The role of soil properties in regulating non‐equilibrium macropore flow and solute transport in agricultural topsoils

Abstract

Dual-permeability models can account for the strong influence of soil macropores on contaminant transport, but their predictive application is hampered by the difficulty in estimating a priori values for the rate coefficient controlling lateral mass exchange between the two flow domains. Our aim was to investigate the possibility of estimating the mass transfer coefficient in the dual-permeability model MACRO from fundamental soil properties. To this end, we calibrated MACRO against transient chloride leaching tests carried out on 33 undisturbed soil columns taken from the topsoils of three agricultural fields, characterized by a wide range of texture and organic matter content. The global search algorithm SUFI was used to derive optimum values of the mass transfer coefficient in each column. A Monte Carlo procedure was carried out on two columns to investigate the stability of the estimates in the presence of potential errors in fixed parameters. Despite such errors, c. 50% of the variation in mass transfer for this data set could be explained by two fundamental soil properties: the geometric mean particle size and the organic matter content. Soils of coarser texture and larger organic matter content were characterized by stronger lateral mass exchange and therefore weaker macropore flow. Harrowing and a 6-year grass ley also reduced the extent of non-equilibrium transport. Our results suggest that a robust functional description of the effects of soil structure on chemical transport is possible for predictive management applications of dual-permeability models across larger areas (i.e. mapping leaching risks at field, farm and catchment scales).

A review of model applications for structured soils: a) Water flow and tracer transport

“…The latter authors conceptually linked soil structure development to the predominant state of saturation of a soil horizon as depending on its location in pedon and landscape position. Jarvis et al (2007) analyzed the variation of the mass transfer coefficient, d, in the DPM MACRO as a function of the geometric mean particle size and the organic matter content. First, SUFI (Abbaspour et al, 1997) was utilized to identify d for chloride BTCs observed for 33 intact topsoil columns sampled from 3 different field sites.…”

Development of a method for estimating the likelihood of crack flow in Canadian agricultural soils at the landscape scale

“…Desiccation cracking is an important phenomenon in soil, and its significance in facilitating the movement of water and contaminants through the soil, especially in clayed soils, has long been acknowledged (Jamieson et al 2002;Jarvis 2007). It is therefore imperative to include crack flow into the transport-hydrology component of water quality models and risk indicators.…”

“…Tillage for seedbed preparation may also result in uniform water flow in the tilled layer (Jarvis et al 2007), reducing likelihood of CF.…”

“…Briefly, the amount of contaminant transported by PF is affected by its mass and initial position in the soil relative to water, and by the timing, the duration and the intensity of rainfall after the chemical has been applied (Edwards et al 1993;Saini et al 2003;McGrath et al 2008). Preferential flow appears to be one of the most important pathways for the transport of strongly adsorbed and fast degrading agricultural chemicals (Villholth et al 2000;Vereecken 2005), phosphorus (Ga¨chter et al 1998;Jarvis 2007), and pathogens (Jamieson et al 2002;Tallon et al 2007) to tile drains, surface waters and groundwaters.…”

“…Another important and unwanted consequence of PF on agricultural soils is that it may lead to water (Thomas and Phillips 1979) and nutrient (Smalling and Bouma 1992) shortage to crops. The amplitude of PF for contaminant transport under various climate, soil and agricultural conditions is summarized in review papers (Flury 1996;Hendrickx and Flury 2001;Nieber 2001;Jarvis 2007;Clothier et al 2008). Briefly, the amount of contaminant transported by PF is affected by its mass and initial position in the soil relative to water, and by the timing, the duration and the intensity of rainfall after the chemical has been applied (Edwards et al 1993;Saini et al 2003;McGrath et al 2008).…”