Comparison of 1D and 2D modelling of pesticide transfer in a tile-drained context. Application to la Jaillière site.
Comparaison de modélisation uni et bi-dimensionnelle des transferts de pesticides dans un contexte drainé par tuyaux enterrés. Application au site de la Jaillière
Résumé
One dimensional physically based models are widely use as regulatory tools for pesticide leaching in the E.U. However, in the particular case of drained soils, water and solute flow are mainly influenced by the 2D-spatial dynamics of the drained water-table (Paris, 2004). On the other hand, preferential flow is often invoked (Jarvis, 2007) to explain the observation of quick pesticide transfer to the drain. The main objective of this study is to evaluate the role of the drained water table and preferential flow in pesticide export, comparing a one dimensional model and a two dimensional model. The la Jaillière experimental station is located in the western part of France (47°27' N, 0°57' W). The site has been monitored for pesticides since 1994 and provides an important data set on pollutant transfers. It is currently used as a drainage scenario for European pesticide registration. The soils are predominantly stagnic luvisol, occurring on a gently sloping plateau. The fields are tile drained according to French criteria (0.9 m deep and 10-12 m drain spacing). The study is focused on one plot of the domain, called "T4", whose characteristics are provided in Table 1. Table 1 : Main soil characteristics of the T4 plot. Horizon Thickness Clay (%) Silt (%) Sand (%) O.M (%)a Bulk density (g.cm3) pH (water) Structure Ap 30 20.8 44.6 34.6 2.17 1.55 6.3 Blocky E 18 25.9 41.3 32.8 0.77 1.63 7 Blocky Bt 17 49.2 35.3 15.5 0.46 1.7 5.6 Prismatic Bt/C 45 42.7 35.8 21.5 0.36 1.7 4.9 Blocky The fate of two substances with widely differing characteristics was investigated. The first, isoproturon, is moderately sorbed (Koc : 124 L.Kg-1) and weakly persistent in soils (DT50 : 15 Days) whereas the second, diflufenican has a high sorption capacity (Koc : 2000 L.Kg-1) and is slightly persistent in the environment with DT50 exceeding 140 days. Both substances are applied during autumn on winter-wheat. The two models tested in this study were MACRO 5.2 (Larsbo and Jarvis, 2003) and HYDRUS-2D (Simunek et al., 1999). MACRO is a one dimensional physically based model with the soil porosity divided into two flow domains (macropores and micropores). Richard's equation and the classic convection-dispersion equation (CDE) are used in the matrix, for water and solute transport, respectively. In the macropore domain, a kinematic wave approach is used for water flow and convection for solute transport. HYDRUS-2D is a two dimensional physically based model which allows users to choose from a range of hydraulic models (e.g single porosity, mobile-immobile water). In order to investigate preferential flow impact on pesticide transport, we decided to test a single porosity model and a dual permeability model developed by Gerke and van Genuchten (1993) using Richard's equationand the CDE in both domains of porosity. According to EU procedures (FOCUS, 2001), models were first calibrated against water flow and bromide for which hourly data sets are available. Bromide has been use in several studies to investigate and calibrate the mass transfer coefficient (e.g. Stenemo and Jarvis 2007; Kohne et al., 2006) and other parameters (tortuosity, disperivity, etc) which were evaluated here. Results show that both models underestimated drainage during the intensive drainage season (IDS) without appropriate calibration. Another difficulty of the site is to represent the role of the groundwater table on drainage intensity and depletion. Simulation of bromide shows discrepancies between models, MACRO can't' represent simultaneously the dynamic of both water and bromide as shown on Fig. 1. Thus, the mass transfer coefficient can not be estimated with certainty. On the other hand, HYDRUS-2D represents the water and solute dynamics well for both options. Consequences for pesticide export are assessed using the mass transfer coefficient calibrated with HYDRUS-2D and MACRO.
Domaines
Sciences de l'environnement
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