Equivalent vadose zone steady state flow: An assessment of its capability to predict transport in a realistic combined vadose zone-groundwater flow system

The applicability of an equivalent steady state vadose zone flow model to a realistic transient three-dimensional, heterogeneous, combined vadose zone-groundwater flow system was tested here. Two cases pertinent to semiarid regions and the presence (or absence) of an irrigated crop were analyzed and are presented in detail using the hydraulic properties from the Bet Dagan trench. In addition, the case of a more humid region characterized by additional summer precipitation and soils of different textures was analyzed and is discussed briefly. Results of the analyses suggested that when the water table is located at sufficiently large distance from the soil surface, the vadose zone can be divided conceptually into two distinct zones: a highly transient, near-surface zone and a deeper, quasi steady state zone. Under these conditions, an equivalent steady state influx, obtained by averaging the cumulative flux of the net applied water over the relevant time period, may provide a relatively good approximation of the cumulative water flux that crossed the water table. As for solute transport, in all cases examined, the trajectories of the centroid of the solute plume and the mean and the standard deviation of the solute breakthrough curves at a given horizontal control plane, CP (expressed as functions of the cumulative flux crossing the pertinent horizontal CP), associated with the transient flows were in relatively good agreement with their counterparts associated with the equivalent steady state flows, particularly under noncropped conditions and when the soil texture is coarser. The main finding of this study is that for the case in which the water table is located at sufficiently large distance from the soil surface, equivalent steady state vadose zone flow may reconstruct the true'' solute BTC (or the solute travel time PDF) at a horizontal CP located in the vicinity of the water table depth. The latter finding applies to heterogeneous soils of different textures and to different atmospheric forcing conditions at the soil surface and in the presence (or absence) of vegetation and despite the fact that features of the transport unique to vadose zone transient flow (induced by the temporal fluctuations in the water table depth) cannot be reproduced by the equivalent steady state vadose zone flow.