Hydrological Effects of Forest Wetland Drainage

1990

S. 37-45

4 Abb., 1 Tab., 9 Lit. Ang.

Unselbständiges Werk

200054440

Modern forest drainage in Sweden is changing. Earlier drainage mainly concerned virgin peatlands. Now, drainage concerns forested wetlands and clear-felled areas. In Sweden there are 6.4 Mha peatlands. Of these, non- forest productive lands constitute 4.6 Mha and forested peatlands 1.8 Mha. Forest wetlands also include moist mineral soils amounting to about 5 Mha. Together, the forest wetlands constitute 40% of the entire forest and peatland area. Forest drainage began in the late 19th century. One activity peak was reached in the 1930s and another in the early 1980s. For a few years during these periods more than 50000 ha/year were drained. Today a recent political awakening due to overall environmental impacts also influences forest drainage. A result of this is decreasing drainage activities during the very last years. The drainage activities affect, among other things, the quantity and chemical composition of runoff. Improved hydrological conditions are found but the effect might also be hazardous. Drainage influences runoff, with both increasing and decreasing mean and high discharges, while the low discharges mainly increase. The changes of chemical composition of runoff are affected by the lagg-ditches penetrating the mineral soil beneath the peat. This often results in increased pH, alkalinity and concentrations of cations but also of sulphate. Effects on phosphorus and nitrogen vary with site. The flow of nitrogen mostly increases with accelerating outflows of inorganic fractions with time, at least during the first five years after drainage. Several precautions have to be undertaken in order to reduce the negative environmental impacts. By doing this the favourable effects of drainage on both forestry and environment could be gained without unnecessary hazards.