Testing hidden assumptions of representativeness in reach‐scale studies of hyporheic exchange

Abstract

Field studies of hyporheic exchange in mountain systems are often conducted using short study reaches and a limited number of observations. It is common practice to assume these study reaches represent hyporheic exchange at larger scales or different sites and to infer general relationships among potential causal mechanisms from the limited number of observations. However, these assumptions of representativeness are rarely tested. In this study, we develop numerical models from four segments of mountain streams in different geomorphologic settings and extract shorter reaches to test how representative exchange metrics are in shorter reaches compared to their reference segments. We also map the locations of the representative reaches to determine if a pattern exists based on location. Finally, we compare variance of these shorter within-site reaches to 29 additional reaches across the same basin to understand the impacts of inferring causal mechanisms, for example, the expectation that wide and narrow valley bottoms will yield different hyporheic exchange patterns. Our results show that the location and length strategy of the study reach must be considered before assuming an exchange metric to be representative of anything other than the exact segment studied. Further, it is necessary to quantify within and between site variations before making causal inferences based on observable characteristics, such as valley width or stream morphology. Our findings have implications for future field practices and how those practices are translated into models. Plain Language Summary: Hydrology models are used to generalize processes and make predictions about how water moves through the environment. For example, models are used to study how water moves in and out of streambeds because processes occurring in the streambed can significantly impact the water quality. These models assume that data and observations from short sections of streams can represent longer portions of a stream or similar streams in different places. However, short sections of the stream may not accurately represent the conditions of the whole site that is being modeled, meaning that the assumptions and ultimately the models are misrepresenting the process. To test this, we compared how water was exchanged between a long segment of a stream and its subsurface to exchanges occurring in a shorter section of the stream and its subsurface. We found that only some of the shorter sections of the stream we modeled accurately represented the longer segment of the stream from which it was taken. These findings are important for future field studies as they demonstrate how scientists need to consider several aspects of the stream before they choose a section of a stream as the basis of their assumption for a model.

Citation

Becker, Paige S.; Ward, Adam S.; Herzog, Skuyler P.; Wondzell, Steven M. 2023. Testing hidden assumptions of representativeness in reach‐scale studies of hyporheic exchange. Water Resources Research. 59(1): 113. https://doi.org/10.1029/2022WR032718.