PL0516 - Cranfield University
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Mathematical modelling at a range of complexities has been given a prominent role in the fate and behaviour section of the registration process by Council Directive 91/414/EEC concerning the placing of plant protection products on the market. Although modelling studies are frequently submitted as part of regulatory data packages, the weight which these are afforded is restricted by a lack of information on the relative strengths and weaknesses of current models. In 1995, SSLRC reported on an evaluation of the use of pesticide leaching and runoff models available at the time (Brown & Hollis, 1995). The main models evaluated were LEACHP, PRZM-2 and VARLEACH and the project concluded that these models may be used to predict residues of pesticides in topsoil, but are not able to adequately simulate leaching of pesticides to depth. The main reason for this failure was identified as the importance of preferential flow in determining the extent of pesticide leaching and the lack of any description of this process in the three models. The MACRO model which includes a mechanistic description of flow through both micropores and macropores was briefly evaluated and found to give improved predictions of pesticide leaching relative to non preferential flow models. Parameter estimation was identified as a major problem in using MACRO and the need for much wider validation was proposed.
There is now evidence to suggest that preferential flow may be an important process for pesticide transport through a wide range of soils including both clays (Harris et al., 1994; Johnson et al., 1994; Brown et al., 1995a, b) and intermediate soils (Flury et al., 1995; Aderhold & Nordmeyer, 1995; Brown et al., 1997). A number of mathematical models have now been developed to simulate preferential flow and its influence on pesticide fate. The incorporation of such models into the regulatory process appears desirable, but evidence of their predictive ability is required and concerns over difficulties with robust selection of a number of key input parameters need to be addressed. In 1995, the FOCUS leaching group (Boesten et al., 1995) stated that:
- "Current models that consider macropore flow require that soil parameters be obtained by calibration. More advances are needed before predictions of macropore flow can be made using soil parameters in existing data bases."
The FOCUS surface water group (Adriaanse et al., 1997) were only slightly more optimistic in their appraisal:
- "Outputs from the macropore flow models MACRO and CRACK-NP are sensitive to parameters related to the macropore region ... which are in turn difficult to estimate. This may lead to high levels of predictive uncertainty compared to the use of models in non-structured sandy soils.”
The aim of this study was to evaluate the predictive ability of preferential flow models against pesticide datasets for a range of UK conditions and to assess the extent to which the concerns raised above have been addressed by recent developments in this field such as the database management tool, MACRO_DB.
Evaluation of the use of preferential flow models to predict the movement of pesticides to water sources under UK conditions (pdf, 101 pages).
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