In the 20th
century, tile drainage was installed in large parts of the
agricultural land and now more than half of the study area is
used for agriculture, most of which is drained by ditches and/or
tile drainage (Van der Grift and Griffioen, 2008). Present day
groundwater levels range from about 0.25 to 4.0 m below the
soil surface. About 1/3 of the area has an average groundwater
level that is within 1 m below the soil surface as well as an
average highest groundwater level within 0.5 m below the soil
surface.