Metodik för bestämning av åkerdikens status : fallstudie av ett dike i Munktorps socken

Abstract: Agricultural drainage ditches are links between fields, lakes and oceans, having the potential to transport e.g. sediment and nutrients. When in a good condition, drainage ditches can act as sinks for e.g. phosphorus and sediments and prevent eutrophication downstream. The aim of this study was to determine the condition of one agricultural drainage ditch and to come up with suggestions for appropriate measures. The aim was also to determine which of the ditch’s properties that were possible to evaluate visually, which processes they represented as well as which properties required complementary physical analysis. This in order to contribute to the work of developing a method for determining the status/condition of agricultural drainage ditches. Ten sections of the ditch were evaluated visually according to the Madras method. Soil samples were also taken from the fields right next to the ditch and from the middle of the ditch bank from each section. The soil type was also determined. Sediment samples were analysed for texture, micro aggregates, organic material and P-AL. Water samples were analysed for turbidity and different phosphorus fractions. The results from the Madras evaluation showed different conditions of the ditch’s sections where one section was in a bad condition and needed measures. The condition of this section was most likely due to erosion from surface runoff and from bank failure. This was probably caused by unsatisfactory dimensions for today’s larger precipitation. A clean out is recommended as well as starting to use protection zones on the fields next to the ditch section. The Madras method was efficient in determining the ditch’s ability to divert water and could probably be used by farmers. To determine whether the ditch was a source or a sink of phosphorus and sediment physical analysis were required. The results showed that some sections of the ditch may have functioned as sediment sources but that the ditch in general was a sediment sink because the sediment depth was larger upstream than downstream. Whether or not the ditch was a source or sink of phosphorus was more difficult to determine. The phosphorus concentrations in the water were not unusually large. However, in order to decide in the matter analysis of the ditch’s phosphorus retention capacity should also be carried out.