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

Abstract

[1] 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. Citation: Russo, D., and A. Fiori (2008), Equivalent vadose zone steady state flow: An assessment of its capability to predict transport in a realistic combined vadose zone -groundwater flow system, Water Resour. Res., 44, W09436,

A scale‐invariant property of the water retention curve in weakly heterogeneous vadose zones

“…It is a matter of fact that a correct modelling of the spatial variability of the hydraulic properties results in a highly reliable prediction of flow and transport processes (see, e.g., Neuman & Tartakovsky, ; Dagan, Fiori, & Jankovic, , and references therein). In particular, these and other studies (such as those of Russo & Fiori, ; Zarlenga, Fiori, & Russo, ) have emphasized how the spatial variability of a vadose zone eventually can be incorporated within upscaled models. As a consequence, a central problem in the hydrology of a vadose zone is whether the laboratory scale Richards equation can be used tout court at larger scales (Beven, ).…”

Two‐scale analytical homogenization of Richards' equation for flows through block inclusions

“…Even though this is not the case in this paper, we note that other parameters used to describe the retention curves, including k r , may have an explicit dependence on the spatial variable x and their variability may also have a strong influence on the flow behavior (see, e.g., Russo and Fiori [2008] and literature therein).…”

Effect of pulse release date and soil characteristics on solute transport in a combined vadose zone‐groundwater flow system: Insights from numerical simulations

“…Quantitative descriptions of solute transport in the combined flow system are essential for the analysis of groundwater contamination, and are fundamental prerequisite for risk assessment of groundwater contamination hazard. For given boundary and initial conditions and for a soil of a given texture, water flow and solute transport in the combined flow system are controlled by the heterogeneity of the soil hydraulic properties as demonstrated by simulations [e.g., Foussereau et al , 2001; Russo et al , 2001; Russo and Fiori , 2008] and analyzed in a theoretical stochastic framework [e.g., Destouni and Graham , 1995; Russo and Fiori , 2009].…”

“…The issue of an equivalent steady state (ESS) vadose zone flow which is able to represent the transient flow process was investigated in recent years by a few authors, e.g., the one‐dimensional, homogeneous numerical analysis of Salvucci and Entekhabi [1994], and the three‐dimensional, heterogeneous numerical analysis of Russo and Fiori [2008]. In particular, the latter showed that the cumulative flux of the net applied water, Qnaw = Qin − Qet , (where Qin is the cumulative water flux entering the soil surface, Qet = Qtr + Qev , and Qtr and Qev are the cumulative fluxes of the actual transpiration and evaporation, respectively), can be used as an approximation for the cumulative flux crossing the water table, Qwt .…”

“…The present analysis is based on detailed, three‐dimensional (3‐D) simulations, viewed as “numerical experiments”, also employed in previous studies [e.g., Russo et al , 1998, 2001, 2006; Russo and Fiori , 2008; Fiori and Russo , 2007, 2008]. This approach may provide detailed information on the consequences of soil‐, climate‐ and plant‐characteristics for the transport of solutes in the combined flow system under quite realistic conditions.…”