The effect of additional subsurface drainage on water discharge and nutrient load on clay soil

In the southern parts of Finland, 75% of the arable land area has subsurface drainage. Most of the subsurface drainage systems have been installed 40-50 years ago. At present, there is need to improve drainage efficiency of the old drainage systems. The aim of this study is to find out how the additional subsurface drainage affects groundwater table depth, discharge, nutrient load and crop yield. The study has been carried out at the experimental site in Jokioinen, in south-western Finland. The soil is heavy clay and the mean slope is 1%. The drain depth was 1.1 m and gravel was used as envelope material. Drain flow and tillage layer runoff from each field section have been continuously measured since May 2007. Concentrations of total nutrients and suspended solids have been determined from flow weighted composite water samples. Depth of the groundwater table and soil moisture have also been measured. According to the results, additional drainage decrease of groundwater level following snow melt and rainfall events was more rapid than before drainage. Additional drainage increased drain outflow and also nutrient loads from the drains. The results suggest that additional drainage did not affect the nutrient concentrations, apart from the peak in total nitrogen concentration in the first year after additional drainage.