Eero Pelkonen

Padotuksen vaikutuksesta pohjavesipinnan syvyyteen ja metsäojien kuntoon

Pelkonen E. Padotuksen vaikutuksesta pohjavesipinnan syvyyteen ja metsäojien kuntoon.

English title: Effect of damming on water table depth and ditch condition

Tiivistelmä

Since 1969 the effect, as well as the need and practical possibilities, of runoff regulation in peatland areas drained for forestry have been studied experimentally by the author (Pelkonen 1975, 1976, 1979). The drainage requirements of tree stands growing on peatlands have been shown to vary considerably during the growing season. When the water table was kept close to the soil surface during spring and early summer by damming (Fig. 1), no harmful effects were noticed. Instead, quite opposite tendencies were detected during dry and warm summers. As the project is now being expanded, some further aspects of damming are dealt with. Water consumption by trees in June usually exceeds the water input as rainfall. Hence the water table should be high. Owing to the greater oxygen requirements of the roots as well as the higher probability of high-rainfall periods in late summer and autumn, the water table should be deeper than in spring and early summer (Fig. 2). In 1976, a 15-ha peatland area was drained using 110-cm deep open ditches with a spacing of 20 m between the ditches. This is a rather effective combination compared with Finnish drainage norms. Peat dams fitted with plastic discharge tubes were constructed in part of the ditches (Fig. 3). The water level in the ditch was raised by plugging the inlet end of the plastic tubes in late autumn. By keeping the inlets closed untill the end of June, the water table was maintained at depth 15—20 cm above that in the undammed strips during early summer (Fig. 4). In spite of practically constant rain fall during July and August, damming did not prevent the watertable from dropping sufficiently during late summer. In order to obtain the maximum effect from damming, the ditch network described above was planned so as to have as small a slope as possible. During damming, the ditches were almost completely filled with water. As a consequence, no vegetation appeared in the ditches during the first four years (Fig. 7 and 8). In the undammed ditches the development of vegetation was fast (Fig. 5 and 6). Owing to the slight slope of the ditch network, no direct conclusions regarding the positive effect of damming on the deterioration of open ditches can be drawn. It is well known, however, that the problem of ditch deterioration is most pronounced in areas characterized by insufficient slope. As the construction of regulated ditch networks is easily carried out in such areas, the positive effect on ditch condition might prove one of the most important arguments when considering their practical use.

Vastaanotettu 31.10.2017 Julkaistu 1.1.1980

Katselukerrat 1130

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