A.C. Armstrong

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.

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.

The Validation of Pesticide Leaching Models

The validation of pesticide leaching models presents particular problems where the number of model predictions is far in excess of the observed data. Normally, however, there are more frequent field observations for other parameters (notably the site hydrology) than for pesticide concentrations in either water or soil. A five-stage validation procedure which takes advantage of the most frequently available observations and which tests each of the components of the model in a cumulative way, is thus advocated: Stage 1: Parameterisation of the model using only independently measured parameters. Stage 2: Hydrological validation: the validation of the predictions of water movement and water content of the soil. Stage 3: Solute movement validation: where field data are available for solutes other than pesticide, the model should first be validated for them, especially if they are more abundant than the pesticide observations. Conserved solutes such as chloride or bromide are preferred, although nitrate may be used for short periods. Stage 4: Pesticide fate in the soil: models should use parameters of pesticide fate derived from independent studies. Stage 5: Pesticide leaching: only in the last stage are the relatively small number of pesticide observations compared with the model predictions with respect to patterns and orders of magnitude of occurrence. With this scheme, the results of each stage are carried forward to the next, and confidence in the model is built with each stage. This is illustrated using the CRACK-P model and hydrological, nitrate and pesticide data from the Brimstone Farm Experiment Oxfordshire, UK.

DITCH: a model to simulate field conditions in response to ditch levels managed for environmental aims

Abstract Wetland areas are frequently managed by manipulating the water levels in the surrounding ditches, with the aim of restoring or enhancing the wetness of the site, especially for the ecological requirements of some bird populations. The extent to which water tables in the centre of fields can be controlled by this action can be simulated through the use of a model, Drain Interaction with Channel Hydrology (DITCH), based on drainage theory. DITCH successfully predicted the pattern of water table behaviour on two wetlands sites, in the Norfolk Broads and Somerset Levels areas in the UK. The model can also be used to predict the strength of the soil surface and the extent of surface flooding. When used to examine the effects of alternative ditch management regimes within the two test areas, the model shows that the effects of ditch management options are not easily converted into impacts in the centre of the fields. If the effects are not sufficient, then hydrological manipulation of sites to achieve or improve wetland status may require more active intervention.

Evaluating pesticide leaching models: the Brimstone Farm dataset

Abstract This paper describes the Brimstone Farm dataset used for the comparison of pesticide leaching model performance. The tasks set to the modellers are then described. The soil of the site is heavy clay, in which cracks have a major effect on the hydrology. Data are presented for the leaching of isoproturon and mecoprop from two contrasting plots. Contrasting soil moisture states, resulting from different cropping, result in higher drainage rates from the wetter plot, which were associated with higher and earlier losses of isoproturon to the drains. No mecoprop was detected in the drainflows from this site.

Development and testing of a model for predicting tillage effects on nitrate leaching from cracked clay soils

Both water movement and nitrate leaching in structured soils are strongly influenced by the nature of the macro-porosity. That macro-porosity can however also be manipulated by choice of tillage operations. In order to investigate the potential impacts of tillage on rates of nitrate leaching from structured soils, a model specific to these soils, CRACK-NP was developed. The model, its application and validation for an experimental site on a heavy clay soil (Verti-Eutric Gleysoil) at Brimstone Farm, Oxfordshire, UK, is described. The model considers the soil as a series of aggregates whose size is also the spacing of the macro-porosity. Water and solutes move in the macro-pores, but within the peds they move only by diffusion, internal infiltration and root uptake (evaporation). The model reflects the influence of diffusion limitation in the release of solutes to by-passing water. The model was then used to investigate the influence of variable ped spacings which were created by variations in tillage practices. The results both from the model and from the field data demonstrated that finer soil structures, which have larger surface contact areas and shorter diffusion path lengths, present greater opportunities for interaction between peds and the water moving around them, and so release more nitrates through the drainage waters.

Mapping the risk to groundwater resources from farm waste stores in England and Wales

Unlined farm waste stores present a potential, but unquantified, risk to groundwater. Field studies of pore water retrieved from beneath active slurry stores indicate that there is a potential for pollutants to be transported down through the unsaturated zone of the aquifer. The highest risk to groundwater is where the water table is shallow, and/or where there is no protection offered to the aquifer by low-permeability soil or drift layers. A preliminary screening tool is presented, which highlights vulnerable areas of aquifer based on the protection offered by superficial layers. Two different approaches were applied, one utilizing soils data and the other making use of superficial geology data. Some of the differences between the approaches are considered, and their influence on the resulting risk assessment are discussed.