Impacts of forest drainage on floods

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Abstract
Forest drainage networks are open ditches excavated for lowering the groundwater level in wetland areas to improve growing conditions for the trees. This procedure creates longer discharge surfaces for input to groundwater and shorter pathways for groundwater and overland flow to the channel system, which in turn, changes the basic runoff processes in the catchment. The hydrological effects of forest drainage have been the subject of studies for many years but the conclusions reached are conflicting because of the diversity of conditions as well as the constraints of data availability and the limited accuracy of the methods applied. This study applied three techniques (the control basin method, as well as conceptual and distributed models) for examining whether or not forest drainage increased peak flows. Small basins were investigated, and estimated parameters were transferred to a larger basin. This paper briefly presents the case study areas, the research concept and the methods applied. The computations showed that the peak flows decreased in general after drainage works. According to all three methods, the effect on the peak flows was larger after dry periods. Event simulations made by the Système Hydrologique Européen (SHE) model suggested that the highest peak flows could be increased by drainage in cases of intensive rainstorms in catchments with already high (close to the soil surface) groundwater level. The general tendency of the drainage effects was that a lowered groundwater level had greater influence on peak flow formation than the increased channel conveyance capacity in the drained catchments.