THE EFFECTS OF FOREST CLEAR-CUTTING AND SCARIFICATION ON THE WATER-QUALITY OF SMALL BROOKS

The Nurmes-study has monitored water quality and hydrology of brooks in six basins since 1978. All brooks were investigated in their untreated state for five years and two basins throughout the whole study period. The first stage of clear-cutting and forestry drainage was carried out in 1983, followed by scarification (forest ploughing, hummocking and drainage) in the clear-cut areas in 1986. These standard forestry practices were carried out in two of the basins (Murtopuro and Kivipuro) while the third (Liuhapuro) remained untouched. The brooks are naturally acid (pH 4.1-6.3) with low temperature (max. 13.5-degrees-C) and conductivity. High amounts of organic matter and iron are typical, as well as a low average concentration of suspended solids. The reduction of the tree cover caused a significant increase in water temperature. The clear-cutting and subsequent scarification increased the phosphorus (P(tot), PO4-P), nitrogen (N(tot), NO3-N, NH4-N) and iron (Fe) concentrations. The most harmful effects were the noticeable increase in suspended solids, the 3-year mean being 83 tn km-2 a-1, over 200 x that during the pretreatment period (0.4 tn km-2 a-1). After scarification the amounts of dissolved organic matter were diminished. A protective zone along the brook clearly reduced the impacts of clear-cutting and scarification.