Numerical analysis of flow and transport from a multiple‐source system in a partially saturated heterogeneous soil under cropped conditions

2007

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[1] The paper aims at analyzing subsurface flow and streamflow generation in steep hillslopes after rainfall. The principal scope of the work is to assess the role played by the spatial variability of the hydraulic properties on the subsurface flow patterns and hillslope discharge. We try to address this task through a three-dimensional numerical model which solves the Richards equation under prescribed initial and boundary conditions. Along an established practice, the relevant hydraulic parameters are modeled as random space functions with given statistical properties. Realistic features of the spatially variable formation parameters and the precipitation data are incorporated into the simulation model. It is found that spatial heterogeneity has a significant impact on the movement of water in the hillslope and the streamflow generation. In particular, the heterogeneous systems are more sensitive to rain input and display an increase of both the water table and saturation which is faster than the homogeneous case. The same is for water velocity, which is everywhere larger when heterogeneity is considered. As a consequence, the dynamics of subsurface stormflow (recognized as the leading mechanism for streamflow generation in the simulations) and the outflow discharge produced by the hillslope after a rain event are enhanced by heterogeneity. Despite of the possible lack of generality of the numerical experiments, they provide useful insight into the complex dynamics occurring in hillslope systems. The simulations show the importance of including realistic spatial heterogeneity in hillslope hydrology studies.

Section: Domain Setup and Mathematical Frameworkmentioning

confidence: 99%

Numerical analyses of subsurface flow in a steep hillslope under rainfall: The role of the spatial heterogeneity of the formation hydraulic properties

Fiori

2007

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“…Nevertheless, we believe that the examined cases are quite significant for many applications and the derived conclusions might have a relatively broad range of validity; results of analyses of flow and transport in similar flow systems, using additional independent realizations of the input soil properties and the applied weather pattern [Russo et al, 2006], and a recent analysis of a different flow system (A. Fiori and D. Russo, Travel time distribution in a hillslope: Insight from numerical simulations, submitted to Water Resources Research, 2008), suggest that the use of an equivalent steady state solution for modeling transient problems in the vadose zone -groundwater flow system can be quite robust.…”

Section: Summary and Concluding Remarksmentioning

confidence: 99%

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

Fiori

2008

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[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,

“…Details of the numerical methods used to approximate the relevant partial differential equations are given in Russo et al [1998aRusso et al [ , 2001Russo et al [ , 2006Russo et al [ , 2013. The numerical scheme used for the solute transport, originally developed to solve the one-region ADE (2), was modified in order to solve the two-region, mobile-immobile transport equation (4) subject to the pertinent boundary and initial conditions [Russo et al, 1998a].…”

Section: The Numerical Approachmentioning

confidence: 99%

“…The intensive computational demand associated with the 3-D simulations required to achieve the objectives of this study, led us to adopt the ''single realization '' approach [e.g., Ababou, 1988;Russo, 1991;Polmann et al, 1991;Russo et al, 1998bRusso et al, , 2001Russo et al, , 2006Russo et al, , 2013Tseng and Jury, 1994], which, in turn, employs the Ergodic hypothesis [Dagan, 1989]. Fulfillment of the ergodicity requirements, in turn, requires a flow domain that spans a sufficient number of the correlation length scales of the relevant soil properties in the vertical and the horizontal directions, I v and I h , respectively.…”

Section: The Numerical Approachmentioning

confidence: 99%

“…The analyses of field-scale transport [e.g., Russo, 1991;Russo et al, 1994Russo et al, , 1998bRusso et al, , 2001Russo et al, , 2006Russo et al, , 2013Tseng and Jury, 1994;Harter and Yeh, 1996;Roth and Hammel, 1996;Foussereau et al, 2001;Botros et al, 2012] are traditionally based on the classical, one-region, advection-dispesion equation (ADE), which, in turn, assumes that the entire water-filled pore space is mobile.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

On the mechanism of field‐scale solute transport: Insights from numerical simulations and field observations

Laufer

Gerstl

et al. 2014

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Field-scale transport of conservative and reactive solutes through a deep vadose zone was analyzed by means of two different model processes for the local description of the transport. The first is the advection-dispersion equation (ADE) model, and the second is the mobile-immobile (MIM) model. The analyses were performed by means of three-dimensional (3-D), numerical simulations of flow and transport considering realistic features of the flow system, pertinent to a turf field irrigated with treated sewage effluents (TSE). Simulated water content and concentration profiles were compared with available measurements of their counterparts. Results of the analyses suggest that the behavior of both solutes in the deep vadose zone of the Glil Yam site is better quantified by the MIM model than by the ADE model. Reconstruction of the shape of the measured solute concentration profiles using the MIM model required relatively small mass transfer coefficient, c, and relatively large volume fraction of the immobile water h im . This implies that for an initially nonzero solute concentration profile, as compared with the MIM model, the ADE model may significantly overestimate the rate at which solutes are loaded in the groundwater. On the contrary, for an initially zero solute concentration profile, as compared with the MIM model, the ADE model may significantly underestimate solute velocities and early arrival times to the water table. These findings stem from the combination of relatively small c and relatively large h im taken into account in the MIM model. In the first case, this combination forces a considerable portion of the solute mass to reside in the immobile region of the water-filled pore space, while the opposite is true in the second case.