Climate and land use effects on hydrologic processes and water allocation in a primarily rain-fed, agricultural watershed

Abstract

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] There is a need to raise our understanding of the impact of climate variability and change on hydrologic processes at the watershed scale. This is important, particularly for land managers and policymakers, in making better-informed decisions to assess adaptation strategies and to ensure that all sectors and populations can meet projected water demand. The Missouri Salt River Basin was chosen for this study due to its unique soil and agriculture-dominated land use. It is dominated by high clay content soils, making it sensitive to changes in the hydrologic condition. While numerous studies have examined hydrologic processes around this region, only a few have analyzed linkages between climate and the consequence of these changes to water allocation. One of the greatest potentials to maintain viable crop and livestock economies is to continue making gains in production efficiency, particularly in the area of rain-fed crops with the potential of increasing irrigation. Therefore, the objective of this study is to: (1) evaluate the impacts of potential climate and land use changes on the hydrologic components of the agriculturally dominated Salt River Basin; (2) evaluate the impact of climate change to agriculture management in this watershed, and determine if land use change can mitigate the climate change impacts on hydrological processes; (3) evaluate the impacts of potential climate changes on the water supply and demand of the Salt River Basin using integrated hydrological model and water allocation model approach; (4) determine if future water supply can meet the Salt River Basin catchment demands, and evaluate the future water competition among different sectors in the Salt River Basin using scenario based approach. Temperature and precipitation projections for two representative concentration pathways (RCP 4.5 moderate CO[2] level and RCP 8.5 high CO[2] level) were obtained from nineteen general circulation models statistically downscaled to better represent local conditions. These data, along with soils, land cover, land management, and topography, were input to the Soil and Water Assessment Tool (SWAT), a process-based hydrologic simulation model, to evaluate hydrologic impacts. Possible outcomes for the near (2020-2039) and far (2040-2059) future scenarios were determined. Combined climate and land use change scenarios showed distinct annual and seasonal variations in hydrological processes. Annual precipitation was projected to increase from 4% to 7%, which resulted in 14% more spring days with soil water content equal or exceeding field capacity in mid-century. However, 07 precipitation was projected to decrease -- a critical factor for crop growth. Higher temperatures led to increased potential vapotranspiration during the growing season, resulting in an increased need for irrigation by 38 mm. Analysis from multiple land use scenarios indicated that converting crop and pasture land to forest coverage can potentially mitigate the effects of climate change on streamflow, thus insuring future water availability. Using hydrologic output simulations from SWAT, evaluation of water allocation strategies was performed using the water evaluation and planning (WEAP) model. By selecting priority water use strategies, WEAP enabled review of potential conflicts among users through scenario-based approaches. Operating on the principle of water balance accounting, a range of inter-related water issues facing water users, including multiple water sources, sectoral demand analyses, water conservation, water allocation priorities, and general reservoir operations, were evaluated. For this study, scenarios with different rate of irrigation expansion for crop areas were evaluated. The Ag Census data from 1997, 2002, and 2007 were analyzed to obtain the historical reported numbers of livestock in each county within the watershed. The historical livestock numbers combined with USDA agricultural projections to 2027 were used to project inventory for 2060. The results indicated that future water shortages will become more prominent in the SRB under projected climate conditions. Without any change irrigation area, the future unmet could double as a consequence of climate change from 3 million m3 to 6 million m3. Increased irrigation equal 10% of crop land results in 38.5 million m3 of unmet water demand. If water from Mark Twain can be withdrawn for agriculture purposes, the unmet demand would lower by 30% compared with the baseline period. However, under prolonged drought period, the impact of the Mark Twain Lake is limited. Finally, under all considered scenarios public water supply is not a source of water vulnerability in this region.