at the regional-scale with 30% MRB cropland pixels under a significant increasing trend at pixel-scale. The existing increasing trends can be explained by the shift in agricultural practices, increased irrigated cropland area, higher productions, moisture regime shifts, and decreased risk of farming in the dry areas. Moreover, the decreasing trend pixels could be the result of the dynamic conversion of wetlands to croplands, decreased and improved irrigation and water management practices in the region. Overall, both studies highlight the potential of Landsat imagery and remote sensing-based ETa modeling approaches in generating historical time-series ETa maps over a wide range of elevation, vegetation, and climate