Accounting for Groundwater Use and Return Flow Improves Modeling of Water Management
Water supply and demand vary over location and time. Water management aims to better align supply and demand through reservoir operations that regulate streamflow. These practices, however, can have significant effects on water resources, river discharge to oceans, and evapotranspiration from the land to the atmosphere. A research team led by scientists at the U.S. Department of Energy’s Pacific Northwest National Laboratory included groundwater use and return flow in PNNL’s integrated water model to better represent water management in Earth system models.
This research showed that groundwater use and return flow have pronounced effects on redistributing water resources for irrigation and non-irrigation use. The scientists’ addition of groundwater use and return flow modules to the integrated water model significantly reduced the water supply deficit simulated by the model, providing a more realistic representation of water management in the United States in recent decades.
Both surface water and groundwater have been used to meet water demand in many regions of the United States. In the Eastern U.S., a significant fraction of water withdrawn from rivers goes into thermoelectric power generation, and the unconsumed water returns to the streams. Realistic representations of sectoral (irrigation vs. non-irrigation) water withdrawals and consumptive demands, and their allocation to surface and groundwater sources, are important for more accurate modeling of the integrated water cycle. In this study, researchers improved the representation of water management in an Earth system model with a spatially distributed allocation of irrigation and non-irrigation demands to surface and groundwater systems simulated by a regional integrated assessment model. They evaluated the integrated modeling framework by analyzing the simulated regulated flow and irrigation vs. non-irrigation supply deficit in major hydrologic regions of the lower 48 United States. Scientists used decreases in historical supply deficit to evaluate model improvement in representing irrigation and non-irrigation supply. The team also assessed regional changes in both regulated flow and unmet irrigation and non-irrigation demands, resulting from individual and combined additions of groundwater and return flow modules. According to the results, groundwater use has distinct regional and sectoral effects by reducing water supply deficit. The effects of return flow showed a clear east-west contrast in the hydrologic patterns, demonstrating the return flow component—combined with the irrigation vs. non-irrigation demands—largely influences where water resources and deficits are redistributed. These analyses highlight the need for spatially distributed representation of irrigation-related water management practices to capture the regional differences in inter-basin redistribution of water resources and deficits.