Nicholas Jarvis

Indirect estimation of near-saturated hydraulic conductivity from readily available soil information

Application of process-based water flow and solute transport models is often hampered by insufficient knowledge of soil hydraulic properties. This is certainly true for dual- or multi-porosity models that account for non-equilibrium flow of water in macropores, where the saturated ‘matrix’ hydraulic conductivity is a particularly critical parameter. Direct measurement is possible, but this is impractical for larger scale studies (i.e. catchment or regional), where estimation methods (pedotransfer functions) are usually required. This paper presents pedotransfer functions for hydraulic conductivity at a pressure head of � 10 cm, K10, based on measurements of near-saturated hydraulic conductivity made with tension infiltrometers in 70 soil horizons at 37 different sites in

Simulation of dichlorprop and bentazon leaching in soils of contrasting texture using the MACRO model

Abstract This paper presents an application of a physically based two‐domain model of water and solute transport in macroporous soil (MACRO) to lysimeter leaching experiments using two herbicides (dichlorprop and bentazon) and five soils with textures ranging from sand to clay. The model, which accounts for nonequilibrium transport of pesticide in soil macropores, is briefly described. Parameter estimation for the model simulations is then discussed. The model simulations showed that MACRO was able to reproduce the pattern of leaching in the clay‐ and loam‐textured soils, providing macropore flow was accounted for. When run in one flow domain, the model failed to reproduce the observed pattern of leaching in all five soils. It was concluded that preferential flow influenced the pattern and amount of herbicide leached, not only in the structured clay and loam soils, but also in two sandy‐textured soils.

Pesticide leaching data to validate simulation models for registration purposes

Abstract A data set originating from three pesticide leaching experiments conducted in undisturbed soil monoliths is described. Leaching of the herbicides dichlorprop and bentazon was measured in five different soils, ranging from loamy sand/sand through sandy loam to silty clay. Information on degradation and adsorption of the herbicides and soil physical and hydraulic properties is also included in the paper. This data was used for a model validation exercise, aimed at comparing and evaluating existing pesticide leaching models from the point of view of regulatory purposes. Larger amounts of dichlorprop were leached than bentazon reaching an average amount of 3.22 g a.i./ha (0.20% of applied dichlorprop) during the eight month study period. The largest leaching loss was found in a clay soil, which was explained in terms of macropore flow.

Simulation of soil water dynamics and herbicide persistence in a silt loam soil using the MACRO model

Abstract This paper presents an application of the model MACRO to measurements of soil water dynamics and persistence of three herbicides (metamitron, methabenzthiazuron and simazin) in a silt loam soil at Krummbach, Germany. The model is briefly described and parameter estimation for the simulations discussed. Soil water dynamics were accurately predicted by the MACRO model following calibration of model parameters related to crop water uptake. Occasional discrepancies between model predictions and measurements were attributed to temporal variation in the hydraulic effectiveness of macropores at the soil surface and also to the use of inappropriate soil water retention curves in subsoil horizons. Soil temperatures were mostly predicted to within ±3°C, except for winter periods characterized by snow and soil freezing. Field-measured persistence of the herbicides in the topsoil was closely predicted by the model using half-lives measured in the laboratory.

Identification of key climatic factors regulating the transport of pesticides in leaching and to tile drains

BACKGROUND Key climatic factors influencing the transport of pesticides to drains and to depth were identified. Climatic characteristics such as the timing of rainfall in relation to pesticide application may be more critical than average annual temperature and rainfall. The fate of three pesticides was simulated in nine contrasting soil types for two seasons, five application dates and six synthetic weather data series using the MACRO model, and predicted cumulative pesticide loads were analysed using statistical methods. RESULTS Classification trees and Pearson correlations indicated that simulated losses in excess of 75th percentile values (0.046 mg m(-2) for leaching, 0.042 mg m(-2) for drainage) generally occurred with large rainfall events following autumn application on clay soils, for both leaching and drainage scenarios. The amount and timing of winter rainfall were important factors, whatever the application period, and these interacted strongly with soil texture and pesticide mobility and persistence. Winter rainfall primarily influenced losses of less mobile and more persistent compounds, while short-term rainfall and temperature controlled leaching of the more mobile pesticides. CONCLUSIONS Numerous climatic characteristics influenced pesticide loss, including the amount of precipitation as well as the timing of rainfall and extreme events in relation to application date. Information regarding the relative influence of the climatic characteristics evaluated here can support the development of a climatic zonation for European-scale risk assessment for pesticide fate.

