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.