Flöden av kväve och fosfor i Forshällaåns avrinningsområde

The objective of this work was to examine the flows of nitrogen and phosphorus in the Forshällaån watershed, which covers an area of 27,4 km 2 and is situated between Uddevalla and Ljungskile in the province of Göteborgs- and Bohuslän, in southwest Sweden, latitude 58°18' N and longitude 11°56' E. 28 % of the watershed is agricultural land, the remaining is forest and some unproductive areas with bare bedrock. The dominant soil types on the agricultural land are sandy loam, silty loam and clay loams, but sand and clay also occur. The forested areas are located on shallow morainic tills in higher areas. Average annual precipitation and runoff are 900 mm and 425 mm respectively. Nitrogen deposition is estimated at 20 kg ha -1 year -1 . For the calculations, results from literature studies, interviews with farmers and estimations of plant nutrient balances on the farms in the watershed and an inventory of the household waste water systems made by the Uddevalla community have been used. Total leaching from the forest area (1960 ha) was estimated to 5.9 Mg N and 0,08 Mg P year -1, viz 3 kg N and 0,04 kg P ha -l year -l , but these figures are fairly approximate. With an increase in N-leaching of 1 kg ha -l year -l the total leaching from forest land would reach 7,9 Mg year -l. The agricultural land (458 ha) contributes with approximately 4.8 Mg N each year, which is 44 %, of the total nitrogen loss. Thus, the agricultural land is the most important source of nitrogen losses. Extensively used agricultural areas (320 ha) contributes with approximately l.3 Mg N year -1. These estimations have been done with good precision. It is difficult to give a precise figure on the contribution of erosion to N and P losses. Estimates of the nutrient losses caused by erosion have been made only for agricultural land with annual crops (200 ha). The estimated losses of P and N are about 0.20 and 0,8 Mg per year, respectively. Phosphorus is lost mainly through household waste water, which contributes with 0.52 Mg per year or 65 % of the total losses. Nitrogen lost from household waste water is calculated to about 1.9 Mg per year. In the forest areas large clear cuts and ruts should be avoided in order to reduce leaching of plant nutrients. In cultivated areas, fertilizing should be adapted to the uptake by tile crops and the manure should be spread over the whole area. Fallow should be avoided. Ploughing should be done as late as possible, preferably in the spring, and in direction perpendicular to the slope. Concerning tile household waste water the best for the water quality would be to use moulder toilets whenever possible.