Nick Jarvis

Review and comparison of models for describing non-equilibrium and preferential flow and transport in the vadose zone

Abstract In this paper, we review various approaches for modeling preferential and non-equilibrium flow and transport in the vadose zone. Existing approaches differ in terms of their underlying assumptions and complexity. They range from relatively simplistic models to more complex physically based dual-porosity, dual-permeability, and multi-region type models. A relatively simple dual-porosity flow model results when the Richards equation is combined with composite (double-hump type) equations for the hydraulic properties to account for both soil textural (matrix) and soil structural (fractures, macropores, peds) effects on flow. The simplest non-equilibrium flow model, a single-porosity model which distinguishes between actual and equilibrium water contents, is based on a formulation by Ross and Smettem [Soil Sci. Soc. Am. J. 64 (2000) 1926] that requires only one additional parameter to account for non-equilibrium. A more complex dual-porosity, mobile–immobile water flow model results when the Richards or kinematic wave equations are used for flow in the fractures, and immobile water is assumed to exist in the matrix. We also discuss various dual-permeability models, including the formulation of Gerke and van Genuchten [Water Resour. Res. 29 (1993a) 305] and the kinematic wave approach as used in the MACRO model of Jarvis [Technical Description and Sample Simulations, Department of Soil Science, Swedish University of Agricultural Science, Uppsala, Sweden (1994) 51]. Both of these models invoke terms accounting for the exchange of water and solutes between the matrix and the fractures. Advantages and disadvantages of the different models are discussed, and the need for inter-code comparison is stressed, especially against field data that are sufficiently comprehensive to allow calibration/validation of the more complex models and to distinguish between alternative modeling concepts. Several examples and comparisons of equilibrium and various non-equilibrium flow and transport models are also provided.

A simple empirical model of root water uptake

An empirical sink function describing water extraction by roots is proposed. Water uptake is given as a function both of the potential transpiration rate and a weighted stress index which accounts for the effects of the vertical distributions of roots and soil water content. The model was tested in the field using measurements of water content in a clay soil in which oats, spring rape and mustard crops were grown under mobile shelters. Measured changes in soil water content were considered primarily due to root water uptake, since recharge was prevented and the redistribution of water assumed negligible. Best-fit parameter values were found by optimization during one experimental year. Model predictions compared well with measurements in two further years (r2 values of 94 and 81%), with no change in the optimized parameter input values.

Comparison of the performance of pesticide-leaching models on a cracking clay soil: results using the Brimstone Farm dataset

Abstract The leaching of the pesticide isoproturon from the macroporous clay soil at Brimstone Farm was modelled using four alternative models (MACRO, CRACK-NP, SIMULAT and PLM). Model results are presented for two test periods, the whole of one winter for which daily observations are available, and a short subset for which hourly data were presented. The best results are those given by MACRO with an expert user, although satisfactory results were also obtained from CRACK-NP and for the longer test period by PLM. SIMULAT was less successful in modelling the site because it did not include an adequate representation of the site hydrology, it was unable to predict the leaching of pesticide. MACRO was also used by a second modelling group who were less familiar with both the code and the site. Although the initial uncalibrated runs from this group were poor, the final calibrated results were almost as good as those derived by the ‘expert’ user. The simulations showed the difficulty of deriving adequate representations, even where relatively complete soil physical data are available. A shortcoming of the dataset provided was the lack of detailed soil moisture observations, particularly to define the initial conditions. From a well-monitored site, many observations of site hydrology (water table position, drainflow and surface flow) were available, but significantly, fewer pesticide concentrations in either the soil or the discharges were available. Models could thus be evaluated only in terms of their ability to predict the magnitude and timing of major pesticide leaching events.

Comparison of pesticide leaching models: results using the Weiherbach data set

Abstract The leaching of the herbicide isoproturon and the water tracer bromide in the profile of a silty loam soil at the Weiherbach Catchment area in Germany was simulated by eight different persons using the models MACRO, SIMULAT, LEACHP, WAVE, GLEAMS and PELMO (only bromide). Experimental results from different laboratory studies and from a field study were available to parameterise the models. Model results concerning water contents, tracer and pesticide profiles were provided for two different test periods in the field (winter and spring). The results for the behaviour of the herbicide and the tracer in the field study in spring were not known beforehand to the model users. Therefore this is considered to be a blind test. After the model users had estimated the parameters and presented the modelling results a consensus parameter set was provided by the data set co-ordinator to reduce the subjective influence of individual users and to identify the effect of different model concepts. The most striking points of the modelling exercise were the diversity of parameters estimated by different modellers from the same basic data set and the problems of simulating the field behaviour using laboratory derived pesticide parameters. An overwhelming influence of the individual users on the model results was remarkable for the Weiherbach data set. It was not possible to estimate, from the laboratory data, alone a parameter combination suitable for describing the field behaviour of isoproturon satisfactorily. The use of literature data or calibration to the field results of the winter period did improve the simulation results significantly.

