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 m(3) 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. Plain Language Summary Volumes of water reside within landscapes for varying amounts of time before they are discharged to a stream. That length of time determines how long the water has to interact chemically with soil and rock, and therefore influences the chemistry of water that ends up in stream channels. Quantifying the full range and variability of those travel times remains a challenge. We built an experimental hillslope, which allows us to keep track of all the water that enters and exits the soil-something that is difficult to accomplish in open environmental systems. We introduced chemically distinct water into the hillslope at specific points in time and followed the movement of that water within, and upon exit from the soil. We discovered that the water being discharged from the hillslope tends to have resided in the landscape for shorter lengths of time when the hillslope is very wet (like a wetted sponge) than when it is very dry (like a dry sponge). This insight helps us understand how different rainfall regimes, and the associated wetness of the landscape, can potentially influence water transit times through the landscape, and their relationship with stream chemistry.