USING HYDROGRAPH SHAPE METRICS TO RELATE HYDROLOGY, BIOLOGY, AND CLIMATE CHANGE

Hydrologic measurements and models are well-suited for characterizing groundwater and surface water systems, but simple abiotic indicators may not answer the hydroecological questions being asked by society. 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 climate-change 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.

An example from the USGS Trout Lake Water, Energy, and Biogeochemical Budgets site is presented where the Nature Conservancy Index of Hydrologic Alteration (IHA) software suite was used to relate macroinvertebrate abundance and richness to present day and potential climate-changed streamflow. 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 field data. 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 climate-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 climate change scenarios. The results suggest that hydrographic shape metrics hold promise for helping to translate climate change hydrology to groundwater dependent ecosystem health.