Artikkelit kirjoittajalta Erkki Ahti

This study aimed at finding out the effects of maintaining ditch systems by ditch cleaning and complementary ditching on volume growth of drained Scots pine (Pinus sylvestris L.) stands. Volume growth during the first ten post-treatment years was increased by 0.16 m3 ha-1 a-1 after ditch cleaning, 0.36 m3 ha-1 a-1 after complementary ditching, and 0.48 m3 ha-1 a-1 after the combined treatment. The average growth reactions during the second five-year period were considerably higher than during the first one. No drastic reductions in stand growth, however, occurred in the untreated plots during the post-treatment period of ten years.

• Lauhanen, The Finnish Forest Research Institute, Kaironiementie 54, FIN-39700 Parkano, Finland Sähköposti: ei.tietoa@nn.oo
• Ahti, Sähköposti: ei.tietoa@nn.oo

The effects of ditch network maintenance on runoff water quality was studied at 23 sites in different parts of Finland. The study included a calibration period of 1–3 years before maintaining the ditches and six years after. No observations were made during winter. After ditch maintenance, which involves either cleaning of old ditches and/or digging of complementary new ditches, the concentrations of suspended solids in runoff water increased immediately. At sites where the ditches cut into fine-textured subsoil, runoff continued to have increased suspended solids concentrations throughout the whole six-year period. However, if the bottom of the ditches consisted of coarse mineral subsoil or peat, the annual mean concentration of suspended solids returned to pre-treatment levels in 5–6 years. Concentrations of mineral nitrogen, especially NH4-N, increased while the concentration of organic nitrogen decreased after ditch network maintenance. These changes persisted for the whole six-year period. The overall effect of these changes resulted in a slight lowering of total dissolved nitrogen concentrations. With the exception of a few sites, runoff water pH increased after ditch maintenance and remained high during the 6-year period. Concentrations of DOC decreased at all sites after ditch maintenance and was still at a low level after six years. Concentrations of base cations (Ca, Mg, K, Na) increased significantly after ditch maintenance and were still high after six years. High concentrations of Al and Fe immediately after the digging operations were observed in a few sites. Concentration of total dissolved P did not change much and tended to decrease rather than increase.

• Joensuu, Forestry Development Centre Tapio, Soidinkuja 4, FIN-00700 Helsinki, Finland Sähköposti: ei.tietoa@nn.oo
• Ahti, Sähköposti: ei.tietoa@nn.oo
• Vuollekoski, Sähköposti: ei.tietoa@nn.oo

Runoff water from 75 Finnish ditch networks was sampled for chemical analysis in 1990-1992. In total, 2815 samples were analyzed. Higher mean and median concentrations of total dissolved phosphorus were observed than reported earlier. Except for phosphorus, concentration of most elements increased with increasing site fertility. No clear relationship between phosphorus concentration and the site characteristics could be detected. The median concentration of suspended solids in runoff water from old ditch networks was as low as 2.4 mg l-1.

• Joensuu, Forestry Development Centre Tapio, Soidinkuja 4, FIN-00700 Helsinki, Finland Sähköposti: ei.tietoa@nn.oo
• Ahti, Sähköposti: ei.tietoa@nn.oo
• Vuollekoski, Sähköposti: ei.tietoa@nn.oo

Runoff from 16 artificial minibasins was sampled after fertilizer application before snowmelt in March and after it in May. Extremely high concentrations of phosphorus, potassium and ammonium were observed during the snowmelt period in April. Key words: fertilization, leaching, peatland, phosphorus

• Ahti, Finnish Forest Research Institute, Department of Peatland Forestry, P.O. Box 18,01301 Vantaa, Finland Sähköposti: ei.tietoa@nn.oo
• Paarlahti, Sähköposti: ei.tietoa@nn.oo

In a drained area, the average flowing distance to a ditch is directly correlated to the ditch spacing. Accordingly, the flowing time and ditch spacing are directly correlated. Because the natural state is approached as ditch spacing is wide enough it is obvious that surface runoff is speeded up by drainage. If the runoff curve corresponding to a certain hydrograph is known for a drained peatland area with a 40 m ditch spacing, theoretical runoff curves corresponding to the same hydro-graph can be constructed for any alternative spacing (fig. 1). By constructing a runoff curve for a large alternative spacing, for instance 400 m, an estimate of the minimum change caused by drainage can be obtained. During a long spell of rainfall with a constant rain intensity, an equilibrium is achieved in which the surface runoff from a drained strip equals to the amount of precipitation falling on to it in unit time. Theoretically, the time required for achieving the equilibrium is independent of the rain intensity (Mustonen 1963) and directly correlated to the ditch spacing. Consequently, theoretical surface runoff curves corresponding to rainfalls with constant intensity can be constructed if the equilibration time (t ) is known for a drained peatland area. The outlines presented above might be used for estimating the effects of forest drainage on flooding provided that the runoff curves for the points susceptible to flooding are known (fig. 2). The most difficult part of the estimation process is to separate the drained-area-runoff from the totalrunoff curves. Further, it may be labourious to determine an average t value for the drained parts of a catchment area. The model may be critisized because infiltration and evaporation effects have been neglected, and because it is based on rainfalls with a constant intensity. It is probable, however that peak runoff is not much influenced by infiltration end evaporation if rainfalls causing floods are considered. The disturbing fact that the intensity of a rainfall usually varies with time can be partly eliminated by using the runoff distribution curve method (Mustonen 1963), which makes it possible to separate the influences of different rain intensities from a runoff curve. The ideas dealt with in this paper are based on the hydrological experimentation carried out by the Department of Peatland Forestry at the Finnish Forest Research Institute (Huikari 1959, Huikari et.al. 1966, Ravela 1967). The aim of this experimental activity is to understand the physical interdependencies connected with the hydrological influences rather than to determine the magnitude of the influences by direct observations and using time-series statistics. It is obvious that the empirical approach, based on representative experimental fields and decades of field measurements, proves unrealistic if the limited research capacity and the extensive drainage activity carried out in Finland are considered.

• Ahti, Sähköposti: ei.tietoa@nn.oo

• Ahti, Sähköposti: ei.tietoa@nn.oo

• Ahti, Sähköposti: ei.tietoa@nn.oo

• Ahti, Sähköposti: ei.tietoa@nn.oo