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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 hydrograph 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 drainedarearunoff 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 timeseries 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.

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