Sources of error in model predictions of pesticide leaching: a case study using the MACRO model

Abstract Uncalibrated predictions of soil water balance, water content, non-reactive solute transport (bromide) and pesticide leaching (bentazone) made by three users of a comprehensive mechanistic model (MACRO) are compared to measured data obtained for a sandy soil at Vredepeel in the Netherlands. The objective was to assess the significance of different sources of error for making predictions of pesticide leaching. Objective statistical indices were used to compare the simulations made by different users and to evaluate overall model performance. All three users predicted very similar water balances. Soil water contents were in good agreement with the measurements, with the simulation based on measured hydraulic functions giving somewhat better predictions than those based on automatic estimation procedures (pedo-transfer functions). Bromide movement was also satisfactorily predicted by all three users despite an inability to reproduce the strong retention near the soil surface caused by finger flow. Bentazone dissipation in the field was severely underpredicted by all three users based on laboratory measurements of degradation. This error overshadowed the effects of differences in parameterisation between users.

Application of the model MACRO to water movement and salt leaching in drained and irrigated marsh soils, Marismas, Spain

Abstract This paper presents an application of a two-domain model of water flow and solute transport in macroporous soil (MACRO) to field experiments in drained and irrigated, saline heavy clay soils under cotton. Model predictions are compared to detailed measurements of the soil water balance and leaching of chloride to field drains made during a 17-day period following two successive sprinkler irrigations. The model was able to reproduce the measured drain hydrograph and the observed response of the water table, providing calibrated, rather than directly measured, saturated hydraulic conductivity values were used. At the end of the first irrigation, the soil profile was fully recharged, with the water table only 10 cm below the surface. In the 2 weeks following irrigation, soil water extraction by the crop was largely restricted to the upper 30 cm of soil, presumably due to excessive salt concentrations in the subsoil. Indeed, the model simulations indicated a reduction in transpiration below the potential rate after only 1 week, with an accumulated water deficit of 60 to 70 mm. A strong dilution of the chloride concentrations during peak drain discharges was observed and was also predicted by the model. This dilution was related to the rapid infiltration of irrigation water of low salt concentration in the cracks. In two flow domains, the model precisely matched observed leaching losses of chloride during peak drain discharges, but underestimated by ≈ 25% the accumulated loss of chloride 1 week after the irrigation. This was entirely due to an underestimation of chloride concentrations in the outflow during the late stages of recession. During such recession flows, the chloride concentrations in the drain outflow (≈ g·l−1) were most likely influenced by inflows of saline shallow groundwater from surrounding areas. In one flow domain, the model predicted qualitatively similar patterns of drain outflow and chloride leaching. Nevertheless, the model performed less well when bypass flow was not taken into account, with chloride leaching overestimated during peak discharges, despite a total drain outflow which was 10 mm less than in the two-domain case.

Dual-porosity and kinematic wave approaches to assess the degree of preferential flow in an unsaturated soil

Abstract The purpose of this study was to assess the degree of preferential flow in an unsaturated soil column using two different models: the dual-porosity model, MACRO, and the kinematic wave approach (KWA) based on boundary-layer flow theory. The soil column experiments consisted of six infiltrations with intensities varying from 15 to 101 mm h−1. Bromide solution was also infiltrated at an intensity of 79 mm h−1 and a concentration of 80 mg l−1. Both MACRO and the KWA indicated the absence of pure preferential flow. The KWA indicated intermediate flow with dispersion of the wetting front with depth, whereas MACRO indicated flow dominated by the diffusion of capillary potential. These results shed light on the transition between flows dominated by momentum dissipation and by diffusion of capillary potential. The absence of pure macropore flow in the structured sandy soil is mainly due to efficient lateral mass exchange in this material.

Analysis of Inverse Procedures for Estimating Parameters Controlling Macropore Flow and Solute Transport in the Dual-Permeability Model MACRO

