The transport of pesticides and other chemicals through macropores has been widely observed and predicting it is a challenge. This article considers a simplified two-layer model, similar to overland flow models in which the processes of adsorption and desorption are separated. For the layer near the surface, or the mixing layer, the solute concentration in the layer is equal to that in the percolating water (including preferentially moving water). In the lower profile, the flow is partitioned between matrix and preferential flow. The solute concentration of the matrix flow is characterized by the soil condition near the outlet point, whereas the preferential flow is represented by the solute concentration in the mixing layer. The closed form equation, exhibiting exponentially decreasing macropore flow solute concentrations, is tested against solute breakthrough curves using three independent sets of experimental data. The predicted depths of mixing between 5 and 25 cm are physically realistic and the closed form is shown to reproduce the form of experimental data, particularly under conditions of significant macropore flow. Although highly simplified, the physically based model yields a framework for predicting solute concentration for preferentially moving water.
tion of salt in the soil, salt leaching from the root zone should be conducted. In regions where the rainfall is Development of an impermeable layer at some depth beneath the low, a leaching fraction (the fraction of applied water soil surface and the presence of a high groundwater level are common that appears as drainage water) is added to the irrigation phenomena in the Yizre'el Valley, Israel. The main objective of this study was to determine the salt and water distributions and salt leach-water to hold the salt concentration in the soil below a ing in a field irrigated by sprinklers, under nonisotropic and high specific value (Rhoades et al., 1973). In contrast, in groundwater level conditions. A field experiment was conducted in regions, where the rainfall is relatively high, the wet a cornfield (Zea mays L.) on a Vertisol (Typic Chromoxerets) with season rainfall can ensure leaching of the salt. subsurface drains. The electrical conductivity (EC) of the irrigation The water percolating below the root zone moves water was 2.5 dS m Ϫ1. The variations in water table level, in EC of downward to the groundwater and may cause the water soil solution and soil saturation paste, and in gravimetric water content table to rise. As the water table approaches the soil along the field were determined at different times. Likewise, corn surface, poor soil aeration and/or high salinity in the yield from various sites across the field was determined at the end root zone may reduce crop yields. Consequently, instalof the growing season (August). The water table level increased lation of a subsurface drainage system, to keep the water sharply in the winter to 49.5 m, and then decreased continuously in table from rising and to allow salt leaching, is commonly the summer despite the irrigation. The EC increased in the downhill direction, more sharply in the deeper soil layers. In the upper part considered to be essential for long-term productivity of the field, the average EC in saturation paste in the 0-to 1.2-m soil (Bradford and Letey, 1992). layer was 1.1 dS m Ϫ1 in March (end of the rainfall season) and 2.1 Under steady-state and isotropic conditions, the salt dS m Ϫ1 in August. Conversely, in the lower part of the field, the ECs distribution in a field is independent of landscape posiin March and August were 4.4 and 3.7 dS m Ϫ1 , respectively. A linear tion and time. Many studies (e.g., Beven and Germann, reduction of the corn yield with increasing EC was observed. The 1981; Meyer et al., 1990; Bradford and Letey, 1992; relatively low level of the groundwater at the upper part of the field Thorburn et al., 1995) have described the water and salt allowed vertical salt leaching by the rainfall. Conversely, the rise of movements and distributions under these conditions. the saline (EC ≈20 dS m Ϫ1) groundwater in the lower part of the field In many cases, particularly in valleys, such as the in the winter with lateral salt movement increased the soil EC. Decline Imperial Valley in California (Grismer and Tod, 1991) of the wat...
Abstract. Modeling of water and solute movement requires knowledge of the nature of the spatial distribution of transport parameters. Only a few of the field experiments reported in the literature contained enough measurements to discriminate statistically between lognormal and normal distributions. To obtain statistically significant data sets, six field experiments at four different sites were performed. Different degrees of macropore and matrix flow occurred at each site. In each experiment a solute pulse was added followed by artificial or natural rainfall. Sixteen thousand spatial distributed fluxes and solute concentrations were collected with wick and gravity samplers. Spatial distributions of solute velocity, dispersion coefficient, water flux, and solute concentration were determined over different timescales ranging from 1 hour to the duration of the experiment. A chi-square test was used to discriminate between the type of frequency distributions. The spatially distributed water and solute transport parameters when averaged over the experimental period were found to fit the lognormal distribution when macropore flow dominates. Otherwise, when only matrix flow occurs a normal distribution fitted the data better. Under no-till cultivation, hourly concentration and water flux are lognormally distributed, while tillage makes the tracer concentration to be normally distributed. Spatial frequency distributions of daily solute concentration change in time: Concentrations were normally distributed when the bulk of the solute broke through with the highest concentrations and lognormally distributed in the beginning and end of the experiment. Daily water flux was found to be lognormally distributed throughout the experiment, but the distribution varied between water applications: Shortly after water application, when wick and gravity pan samplers collected water predominantly from macropores and normally distributed at later times when mostly matrix pores were sampled with wick pan samplers. IntroductionThe quality of groundwater and surface waters is increasingly being compromised through recharge water that still contains significant concentrations of surface-applied chemicals such as fertilizers and pesticides [Clothier et al., 1996]
Concern about the environmental effects of agricultural chemicals through preferential flow has risen in recent years. A simple model is presented describing the processes involved in preferential transport of both soil-adsorbed and non-adsorbed solutes. The model assumes that water and solutes are mixed into an upper soil layer. The water and solutes then flow through macropores to the groundwater. Analysis of the solute breakthrough curves in subsurfact drainage effluent makes it possible to calculate the depth of the mixing layer or the adsorption desorption partition coefficient. Data from a drainage experiment with chloride, 7.4-D and atrazine were used to test the model. The study was performed on a no-till and a conventionally tilled plot. The model and experimental results indicate that only a fraction of the field area participates in transport to the macropores. Differences between breakthrough curves from the conventionally tilled and no-till plots are explained well by the mixing of solutes and water in the upper layer. This simple, physically based model can help us to understand and estimate the environmental threats of herbicides shortly after application.
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