Subsurface stormflow recession analysis of different hillslopes

In many natural landscapes, subsurface stormflow (SSF) is a runoff-producing mechanism which can substantially contribute to the stream’s storm hydrograph. Despite its importance, the hidden (subsurface) processes controlling SSF are still not well understood. To study SSF and characterize associated storage-discharge dynamics, we analyzed the recession behavior of multiple SSF events of three trenched hillslopes in a Black Forest (Germany) catchment.  SSF triggered by natural and artificial rainfall events was measured in three trenches at the bottom of different hillslopes (T1–T3; 11–15 m wide, 1–3 m deep). In addition to SSF discharge (Q), groundwater levels and soil moisture dynamics were continuously monitored upslope of the trench. We extracted SSF recession segments and evaluated a single linear reservoir (1LR) model, a two parallel linear reservoirs (2PLR) model and also a power-law relationship −dQ/dt = aQ^b, where t is time and a and b are fitted parameters.

Recession behavior varied significantly across hillslopes: at T1, most recessions were adequately reproduced by the 2PLR model, whereas at T2 and T3 recessions generally followed the 1LR dynamics. The median 1LR recession timescales (k) were similar for T2 and T3 and about twice as long at T1. Where 2PLR was required, the slow and fast reservoirs differed strongly (median relationship between k_s/k_faround 16 at T1, 11 at T2, 14 at T3). The 2PLR fits show that a transient b > 1 can occur from the superposition of two linear reservoirs: b approaches 1 under clear fast- or slow-flow dominance, but steepens during the transition between the two reservoirs. Consistent with this mechanism, T1 had higher apparent nonlinearity (median b = 2.5) than T2–T3 (median b = 1–1.5). The comparison between natural and artificial rainfall events suggests that event-to-event variability in recession timescales is partly driven by changes in the upslope contributing area feeding the trench. Soil moisture and water table dynamics provide further insights on how evolving hydrological conditions modulate the SSF drainage.