DIGITALNA ARHIVA ŠUMARSKOG LISTA
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ŠUMARSKI LIST 13/2005 str. 202 <-- 202 --> PDF |
PRESENTATION AT THE INTERNATIONAL SYMPOSIUM Šumarski list - SUPLEMENT (2005), 195-201 APPLICATION OF THE METHOD OF RUNOFF CURVE NUMBERS TO EVALUATE THE EFFECT OF FOREST ON SURFACE RUNOFF Miloslav JANEČEK* SUMMARY: The forest, in general, can be regarded as a very efficient vegetation cover. It enhances the water retention capacity of a catchment and protects the soil against erosion. However, because of the extreme variability of hydrological properties of underlying soils, especially their uppermost layers (the forest litter), the forest may sometimes generate a considerable amount of surface runoff causing, among other impacts, soil erosion. The CN´-method and, in particular, the recently developed HydroCAD model (http://www.hydrocad.net), which is based on this method, can be recommended as tools for estimation of probable runoff from the forest catchments up to 10 km . Key words: forest, retention capacity, surface runoff, hydrological con ditions In general, forest stands are considered as the best soil cover markedly contributing to a reduction of surface runoff and erosion. These effects are generally based on: protection of soil from destructive effects of incident raindrops support of water infiltration into the soil improvement of soil consistence reduction of the transporting power of water and surface runoff accumulation retardation of surface runoff and washed soil. It is to note that not all forest stands fully fulfil this function as indicated by the evaluation of forest stands by the method of curve numbers (CN) (Kent - 1971 et al.). Hydrological conditions of forest stands are mainly related with the density of the cover - vegetation, litter, etc. Naturally, leaves, needles, twigs, bark and other residues of vegetation on the forest soil are decomposed to form litter from which the layer of forest floor originates. Evaluation of the forest floor by the CN method is as follows: * Miloslav Janeček, Research Institute for Soil and Water Conservation Praha, Žabovreska 250, 156 27 Praha 5 - Zbraslav, Czech Republic, phone: 00420 257 921 497 e-mail: janecek@vumop.cz If appropriate cultural practices are applied in the forest, the forest floor is porous and its infiltration and accumulation capacity is high. To determine hydrological conditions of any forest it is necessary to know the hydrological group of soil, litter layer, humus layer and its type. According to the CN method there exist four hydrological groups of soils on the basis of minimum rate of infiltration into the soil without cover after long-term saturation. The class of hydrological conditions is determined according to the layer of litter or forest floor and its compaction. If the litter layer is lower than 1 cm, the forest floor is not considered as protected and the class of hydrological conditions is reduced by the coefficient 0.5. Differences in hydrological conditions are generally expressed for forests by the CN method - see Table 3. The above values of runoff curve numbers describe the potential of runoff from the watershed - see the graph in Fig. 2. The above relationships are based on an assumption that the ratio of runoff volume to the amount of torrential rain equals the ratio of the volume of water intercepted during runoff to potential volume that may be intercepted by the soil and cover. |