Scale Issues in the Assessment of Pesticide Leaching Vulnerability for Loamy Structured Soils in Denmark.

Three approaches for vulnerability mapping of pesticide leaching for Denmark are considered. The first method is based on an approach developed at the European spatial scale, the second method relies on mapping soil hydraulic properties at a finer scale derived from a national soil property map. Both methods rely on the findings that leaching of pesticides from well structured soils is sensitive to the occurrence of preferential flow. Finally a third approach is considered in which an integrated catchment scale hydrological model is employed that allows taking into account implications for vulnerability due to change in climate and agricultural land management. In the European scale approach soil types in Denmark are classified using a decision tree structure that accounts for both soil texture as well as the lower boundary condition available from soil survey data at the European scale. At the national scale, soil types are classified directly from a detailed soil property map. The near-saturated hydraulic conductivity (Kh) defined at a soil water potential of -1 kPa has been utilized as a threshold parameter to determine when preferential flow is likely to occur. Kh is associated to each soil type as derived by the European scale method and also derived from a neural network type PTF at the national high spatial resolution. Simulations of pesticide leaching by the one-dimensional numerical water flow and reactive solute transport model MACRO are performed for low and high values of near saturated conductivity. It is expected that loamy soils are vulnerable at both the low and high values as low values indicate risk for preferential flow, whereas high values correspond to a more coarse soil structure with high soil hydraulic conductivity. It appears that the coarse spatial scale of derived soil classes according to the first method 1 does not match the scale at a required finer scale for vulnerability mapping. Finally, the method employing a coupled MACRO-MIKE SHE catchment scale approach shows that climate change resulting in altered rainfall intensities and patterns as well as changed land management are expected to have repercussions for pesticide leaching vulnerability.