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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.

Stream solute tracers are commonly injected to assess transport and transformation in study reaches, but results are biased toward the shortest and fastest storage locations. While this bias has been understood for decades, the impact of an experimental constraint on our understanding has yet to be considered. Here, we ask how different our understanding of reach‐ and segment‐scale transport would be if our empirical limits were extended. We demonstrate a novel approach to manipulate experimental conditions and observe mass that is stored at timescales beyond the traditional reach‐scale window of detection. We are able to explain the fate of an average of 26% of solute tracer mass that would have been considered as “lost” in a traditional study design across our 14 replicates, extending our detection limits to characterize flowpaths that would have been previously unmeasured. We demonstrate how this formerly lost mass leads to predicting lower magnitudes of gross gains and losses in individual reaches, and ultimately show that the network turnover we infer from solute tracers represents an upper limit on actual, expected behavior. Finally, we review the evolution of tracer studies and their interpretation including this approach and provide a proposed future direction to extend empirical studies to not‐before‐seen timescales.

Surface topography can influence flow pathways and the location of runoff source areas and water transport in steep headwater catchments. However, the influence of topography on spatial patterns of residual soil moisture is less well understood. We measured soil volumetric water content (VWC) on 14 dates at 0–30 and 30–60 cm depth at 54 sites on a steep, 10 ha north‐facing forested slope in the west‐central Cascades Mountains of Oregon, USA. Spatial patterns in VWC were persistent over time, and contrary to expectations VWC at 30–60 cm depth was greater on divergent than convergent slopes, especially during wet periods (R² = 0.27,p < 0.001). Vegetation characteristics were assessed for all VWC monitoring locations and soil properties were determined for 13 locations as local factors that affect spatial patterns in VWC. Mean VWC over all dates was negatively correlated to gravimetric rock content (R² = 0.28,p = 0.03) and positively correlated to water storage at field capacity (R² = 0.56,p < 0.01). The variability in rock content in quick‐draining soils influenced soil‐water retention, and by extension, created spatially heterogenous but temporally persistent patterns in VWC. While spatial patterns were persistent, they were not easily explained by surficial topography in a steep, mountainous landscape with rocky, well‐drained soils. Further research is needed to understand if combined soil‐terrain metrics would be a more useful proxy for VWC than terrain‐based wetness metrics alone.