Modelling of macropore flow and transport processes at catchment scale

Macropores play a significant role as a preferential flow mechanism in connection with pesticide leaching to shallow groundwater in clayey and loamy soils. A macropore description based on some of the same principles as those of the MACRO code has been added to the coupled MIKE SHE/Daisy code, enabling a physically based simulation of macropore processes in a spatially distributed manner throughout an entire catchment. Simulation results from a small catchment in Denmark suggest that although the point scale macropore processes have no dominating effect on groundwater recharge or discharge at a catchment scale, they will have significant effects on pesticide leaching to groundwater at a catchment scale. The primary function of macropores in this area is that they rapidly transport a significant part of the infiltrating water and solutes from the plough pan at 20 cm depth some distance downwards before most of it flows back into the soil matrix. This has a very significant effect on the leaching of pesticides from the surface to the groundwater table, because some of the pesticides are transported rapidly downwards in the soil profile to zones with less sorption and degradation. It is concluded that the spatial variations of macropore flows caused by the variation in topography and depth to groundwater table within a catchment are so large that this has to be accounted for in up-scaling process descriptions and results from point scale to catchment scale.