Reglerad dränering

About 45% of the anthropogenic nitrogen loads from Sweden reaching the seas come from arable land. Within the work of the national environmental quality objective "No eutrophication", the Swedish government determined an environmental goal of reducing the nitrogen loads from arable land to the seas south of the sea of Åland by at least 50% compared with the level year 1985 level (to 38500 ton N year⁻¹). This goal should be reached by year 2020. The present programme contains legislation, extension service, economical management, research and development work and monitoring. Research and development work included in the programme are both aiming to promote technical development mainly in the domain of management of manure and growing catch crops. Despite the progress in reducing amount of available N for leaching, the variability in N loads between years remains, mainly due to the great variability in annual runoff. Controlled drainage has the potential to moderate this variability in annual runoff and to a certain degree control the timing of the outflow and even further reduce the amount of available N for leaching. Field experiments conducted in the South of Sweden between 1996 and 2001, show that controlled drainage has a great potential to regulate unnecessary drainage outflow from cropland and subsequently reduce nutrients leaching. However, the applicability of the technique in large scale in Sweden has not yet been evaluated which is the task of this project. The aim of the project was to assess the potential of arable land in the coastal areas of Southern Sweden suitable for controlled drainage. Some topographical and agro hydrological site conditions need to be present to en sure feasible and practical management of a controlled drainage system. The following six conditions indicate if a site is suitable for controlled drainage or not: the site needs to have drainage demand under natural conditions, level topography (slope 2%. Soil maps at a scale of 1:50000, 1:100000 and 1:1 000000 were used to classify the soil texture after assumed hydraulic conductivity into three classes; high, medium and low hydraulic conductivity. The depth of observations was 0.5 m. In order to identify land use, maps at a scale of 1:50 000 were used. After the data had been reclassified with the geographic information system software Arc info 8.2, the information was merged. The land identified as cultivated land was then further divided into five classes, classes l to 4 and other cultivated land, with decreasing suitability for controlled drainage. More than 720000 ha of cultivated land were included in the study of which 21 % was classified as highly suitable, 13% suitable regarding topography and hydraulic conductivity and 6% with some suitability. Since not all the land require drainage the total number of 289 000 ha with suitability will after more detailed investigations be reduced. Additional data will be brought into the evaluation, such as existing information on the drainage situation of the land. Furthermore, validation of the result will be performed by field survey in small catchments