Upland Hillslope Groundwater Subsidy Affects Low-Flow Storage–Discharge Relationship

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.