This macropore flow algorithm considers soil properties and hydrologic factors that control macropore flow in Quebec soils. This study incorporates a macropore flow algorithm into SWAT, creating a new version of the model that is called the SWAT-MAC (Quebec version 2).
Thus, a new algorithm was needed to partition percolating water into macropore and matrix flows, considering the factors that control macropore flow in non-Vertisolic soils of the cold, humid temperate region. Moreover, neither SWAT nor SWAT-QC explicitly describe macropore flow for non-Vertisolic soils ( Neitsch et al., 2011). However, surface runoff was still over-predicted and subsurface drainage was still under-predicted ( Michaud et al., 2008). Modifications to the SWAT-QC model increased the tile drainage from 200 mm per year in Quebec. A greater proportion of water flows through macropores than the soil matrix, and the high water volume contributes to subsurface transport of P, particularly in fine-sized sediments ( de Jonge et al., 2004 Vidon and Cuadra, 2011 Poirier et al., 2012).īecause SWAT (version 2005) overestimated runoff and underestimated tile drain flow in Quebec watersheds, a modified version of SWAT called SWAT-QC was developed to better predict the balance between surface and subsurface drainage ( Michaud et al., 2008). An important pathway for P loss from soil is macropores, which include cracks and soil pores with minimum equivalent diameters of 0.3–0.5 mm ( Jarvis, 2007). In agricultural regions where land is systematically drained by installing subsurface tile drainage, such as in Quebec, Canada ( Gollamudi, 2006 Michaud et al., 2009), it is important to describe accurately subsurface pathways that transport phosphorus (P) into tile drains, since P is a main cause of eutrophication in surface waters.
The Soil and Water Assessment Tool (SWAT) is used widely for water quality modeling at the watershed scale ( Gassman et al., 2007).
We conclude that the percolation algorithm of SWAT-MAC represents the macropore flow pathway and improves the description of water movement through agricultural soils with subsurface drainage systems, which are important for transferring water and nutrients to downstream aquatic systems in cold, humid temperate regions. There are too few observations of regional-specific effects of soil moisture and management practices on macropore flow to correct the algorithm at this time. However, macropore flow was underestimated in the non-growing season and over-predicted during the growing season, which can be adjusted in the macropore flow algorithm by accounting for dynamic macropore connectivity or effective macroporosity. We predict more macropore flow into tile drains under fine-textured soils than coarse-textured soils, which is consistent with experimental observations. The macropore flow algorithm also improved water allocation between the annual surface runoff and subsurface flow in the SWAT-MAC model. Partitioning of subsurface flow between macropore and matrix components was reasonable, compared to a chemical-based hydrograph separation of streamflow in this subwatershed. We produced a new percolation algorithm to distinguish the macropore flow pathway, which was integrated in the SWAT-MAC model and used to predict water flows in a 30 km 2 agricultural subwatershed in southern Quebec, Canada. However, the Soil and Water Assessment Tool (SWAT) describes water flows poorly in land with subsurface drainage because it does not partition water between macropore and matrix transport processes. More water and nutrients from artificially-drained agricultural land reach surface waters by leaching through macropores than by percolating through the soil matrix. 2Institut de Recherche et de Développement en Agroenvironnement, Québec, QC, Canada.1Department of Natural Resource Sciences, Macdonald Campus of McGill University, Ste-Anne-de-Bellevue, QC, Canada.
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