CRACK-NP: a pesticide leaching model for cracking clay soils

Abstract The CRACK-NP model describes the movement of water and solutes (nitrates and pesticides) in cracked clay soils. CRACK-NP differs from other models in that it assumes water moves around peds which sorb and desorb water, but within which there is no upward or downward flux. This model is thus restricted to those soils in which macropore flow is the dominant mode of water movement. The major interaction between macropore and soil matrix components is controlled by the size of the peds, which is a measurable soil parameter. The water movement and solute (pesticide) components of this model are described. Pesticide degrades according to an exponential function, with coefficients dependent on temperature and moisture content. Partitioning of pesticide between adsorbed and soluble phases in the soil is described in the model by the Freundlich equation. The application of the model to the leaching of pesticides at the Brimstone Farm experimental site in Oxfordshire, UK, is described. The model is shown to reproduce the hydrological behaviour of the site for an independent test period, and to replicate some of the characteristics of the pattern of nitrate leaching. Applying the model to the estimation of pesticide leaching for the test periods is identified for the comparison exercise, the model was capable of predicting the timing of the main pesticide leaching peak. For the shorter period for which detailed data were available, the model predicted the pesticide concentrations in the drainwater moderately well.

Field test of a water balance model of cracking clay soils

Abstract This paper describes a field test of a water balance model of cracking clay soils. An important feature of the model is the dynamic treatment of both soil structure (which varies as a function of soil water content) and the crack water balance which is solved as a dynamic equilibrium between input at the soil surface, storage in cracks and uptake into aggregates. Model predictions are compared with measurements of soil water content made by neutron probe for two access tube groups located about 40 m apart in a heavy clay soil (50% clay content) in southern England in two years with contrasting weather (one dry, one wet). The results clearly demonstrated the importance of bypassing flow, with soil water recharge following dry periods occurring nearly simultaneously at all depths in the profile. Root water uptake was also affected by soil structure. For example, very low values of the critical soil air content were inferred (0.5%) and this was thought to reflect preferential root growth in the well-aerated structural porosity. Model predictions and measurements were generally in excellent agreement, although there was an apparent tendency to overestimate the amount of water stored in the surface layers (0.1–0.2 m depth), particularly during autumn soil water recharge periods. This may have been due to simplifications and assumptions in the model, in particular those related to the treatment of rainfall pattern and interception loss, and also the neglect of soil water redistribution in the matrix.

Effects of surfactant use and peat amendment on leaching of fungicides and nitrate from golf greens

Soil water repellency in golf putting greens may induce preferential “finger flow”, leading to enhanced leaching of surface applied agrochemicals such as fungicides and nitrate. We examined the effects of root zone composition and the use of the non-ionic surfactant Revolution on soil water repellency, soil water content distributions, infiltration rates, turf quality, and fungicide and nitrate leaching from April 2007 to April 2008. The study was made on 4-year-old experimental green seeded with creeping bentgrass (Agrostis stolonifera L.) ‘Penn A-4’ at Landvik in southeast Norway. Eight lysimeters with two different root zone materials: (i) straight sand (1% gravel, 96% sand, 3% silt and clay, and 4 g kg−1 organic matter) (SS) and (ii) straight sand mixed with Sphagnum peat to an organic matter content of 25 g kg−1 (SP) were used in this study. Surfactant treatment reduced the spatial variability of water contents, increased infiltration rates and reduced water drop penetration times (WDPTs) by on average 99% in and just below the thatch layer. These effects were most evident for SS lysimeters. Surfactant treatment resulted on average in an 80% reduction of total fungicide leaching, presumably due to reduced preferential finger flow facilitated by decreased soil water repellency. Peat amendment reduced fungicide leaching by 90%, probably due to increased sorption of the fungicides to organic matter. Nitrate leaching was also smaller from surfactant-treated straight-sand root zones, but this effect was not significant.

Surface Runoff of Pesticides from a Clay Loam Field in Sweden

Pesticides stored at or close to the soil surface after field application can be mobilized and transported off the field when surface runoff occurs. The objective of our study was to quantify the potential pesticide losses in surface runoff from a conventionally managed agricultural field in a Swedish climate. This was achieved by measuring surface runoff volumes and concentrations in runoff of six spring-applied pesticides and autumn-applied glyphosate and its metabolite aminomethylphosphonic acid (AMPA). Measurements were performed for 3 yr both during the growing seasons and during intervening winter snowmelt periods on a clay loam field close to Uppsala. During growing seasons, surface runoff was generated on only five occasions during one 25-d period in 2012 when the infiltration capacity of the soil may have been reduced by structural degradation due to large cumulative rainfall amounts after harrowing. Concentrations in surface runoff exceeded Swedish water quality standards in all samples during this growing season for diflufenican and pirimicarb. Surface runoff was generated during three snowmelt periods during the winter of 2012-2013. All of the applied pesticides were found in snowmelt samples despite incorporation of residues by autumn plowing, degradation, and leaching into the soil profile during the period between spraying and sampling. Concentrations of glyphosate ranged from 0.12 to 7.4 μg L, and concentrations of AMPA ranged from 0 to 2.7 μg L. Our results indicate that temporal changes in hydraulic properties during the growing season and when the soil freezes during winter affect pesticide losses through surface runoff.