Clean surface water is largely a result of well-functioning catchment area. Undisturbed soils retain water and prevent overflows and floods and thus erosion. They decrease the release of soil particles, minerals and nutrients to water bodies and maintain better water quality.
Mire drainage, soil preparation and tilling amongst others reduce the ability of the soil to retain water. Mire drainage was most intensive in Finland during a time period from 1950s to 1970s. Mires have been drained for agricultural land in southern and western Finland but more commonly the drainage has been carried out to increase the land area for forestry. At present only 23% of original mire area in southern Finland is still intact. In northern Finland this share is 59%.
In forest sector most of the clear felled areas are mechanically prepared by ploughing, mounding, scarification or harrowing to accelerate forest regeneration. This preparation increases the amount of loose soil particles and nutrients which then leach to water bodies. In addition to this, the amount of water increases in the logging areas because the lack of vegetation decreases the vaporization and the soil ability to absorb water. In latest decades the average annual soil preparation area has been about 120 000 ha, in recent years the area has decreased slightly. In the 2000s lighter preparation methods such as mounding and harrowing have become more common.
Agriculture is the largest source of nitrogen and phosphorous in the water. The reasons for large nutrient discharges are for example excessive use of fertilizers, narrow border strips, intensive tilling and leaving fields without vegetation during winter. The lack of snow cover resulting from warming winters increases the wintertime run-offs in the fields. The amount of annual discharge of nitrogen and phosphorous has remained stable during last twenty years. The phosphorous leakage has been about 2 700 tons per year and that of nitrogen 39 000 tons per year.
About 5.4 million m3 of groundwater is generated in Finland every day. There are 6 020 classified groundwater areas which cover altogether 13 300 km2. More than half of these deposits are important for water supply. Even if there are abundantly groundwater areas in Finland, they are not evenly distributed. The richest groundwater areas are in Lapland, the most meagre in central eastern part of Finland and coastal areas.
Pohjavesialueet. Finnish Environment Institute. [In Finnish]
Finnish Statistical Yearbooks of Forestry. Finnish Forest Research Institute.
National Forest Inventory (NFI). Finnish Forest Research Institute.