How does tile drainage react on precipitation? Results of landscapes in the Northern Hemisphere
Tile drainages are installed to remove excess water from soils, to improve agricultural production. In Germany, tile drainages are present on about 3 M ha of the agricultural land (some 16%) (Werner et al., 1991; Wodsak and Werner, 1994), in Europe from north to south a decline is observable: So 91% of the total agricultural land is tile drained in Finland, 90% in Lithuania and 80% in Latvia (De Cuerva 2006). In Spain artificial drainages (1%) and Portugal tile drainages (0.3%) are of less importance for climatic reasons. Most of the tile drainage systems were installed in the 1960’s to the 1980’s. Tile drainages interfere considerably with the water and nutrient balance of a landscape. The shortened nutrient discharge pathways and the reduced denitrification capacity mean that the drainage systems beneath the groundwater systems are the main pathways for diffuse nitrogen input into rivers (Behrendt et al., 2000; Hirt et al., 2005a, b; Hirt, 2003; Amatya, 2004) and are relevant for the development of floodwaters (Wesström et al., 2000; Wiskow and van der Ploeg, 2003). A reduction in nutrient inputs, especially nitrate is strongly recommended in order to achieve the environmental objectives of the European Water Framework Directive (WFD; Directive 2000/60/EC). For the assessment of such nutrient inputs into river basins, a quantification of the drainage discharge is indispensable. Nevertheless, there are relatively few tile drainage discharge monitoring stations, and it is difficult to quantify the tile drainage discharge for river basins with different hydrological and soil conditions. Thus information is one of the main missing links for modelling the water and nutrient fluxes both on agricultural and forested lands.
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