Simulating Hydrologic and Biologic Response to Land Use and Climate Change
U.S. Geological Survey
Hydrologic measurements and models are well-suited for characterizing groundwater and surface water systems, but simple abiotic indicators may not answer the hydroecological questions related to changes in land use and climate. Thus, understanding how changes in the hydrological system might ripple to the biological system is a critical topic for understanding and protecting groundwater dependent ecosystems, both for present day and potential future conditions. In this work, the effect on the biotic system was evaluated using simulated changes in hydrograph shape metrics (also referred to as hydrologic indices or hydrologic condition metrics). In this approach, the hydrograph is characterized using many different criteria (e.g., low-flow duration, stormflow recurrence), which are then summarized into a set of statistical metrics. Others have shown that some hydrologic metrics responded to urbanization and related to both water quality and biologic field data, and that such relations held for watersheds evaluated in the conterminous United States and internationally. Such results are promising because climate change predictions are commonly reported in abiotic terms, yet societal concerns are often ecosystem focused. The USGS coupled hydrologic model GSFLOW was used to simulate two watersheds in Wisconsin, USA. Model results are processed using the Nature Conservancy Index of Hydrologic Alteration (IHA) software suite to assess possible biological response to present day and changed streamflow resulting from climate and/or land-use change. The low pulse frequency count, defined as the number of flow events where the flow drops below a low-flow threshold, related well to current climate biological field data. In one watershed the relation established between both macroinvertebrate abundance and richness and the low pulse frequency counts simulated using a current-conditions calibrated groundwater-surface water model was then extrapolated to change scenario conditions. The increased temperature scenarios resulted in decreases in expected invertebrate abundance, with the lowest expected quality at a stream site that was periodically dry during some change scenarios. Results from both watersheds suggest that hydrographic shape metrics hold promise for helping translate future changes in climate or land use to ecosystem health.