JH_2004_299_136-158

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Journal of Hydrology 299 (2004) 136–158 www.elsevier.com/locate/jhydrol

Modelling of macropore flow and transport processes at catchment scale Jesper Skovdal Christiansend, Mette Thorsena, Thomas Clausena, Søren Hansenb, Jens Christian Refsgaardc,* a DHI Water and Environment, Hørsholm, Denmark Royal Veterinary and Agricultural University, Copenhagen, Denmark c Geological Survey of Denmark and Greenland, Department of Hydrology, Oster Voldgade 10, DK-1350 Copenhagen, Denmark d Atkins Danmark, Copenhagen, Denmark b

Received 15 August 2002; revised 20 April 2004; accepted 26 April 2004

Abstract 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. q 2004 Elsevier B.V. All rights reserved. Keywords: Preferential flow; Leaching; Pesticide; Catchment; Modelling

1. Introduction The importance of macropores as a preferential flow mechanism for infiltrating water and transport of solutes has been generally recognised during * Corresponding author. Fax: þ 45-3814-2050. E-mail address: jcr@geus.dk (J.C. Refsgaard). 0022-1694/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2004.04.029

a couple of decades (Beven and Germann, 1982; Barbash and Resek, 1996). Macropores play significant roles in many contexts such as leading to more infiltration and thus reducing overland flow and is rapidly transferring pesticides and other pollutants through the soil toward the groundwater. Pesticides have been detected in shallow groundwater worldwide (US Environmental Protection Agency, 1990,


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