Influence of Irrigation and Drainage Practices on Water Resources

Climate change, increasing demand and overuse have resulted in water stress, a condition where available water resources are not enough to meet needs, in many parts of the US. At the same time, increasing seasonal precipitation, and more intensive management practices mean subsurface drainage is expanding in the Midwest, which has resulted in significant nutrient loading to water bodies and changes to the hydrologic balance of river basins. To ensure the sustainability of water resources, it is crucial to understand how much water available, and how agricultural water use is impacted by changing weather, stress and legal conditions. Water use systems must be properly managed to ensure prosperous agricultural production, and secure water resources. The overall goal of this research is to evaluate the sustainability of groundwater resources in the US through better quantification of surface and groundwater use and the interactions with agricultural water management practices. Watershed-scale measures of surface water and groundwater stress were calculated to explore the spatial and temporal variability of water stress in the US. Much of the Western US, which has been in a water stress situation for 35 years (1985-2015), is shifting to increasing surface water use, but both groundwater stress and surface water stress are continuing to rise. An increase in groundwater stress has also been observed in isolated watersheds in the eastern United States. The pattern of rising surface water stress corresponds more closely to changes in water use, whereas the pattern of rising groundwater stress corresponds to changes in water supply. It has been observed that those watersheds that experienced water stress in 1985 have responded by increasing their water withdrawals from other sources, but they have not decreased their agricultural water withdrawals. The decision to invest in groundwater irrigation depends on physical water availability and cost, as well as the right to use the water, as determined by groundwater doctrines. Overall, groundwater withdrawals for irrigation have increased in humid and temperate regions, and irrigation per unit area has also increased across the US. At the same time, the fraction of irrigation coming from groundwater is growing, suggesting a potential shift in the preferred water source. This is especially true in states following absolute ownership, correlative rights, and reasonable use doctrine. Correlative Rights doctrine prevalent in the western Corn Belt and Absolute Ownership doctrine (Indiana) appears to encourage increasing groundwater withdrawals during times of stress. These states increase their irrigation rate and the fraction of groundwater withdrawals for irrigation when less surface water is available. Subsurface drainage is common practice across vast areas of the US’ Corn Belt. Subsurface drainage can provide a better growing environment for crops, removing the excess water from the root zone and improving the trafficability of the soil. However, by lowering the water table, drainage has the potential to reduce groundwater recharge. Controlled drainage allows the user to control the water table by adjusting the height of the outlet to limit the subsurface flow during the non-growing season