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Detecting the Invisible: The first step in addressing streamflow depletion across the U.S. - Andrea Brookfield and Misty Porter, USGS

Streamflow depletion is a reduction in streamflow caused by groundwater pumping that adversely affects water supplies and socio-ecological systems. Streamflow depletion is challenging to measure directly because it is often obscured by other processes such as surface-water diversions, land use and land cover change, and climate change, making it difficult to include in regulatory frameworks. Streamflow depletion can be estimated at the scale of a stream reach using detailed field measurements, but this is very effort-intensive and not feasible for scales beyond a single stream. Although there are analytical, numerical, and statistical models for estimating streamflow depletion at a larger scale, they are often limited by data availability and/or simplifying assumptions. In particular, identifying streamflow depletion is often dependent on knowing when and where groundwater is being pumped, which is rarely available at necessary temporal or spatial scales. In this work, we used national datasets to identify regions with declining baseflow (groundwater contributions to streams), and infer causation using a variety of statistical approaches, hydrologic signatures, and visualizations. Our findings suggested that, if confounding factors can be accounted for, the national streamflow datasets can be used to identify when, where, and why streamflow depletion has occurred. If confounding factors cannot be accounted for, these methods can be used to identify when and where we have decreases in baseflow. However, tying decreases to specific natural or anthropogenic factors remains elusive.

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