Vattensänkningar
en analys av orsaker och effekter
Physical geographyGeomorphologyWater levelsLakesScaniaGeographical information systemsPedologyCartographyClimatologyNaturgeografiGeomorfologiMarkläraKartografiKlimatologiEarth and Environmental Sciences
There are records of lowering of the water level in a lot of the Swedish lakes from the middleof the 1700th century. The aim was gaining more areable land to meet the needs of more foodto support the expanding population. In areas with top soil which contents of clay or loam asin the southwest of Scania in southern Sweden this was proven to be a successful wayexpanding the total outcome of the harvest.The studied area in the northeast of Scania, though, consists of morain with a lot of mires -much less nutritious conditions. When the water level is lowered the atmosphere gets accessto the peat. The dissolving of the organic matter that has been suppressed by the lack ofoxygene below water level continues. This means that the ground itself will sink and (i)approaches the ground water level, and (ii) reduces the essential difference in water level inthe lowered area and the drainage canals as well.The farmers were often shocked by the fact that maintaining the ground dry demanded somuch, not only in hard work but financially as well. Economic reasons made failures ofseveral lowering projects, with omission of the maintenance of the ditches that were supposedto drain the lowered area. Summarily, those projects made several hectars of land not areablebut indeed useless for any kind of cultivating. Ditching goes on in Sweden nowadays as well.Modern draining is due to facilitate the growth for newly planted saplings of spruce or pine informer areable areas or in entirely felled forest areas.This paper contains a description of the reasons for the lowering projects, and ditching,based on a question from the local environmental authorities in the county of Osby, Scania, inSweden: What has happened with the drainage system up- and downstream the lake ofGrässjön, which sits on the border between the counties of Osby and Östra Göinge, duringthe lowering projects and the further ditching in the area?Two projects were completed in the area, in the 1880?s and the 1900?s. Grässjön was a partof a larger lake, Wielången. Today, in the beginning of the 21st century, Grässjön is alsodisappearing due to contribution of high rates of nitrogene from a sewage treatment worksupstreams, northwards near Lönsboda village. The county authorities need to be informed offormer actions in the area to restore the lake as the bird paradise it once was.A variability study of the ditches, rivers and lakes is also made based on paper maps from1869, 1934, 1974 and 1997, in showing the changes that have been done. The length of thestreams doubled between 1869 and 1975, but then shortened by approximately one third until1997. The number of lakes seems to follow the same rules, with a peak in 1975, but for thesum of the areas the peak was 1869 and declined as time went by. The change was mostdramatic in the Ekeshult part of the Skräbe river system. The wet areas became fewer overtime in the whole area. Concerning the wet areas they had declined from 1869 until 1975, buthad grown until 1997. The growth is explained by approximately one sixth of lake areas 1975that had changed category to wet areas on the following map.