Numerical analysis of solute transport during transient irrigation: 1. The effect of hysteresis and profile heterogeneity

Abstract

Most of the existing data on vadose zone field scale solute transport have been obtained from experiments conducted under transient, nonmonotonic water flow. However, the majority of the theoretical analyses of these experiments have used models which assume monotonic steady state water flow and uniform water content for the entire profile. In this study, transport of nonreactive solutes under nonmonotonic, transient water flow is analyzed numerically. The effect of hysteresis on solute transport is evaluated by making the soil hydraulic properties hysteretic using the procedure of Kool and Parker (1987). The effect of profile heterogeneity on solute transport is analyzed by assuming that the medium is scale heterogeneous in a vertical direction, with a random scale factor. Results of the simulations show that under transient water flow, analysis of solute transport data with a steady state water flow model may considerably overestimate the effective vertical pore water velocity. Under nonmonotonic water flow, when the hysteretic characteristics of the soil are important, transient flow models which neglect hysteresis can also seriously overestimate the solute velocity. In addition, failure to account for profile heterogeneity will also overestimate the solute velocity, because both hysteresis and profile heterogeneity change the water content profile and concurrently retard solute transport relative to the movement predicted if the soil water system is considered as homogeneous and nonhysteretic. Analyses of the computed breakthrough curves suggest that direct estimates of the amount of water drained below a given depth may improve the goodness of fit of the solution of the convection dispersion equation with constant effective parameters to the breakthrough curves obtained under transient conditions. The fitted parameters, however, are depth dependent and the resulting effective solute velocity is smaller than the steady state pore water velocity. Wierenga, P. J., Solute distribution profiles computed with steadystate and transient water movement models, Soil Sci. Soc. Am. J., 41, 1050-1055, 1977. Wild, A., and I. A. Babiker, The asymmetric leaching pattern of NO 3 and C1 in a loamy sand under field conditions, Soil $ci., 27, 460-466, 1976.

The effect of hysteresis on three‐dimensional transient water flow during surface trickle irrigation

“…This is because when hysteresis is included in the flow analysis, the relative large change in matrix potential head on reversal from wetting to drying produces only a small change in water content compared to the change predicted from a single wetting Q(H) curve. This is in agreement with results presented by Russo et al (1989) who simulated one-dimension infiltration without considering root-water uptake and evaporation from the soil surface. Figures 3(a, b) to 6(a, b) also show that when hysteresis is neglected and the wetting Q(H) curve is used for both wetting and draining, on reversal the draining process will be much more rapid than with hysteresis included.…”

“…Hysteresis can significantly influence water flow in variably saturated porous media (Vachaud and Thony, 1971;Gillham et al, 1979;Russo et al, 1989). Hysteresis refers to the non-unique relationship between the pressure head, H, and the water content, Q, in the soil water characteristic curve (SWCC).…”

“…Similarly, during evaporation or gravity drainage when the water content and the pressure head are monotonically decreasing, the retention curve can also be described by a unique but different function. When drying reverses into a wetting process and vice versa, Q(H) is no longer unique and hysteresis must be introduced (Russo et al, 1989). When the soil is wetted from the residual water content or drained from the saturated water content, the main wetting or drying curves are followed, respectively.…”

Multiresponse modeling of variably saturated flow and isotope tracer transport for a hillslope experiment at the Landscape Evolution Observatory

“…Various hypotheses have been invoked to explain possible factors that affect the migration and distribution of solutes under unsaturated, heterogeneous conditions, including turbulent mixing due to high rainfall (Havis et al, 1992); solute transfer between mobile and immobile water (De Smedt and Wierenga, 1984); mobile-immobile exchange and hysteresis (Butters et al, 1989;Russo et al, 1989aRusso et al, , b, 2014; lateral mixing due to velocity fluctuations (Russo et al, 1998); isotope effects (Barnes and Allison, 1988;LaBolle et al, 2008;Zhang et al, 2009);variable, state-dependent anisotropy (McCord et al, 1991); non-Gaussian early-time mean tracer plume behavior (Naff, 1990); non-Fickian solute migration at low water contents (Padilla et al, 1999) and for macroscopically homogeneous sand (Bromly and Hinz, 2004); and saturation-dependent dispersivity (Raoof and Hassanizadeh, 2013). In addition, Konikow et al (1997) and Parker and van Genuchten (1984) discuss the importance of boundary condition treatment (e.g., water-solute injection, solute exchange between soil and atmosphere).…”

Soil Hydraulic Properties

“…Neglecting hysteresis leads to significant errors in the water redistribution under transient boundary conditions (Dane and Wierenga, 1975;Gillham et al, 1979, Hoa et al, 1977Viaene et al, 1994;Lehmann et al, 1999;Si and Kachanoski, 2000). Russo et al (1989) and Mitchell and Mayer (1998) investigated the influence of hysteresis on solute transport and found that the magnitude of the deviations between hysteretic and nonhysteretic simulations was not a simple function of single parameters, but rather depended on the combined values of many or all of the hydraulic parameters. Hysteresis is also seen as one of the major obstacles for comparing different measurement methods for the K(θ ) (Stolte et al, 1994;Basile et al, 2003).…”