the best simulation of the data giving the desired parameter set. The best-fit condition is reached by minimizing Because they are objective and reproducible, inverse modeling an objective function, which expresses the discrepancy procedures are increasingly used to identify model parameters that cannot be easily measured. This study investigated the feasibility of between the experimental data and the simulation. Inusing inverse methods to estimate parameters describing macropore verse modeling procedures are thus objective, reproducflow, transport, and transformation processes in the dual-permeability ible, and unambiguous, providing the problem is well model MACRO. MACRO was linked to the inverse modeling package posed (i.e., the solution exists, is unique, and depends SUFI, and we used numerically generated data representing transient continuously on the initial data). Ill-posed problems, leaching experiments for tracers and reactive solutes in microlysimearising from insufficient data in terms of quality and ters (21-cm height). Attention was focused on parameter sensitivity, quantity with respect to the parameters to be estimated, availability of experimental data (flux and resident concentrations), lead to problems of nonuniqueness and unreliable pathe degree of macropore flow in the system, and the significance of rameter estimates (Dubus et al., 2002). experimental errors. Reliable results were obtained in the case of strong macropore flow, but both resident and flux concentrations To date, inverse methods have been widely applied were needed. However, the uncertainty in d, the parameter describing in soil physics to derive soil hydraulic properties and in mass exchange between microporosity and macroporosity, remained large-scale distributed hydrological models (Hopmans large, and the adsorption coefficient could not be estimated accurately. and Simunek, 1999; Madsen, 2003), but very little is curResponse surface analysis showed that this was due to a lack of sensitivity to d and to internal correlation between adsorption and rently known about the possibilities and potential probdegradation parameters. In the case of equilibrium flow, the model lems of inverse modeling techniques applied to macwas overparameterized, and the parameters related to macropore flow ropore flow problems. Durner et al. (1999) showed that were not sensitive enough to be estimated properly. Experimental the parameters of bimodal water retention and hydrauerrors did not affect the feasibility of the procedure, although the lic conductivity functions assumed in some dual-permeuncertainty in the estimates increased. SUFI linked to MACRO apability models could be determined by inverse modeling pears to be a promising tool for optimization of the system parameters against measured water outflows from multistep outflow in soils affected by macropore flow, but the “experimental” design experiments. They claimed that the procedure was roneeds to be improved for reliable determination of the mass exchange bust, leading to unique solutions with limited data (waparameter and the adsorption coefficient. ter outflows only), irrespective of the number of parameters included, providing the underlying model accurately represented the true soil hydraulic properties. Schwartz M flow in the unsaturated zone is a signifiet al. (2000) attempted to estimate the parameters of a cant process that has a major impact on leaching dual-permeability model by inverse modeling on steadyand has been demonstrated in many field experiments state bromide breakthrough experiments on a variably (e.g., Flury, 1996; Jarvis, 2002). A number of models charged tropical soil, where the Br ion could be considaccounting for macropore flow are now available (Jarered as a weakly sorbed reactive solute. They encounvis, 1998; Feyen et al., 1998). The most widely adopted tered great difficulties in obtaining physically realistic concept is to divide the porosity into two or more reestimates of two critical parameters, namely the dispergions, each characterized by a water pressure (or water sion coefficient in the micropores and the fraction of content), water flow rate, and solute concentration. sorption sites in the macropores. They concluded that However, the introduction of additional parameters deinverse procedures are problematic even for the simple scribing the macropore region in such dual-permeability case of steady water flow with four unknown parameters models makes the task of parameter estimation even to estimate and were also pessimistic about the potential more difficult, and this is the main obstacle to the application of macropore flow models. Inverse modeling, also to estimate macropore flow parameters under transient termed automatic calibration, is a promising alternative conditions in the field. These findings highlight the need method to derive parameters that cannot be estimated to investigate the feasibility of inverse procedures beby accurate independent measurement or by expert fore applying them to actual data, to avoid the risk of judgment. Parameter values are derived from the comidentifying physically inappropriate parameter values. parison of model simulations with experimental data, In particular, it remains to be seen whether robust estimation of model parameters regulating macropore flow is possible for transient leaching experiments with reacS. Roulier and N. Jarvis, Department of Soil Sciences, SLU, Box 7014, tive solutes. 750 07, Uppsala, Sweden. Received 14 Feb. 2003. Original Research Paper. *Corresponding author (stephanie.roulier@mv.slu.se). This study focused on the development and testing of an inverse procedure to derive soil hydraulic properPublished in Vadose Zone Journal 2:349–357 (2003). ties and adsorption and transformation parameters in  Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA simulation models which account for rapid nonequi-

Modeling spatial variation in microbial degradation of pesticides in soil.

Currently, no general guidance is available on suitable approaches for dealing with spatial variation in the first-order pesticide degradation rate constant k even though it is a very sensitive parameter and often highly variable at the field, catchment, and regional scales. Supported by some mechanistic reasoning, we propose a simple general modeling approach to predict k from the sorption constant, which reflects bioavailability, and easily measurable surrogate variables for microbial biomass/activity (organic carbon and clay contents). The soil depth was also explicitly included as an additional predictor variable. This approach was tested in a meta-analysis of available literature data using bootstrapped partial least-squares regression. It explained 73% of the variation in k for the 19 pesticide-study combinations (n = 212) in the database. When 4 of the 19 pesticide-study combinations were excluded (n = 169), the approach explained 80% of the variation in the degradation rate constant. We conclude that the approach shows promise as an effective way to account for the effects of bioavailability and microbial activity on microbial pesticide degradation in large-scale model applications.