Predicting rapid herbicide leaching to surface waters from an artificially drained headwater catchment using a one dimensional two-domain model coupled with a simple groundwater model

Pesticide losses to water can present problems for environmental management, particularly in catchments where surface waters are abstracted for drinking water supply. The relative role of different transfer pathways (spray drift, spills, overland flow and leaching from soils) is often uncertain, and there is a need for experimental observation and modelling to ensure that processes are understood under a range of conditions. Here we examine the transport of propyzamide and carbetamide in a small (15.5 ha) headwater sub-catchment dominated by an artificially drained field with strongly undulating topography (topographic gradients >1:10). Specifically, we explore the validity of the "field-scale lysimeter" analogy by applying the one dimensional mathematical model MACRO. Although one dimensional representation has been shown to be reasonable elsewhere, the scale and topography of the monitored system challenge many of the underlying assumptions. MACRO considers two interacting flow domains: micropores and macropores. The effect of subsurface drains can also be included. A component of the outflow from the main drain was identified as originating from an upslope permeable shallow aquifer which was represented using a simple groundwater model. Predicted herbicide losses were sensitive to drain spacing and the organic carbon to water partition coefficient, K(OC). The magnitude of the peak water and herbicide transport and their timing were simulated satisfactorily, although model performance was poor following a period of one month when snow covered the ground and precipitation was underestimated by the rain gauge. Total herbicide loads were simulated adequately by MACRO, suggesting that the field-scale lysimeter analogy is valid at this scale, although baseflow contributions to flow needed to be accounted for separately in order to adequately represent hydrological response.