Water storage dynamics and runoff response of a boreal Shield headwater catchment

Thresholds in terrestrial water storage were quantified to explain differences in observed rainfall-runoff relationships for a 71·5 ha research catchment in northwestern Ontario, Canada. Using terrain analysis techniques, the catchment was partitioned into discrete hydrologic response units (HRUs). Unsaturated and saturated water storage was calculated for depression and midslope HRUs using continuous hydrometric measurements and a depth function for drainable porosity. The relationship between total water storage in these HRUs and catchment discharge was then examined for evidence of threshold behaviour. Piecewise regression analysis (PRA) was used to quantify a breakpoint in the nonlinear storage-discharge relationship, with separate linear regressions explaining the change in discharge with storage above and below this value. Above the breakpoint, a large increase in discharge is associated with a small increase in storage. Our results show that event-scale hydrologic response displays a threshold relationship with antecedent storage and maximum event storage in the terminal depression in the catchment. Our results also suggest that predictions of event runoff improve when storage excesses from upslope depressions are explicitly routed through the catchment taking into consideration storage deficits in downslope HRUs that may impede flow. The application of landscape delineation, hydrometric monitoring and PRA to model S-Q relationships is demonstrated to be an objective means of quantifying the transition between the two distinct hydrologic regimes in this catchment and provides new insight into how S-Q dynamics govern the hydrologic functioning of bedrock-dominated catchments