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- Title
Upland Hillslope Groundwater Subsidy Affects Low‐Flow Storage–Discharge Relationship.
- Authors
Li, Hongyi; Ameli, Ali A.
- Abstract
Large‐scale cross‐site scientific synthesis on low‐flow storage–discharge relation can promote developing transferable hypotheses on the interactions among critical zone attributes and on how such interactions affect catchments' water vulnerabilities. This study leverages cross‐site empirical and theoretical analyses and develops a similarity index, based on the interactions among critical zone attributes, to help determine the less‐explored influence of upland hillslope groundwater subsidy on storage–discharge relation. We show that an increase in the relative extent of upland hillslope groundwater subsidy to low‐flow discharge, occurring through deep slow low‐moving (e.g., bedrock) storage unit, leads to (a) an increase in the nonlinearity of low‐flow discharge sensitivity to storage (β1) and (b) an increase in the convexity of low‐flow storage–discharge relation. Our findings also raise new hypotheses on the applicability of Boussinesq‐based hydraulic groundwater theory at low‐flow condition. Empirical results show that in a portion of our study catchments, particularly in those with a relatively small extent of upland hillslope groundwater subsidy, the theory's proposed range of nonlinearity sufficiently explains the nonlinearity of low‐flow storage–discharge relation. However, in catchments with a strong influence of upland hillslope groundwater subsidy through deep slow‐moving storage unit, the current state of hydraulic groundwater theory, using one single (non)linear representative storage unit, may not be sufficient to explain the large nonlinearity and convexity of low‐flow storage–discharge relation (or the long tail of hydrograph late recession). Considering β1 informs the low‐flow vulnerability of catchments, the findings of this study deepen and generalize our understanding of where low‐flow discharge is vulnerable to storage's change. Plain Language Summary: There is limited knowledge of the ways watersheds generate stream low flow during dry periods, in watersheds with no (or little) streamflow observations and even in watersheds with extensive streamflow observations. This knowledge gap can limit the development of accurate models and hinder assessments of the impacts of global environmental changes on drought vulnerability. This study aims to address this knowledge gap by examining the a less‐explored way in which watersheds may generate a large amount of streamflow during dry periods from upland hillslopes located tens to hundreds of kilometers away from the mainstream. Our study shows that in some environments, this poorly quantified component of water balance may disproportionately (a) affect the relationship between watershed storage and streamflow discharge, which is used to develop a hydrologic model, and (b) contribute to the long tail of streamflow hydrograph during dry periods and thus reducing the vulnerability of drought. Our findings suggest that while the classical theory of streamflow generation can sufficiently explain the ways watersheds generate stream low flow in a portion of watersheds, the large contribution from upland hillslopes to low flow may result in certain watersheds being less low‐flow vulnerable than others. Key Points: Upland hillslope groundwater subsidy increases the nonlinearity of low‐flow dynamics and alters the shape of storage–discharge relationHydraulic groundwater theory explains the estimated low‐flow nonlinearity in a portion (but not all) of the study catchmentsCatchments with a limited extent of upland hillslope groundwater subsidy may be more low‐flow vulnerable than catchments with a larger upland subsidy
- Subjects
UPLANDS; GLOBAL environmental change; WATERSHEDS; GROUNDWATER; BEDROCK; ENVIRONMENTAL impact analysis
- Publication
Water Resources Research, 2023, Vol 59, Issue 10, p1
- ISSN
0043-1397
- Publication type
Article
- DOI
10.1029/2022WR034155