Numerical Experiments on the Age of Seasonal Rain Water in Hillslopes

Solute transport in hillslope is often ruled by the water dynamics at the vadose zone and the groundwater. Solute is mobilized by rainfall, which feds the subsurface components of flow leading to water discharge and pollutograph at the outlet. It is often convenient to analyze solute transport through the travel time analysis. The latter is deeply influenced by the partitioning of the water ages. That is, rainfall typically mobilized pre-event (or “old”) water in the catchment, and the discharged water (and solute) is characterized by an age much larger than the one pertaining to the event water. We analyze solute transport in hillslope through a series of detailed numerical experiments. These are carried out through a three-dimensional saturated/unsaturated model, with realistic assumptions regarding the time- dependent rainfall and evapotranspiration forcing the flow system and the spatial distribution of the hydraulic properties. The scope is to analyze the travel time of rainfall in hillslopes as function of some selected characteristics and the seasonality of rainfall. We show that most of the river discharge is due to old water, i.e. rainfall coming from previous periods, sometimes very distant from the event. Also, each season displays a different travel time distribution for rainfall, as function of a few significant characteristics, like e.g. transpiration and the pre-event groundwater storage. The Equivalent Steady State approximation (ESS) for the analysis of solute transport is also tested.