The mechanistic basis for storage-dependent age distributions of water discharged from an experimental hillslope
Distributions of water transit times (TTDs), and related storage-selection (SAS) distributions, are spatially integrated metrics of hydrological transport within landscapes. Recent works conﬁrm that the form of TTDs and SAS distributions should be considered time variant—possibly depending, in predictable ways, on the dynamic storage of water within the landscape. We report on a 28 day periodic-steady-state-tracer experiment performed on a model hillslope contained within a 1 m3 sloping lysimeter. Using experimental data, we calibrate physically based, spatially distributed ﬂow and transport models, and use the calibrated models to generate time-variable SAS distributions, which are subsequently compared to those directly observed from the actual experiment. The objective is to use the spatially distributed estimates of storage and ﬂux from the model to characterize how temporal variation in water storage inﬂuences temporal variation in ﬂow path conﬁgurations, and resulting SAS distributions. The simulated SAS distributions mimicked well the shape of observed distributions, once the model domain reﬂected the spatial heterogeneity of the lysimeter soil. The spatially distributed ﬂux vectors illustrate how the magnitude and directionality of water ﬂux changes as the water table surface rises and falls, yielding greater contributions of younger water when the water table surface rises nearer to the soil surface. The illustrated mechanism is compliant with conclusions drawn from other recent studies and supports the notion of an inverse-storage effect, whereby the probability of younger water exiting the system increases with storage. This mechanism may be prevalent in hillslopes and headwater catchments where discharge dynamics are controlled by vertical ﬂuctuations in the water table surface of an unconﬁned aquifer.