N. J. Jarvis

MACRO (v5.2): Model Use, Calibration, and Validation

MACRO is a one-dimensional dual-permeability model of variably saturated water flow and reactive solute transport in soil that has been used since the early 1990s as a research tool to investigate the effects of macropore flow on soil hydrology and contaminant transport under transient field conditions. It is also widely used in the form of bespoke versions in pesticide exposure and risk assessments for groundwater and surface waters, e.g., in registration procedures in the European Union (EU). Macropore flow is a highly episodic, fast, non-equilibrium process that can dominate the leaching of reactive solutes in structured soils. This has important consequences for model calibration and validation procedures. Firstly, it means that in addition to water contents and resident solute concentrations, water flows and flux concentrations measured at high time resolution are required. Secondly, it implies that more weight must be placed on flux data obtained during these important but short-lived episodic flow events if parameters controlling macropore flow are to be reliably estimated. Although the choice of approach will vary with the purpose, automatic or hybrid automatic/manual calibration procedures are generally recommended for MACRO, especially global methods that account for uncertainty within a multi-objective framework. Despite the complexity of the processes it attempts to represent, MACRO is a parsimonious model, requiring only five additional parameters to simulate water flow and reactive solute transport, compared to the use of the Richards equation and the advection-dispersion equation. Nevertheless, for practical reasons, the size of the parameter space that can be explored by calibration is often quite limited. This emphasizes the importance of measuring those parameters that can be measured and the role of sensitivity analyses in supporting the choice of parameters to calibrate. This will vary between applications, but at least for structured soils, all five parameters regulating the generation and strength of non-equilibrium flow and transport are usually rather influential.

Measurements and modeling of pesticide persistence in soil at the catchment scale.

Degradation of pesticides in soils is both spatially variable and also one of the most sensitive factors determining losses to surface water and groundwater. To date, no general guidance is available on suitable approaches for dealing with spatial variation in pesticide degradation in catchment or regional scale modeling applications. The purpose of the study was therefore to study the influence of various soil physical, chemical and microbiological characteristics on pesticide persistence in the contrasting cultivated soils found in a small (13 km(2)) agricultural catchment in Sweden and to develop and test a simple model approach that could support catchment scale modeling. Persistence of bentazone, glyphosate and isoproturon was investigated in laboratory incubation experiments. Degradation rate constants were highly variable with coefficients of variation ranging between 42 and 64% for the three herbicides. Multiple linear regression analysis and Mallows Cp statistic were employed to select the best set of independent parameters accounting for the variation in degradation. Soil pH and the proportion of active microorganisms (r) together explained 69% of the variation in the bentazone degradation rate constant; the Freundlich sorption co-efficient (K(f)) and soil laccase activity together explained 88% of the variation in degradation rate of glyphosate, while soil pH was a significant predictor (p<0.05) for isoproturon persistence. However, correlations between many potential predictor variables made clear interpretations of the statistical analysis difficult. Multiplicative models based on two predictors chosen a priori, one accounting for microbial activity (e.g. microbial respiration, laccase activity or the surrogate variable soil organic carbon, SOC) and one accounting for the effects of sorption on bioavailability, showed promise to support predictions of degradation for large-scale modeling applications, explaining up to 50% of the variation in herbicide persistence.

Direct and indirect effects of climate change on herbicide leaching - A regional scale assessment in Sweden

Climate change is not only likely to improve conditions for crop production in Sweden, but also to increase weed pressure and the need for herbicides. This study aimed at assessing and contrasting the direct and indirect effects of climate change on herbicide leaching to groundwater in a major crop production region in south-west Sweden with the help of the regional pesticide fate and transport model MACRO-SE. We simulated 37 out of the 41 herbicides that are currently approved for use in Sweden on eight major crop types for the 24 most common soil types in the region. The results were aggregated accounting for the fractional coverage of the crop and the area sprayed with a particular herbicide. For simulations of the future, we used projections of five different climate models as model driving data and assessed three different future scenarios: (A) only changes in climate, (B) changes in climate and land-use (altered crop distribution), and (C) changes in climate, land-use, and an increase in herbicide use. The model successfully distinguished between leachable and non-leachable compounds (88% correctly classified) in a qualitative comparison against regional-scale monitoring data. Leaching was dominated by only a few herbicides and crops under current climate and agronomic conditions. The model simulations suggest that the direct effects of an increase in temperature, which enhances degradation, and precipitation which promotes leaching, cancel each other at a regional scale, resulting in a slight decrease in leachate concentrations in a future climate. However, the area at risk of groundwater contamination doubled when indirect effects of changes in land-use and herbicide use, were considered. We therefore concluded that it is important to consider the indirect effects of climate change alongside the direct effects and that effective mitigation strategies and strict regulation are required to secure future (drinking) water resources.

A comparison of near-saturated hydraulic properties measured in small cores and large monoliths in a clay soil

Abstract The effect of measurement scale on hydraulic properties close to saturation was investigated in a clay soil. Results from measurements on undisturbed “standard” small soil core samples were compared with results from three large intact soil monolith samples which were assumed to reflect natural soil hydraulic behaviour. An intermediate sample size, which was used to characterize soil water retention relations (θ(ψ)) in each layer in the intact monoliths, was obtained by cutting the intact monoliths in three layers. The small cores were subsequently sampled from the cut monoliths so that all measurements were made on the same soil material. Measurements of θ(ψ) and saturated hydraulic conductivity ( K s ) were compared, as well as the exponential relationships between K s and macroporosity ( ϕ ma ), the latter derived from the θ(ψ) data for small cores and from specific yields for intact monoliths. Between soil water pressure heads of −60 cm and −15 cm the small core data and the cut monolith data showed similar θ(ψ) relations, whereas they diverged closer to saturation. When comparing K s and ϕ ma , small core data were correlated with the intact monolith data although the small cores had smaller values. Thus, the “standard” small cores can satisfactorily predict the investigated soil hydraulic properties for a natural soil at pressure heads equal or less than −15 cm, but they may be in error, especially in soils with macropores, in the pressure head range −15 cm to saturation, i.e. in the largest macropores.

Extended sorption partitioning models for pesticide leaching risk assessments: Can we improve upon the koc concept?

Models used to assess leaching of pesticides to groundwater still rely on the sorption koc value, even though its limitations have been known for several decades, especially for soils of low organic carbon content (i.e. subsoils). This is mainly because the general applicability of any improved model approach that is also simple enough to use for regulatory purposes has not been demonstrated. The objective of this study was to test and compare alternative models of sorption that could be useful in pesticide risk assessment and management. To this end, a database containing the results of batch sorption experiments for pesticides was compiled from published studies in the literature, which placed at least as much emphasis on measurements in subsoil horizons as in topsoil. The database includes 785 data entries from 34 different published studies and for 21 different active substances. Overall, the apparent koc value, koc(app), roughly doubled as the soil organic carbon content decreased by a factor of ten. Nevertheless, in nearly half of the individual datasets, a constant koc value proved to be an adequate model. Further analysis showed that significant increases in koc(app) in subsoil were found primarily for the more weakly adsorbing compounds (koc values<ca. 100-200Lkg(-1)) and that sorption to clay in loamy and clayey-textured subsoil horizons was the main cause. Tests with the MACRO model demonstrated that sorption to clay minerals may significantly affect the outcome of regulatory exposure and risk assessments for leaching to groundwater. The koc concept currently used in leaching models should therefore be replaced by an alternative approach that gives a more realistic representation of pesticide sorption in subsoil. The two alternative models tested in this study appear to have widespread applicability and are also simple enough to parameterize for this purpose.