Evaluating the Effects of Climate Change on Water Supplies and Operations: How Important are Feedbacks between Groundwater and Surface Water Use and Management?

Abstract:

It is well established that groundwater pumping affects surface-water availability by intercepting water that would otherwise discharge to streams and/or by increasing seepage losses from stream channels. Conversely, surface-water management affects groundwater availability by altering the timing, location, and quantity of groundwater recharge and pumping. Analyses of climate change impacts on water resources, however, often fail to account for these interactions, in large part due to a lack of modeling tools capable of simulating interactions between surface-water and groundwater management and use.

Here we use a modified version of the MODFLOW One Water Hydrologic Flow Model (MODFLOW-OWHM) to evaluate the role of feedbacks between surface-water and groundwater management and use in the context of climate change impact and adaptation studies. This modified version of MODFLOW-OWHM was developed by USBR and USGS by incorporating a fully-integrated surface-water operations module within MODFLOW-OWHM. We use a hypothetical) test case to compare direct and indirect effects of climate change on surface-water and groundwater resources. We define direct effects as changes in groundwater and surface-water conditions resulting from the hydrologic response to climate change (e.g., driven by changes in runoff, recharge, and demands resulting directly from climate change); we define indirect effects as changes arising from the anthropogenic response to direct effects (e.g., driven by changes in groundwater pumping in response to climate-driven changes in water demands or surface water supplies).

Results demonstrate that indirect effects on groundwater from changes in demand can outweigh direct effects from changes in supply—viz., indirect effects of increased groundwater pumping due to decreasing surface water supplies can be greater than direct effects of decreasing groundwater recharge. Moreover, under some conditions, indirect effects from changes in groundwater use, and subsequent changes in baseflow and seepage, can significantly exacerbate the direct effects of climate change on surface water. These results, while limited to a simple test case, underscore the importance of accounting for the coupled response of surface-water and groundwater management in climate change analyses.