Markavvattningsföretagens geografi och konsekvenser i Trosaåns avrinningsområde
Agricultural drainage is widely used to manage the amount of water in the ground in order to improve conditions for cultivation. However the effects of drainage companies on water regimes are considerable leading to runoff changes such as altered stream flows, peak flows and water levels. In a greater extent this could imply an increased risk for floods along affected watercourses. This study analyzes the extension of agricultural drainage within the drainage basin of Trosaån in Sweden by ArcGIS mapping. In a statistical analysis, historical variation in streamflow in relation to mean precipitation and mean temperature are investigated for the study area. From a perspective of climate changes and present water rights court rules for Trosaån, consequences of agricultural drainage are discussed in relation to flood risk within the study area. Results indicate an increase in stream flow between 1961-2010 and in the extension of agriculture land affected by drainage between 1940-1989. There has also been increasing trends for mean precipitation and temperature 1961-2010 leading to uncertainties of the influence of each individual variable on stream flow. No implications of present water rights court rules on the flood risk were discovered in this study. However, further research on the effects of drainage on stream flow is important for future flood risk planning since drainage obviously affects natural runoff patterns.Keywords: Drainage company, drainage density, runoff patterns, streamflow, flood risk, climate change, water rights court rules, ArcGIS