DIGITALNA ARHIVA ŠUMARSKOG LISTA
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ŠUMARSKI LIST 13/2005 str. 40 <-- 40 --> PDF |
P. Kantor: POSSIBILITIES OF MOUNTAIN FORESTS IN REDUCING HIGH WATERS AND FLOODS Šumarski list SUPLEMENT (2005) -39 After the fall on the forest soil precipitation water largely infiltrates into it. In addition to the saturation of soil pores and subsequent seepage the infiltrated water gets through the system of water paths to a parent rock (ducts in the forest soil created by roots of trees, animals etc.), runs down and accumulates in the parent rock hollows and depressions. If the parent rock stratum is impermeable the infiltrated water changes into the subsurface runoff. If the parent rock stratum is of the fissure type the water permeates to groundwater participating in underground runoff. And just here, there is a substantial difference in the water-management effectiveness of forests as compared with agricultural soils the infiltration capacity of which being usually markedly lower. Also during rainstorms inducing floods in summer 1997, 1998 and in August 2002, surface runoff in spruce and beech was not decisive (Sach, Kantor, Černohous 2000). And once again, it is necessary to mention that in the given case it refers to ordinarily managed forest stands. Increased surface runoff (although generally not dangerous) is usually noticed only during spring snowmelt in unmixed beech stands where the compact layer of leaves shows lower infiltration capacity than spruce litter. Moreover, the intensity of thawing in leafless beech stands under sunny days is even 30 % higher than in closed spruce stands. Under the usual regime of precipitation, water in soil is drawn by roots of trees to ensure their physiological processes (transpiration). Intensity of transpiration in broadleaves is usually 2 to 5 times higher than in coniferous stands. With respect to markedly higher values of the biomass of spruce stand needles as against the foliage biomass in beech stands differences between transpiration of conifers and broadleaves are usually not marked. This very important finding was corroborated particularly by German studies already 30 to 40 years ago (L a d e f o g e d 1963, M i t s c h e r lieh 1971, Schmaltz 1969). Similarly, mature stands of spruce and beech in the Orlicke hory Mts. consumed for this form of evaporation virtually the same amount of precipitation, viz on average 180 to 200 mm per year. Finally, the certain part of soil moisture is evaporated right from the soil surface - in our case it referred to about 80 mm per year in both types of stands. Data on the water regime of spruce and beech stands on balance plots in the Orlicke hory Mts. are given in Table 1. Thus, it is evident that forest ecosystems are the considerable consumer of precipitation water. Not only findings from the Orlicke hory Mts. but also all available published sources have corroborated substantially higher consumption of water by spruce stands as compared with beech stands - see Table 2. Markedly higher interception in a spruce stand is unambiguously the main cause of these differences. From the viewpoint of reducing high waters and floods the maximum water capacity of forest soils is of exceptional importance. It is the highest amount of water which can be retained by soil. In our actual case in the Orlicke hory Mts. (light-textured sandy-loam up to loamy-sandy Cambisol with a 50 % admixture of soil skeleton), its value amounted to about 270 mm at the soil depth of 70 cm. In the course of summer months, the actual water content in the soil of both stands ranged between 170 and 190 mm not decreasing below 150 mm in the period without precipitation. In the given case, the forest soil of mountain spruce and beech stands was able to retain and accumulate 40 to 60 mm precipitation water, maximally 100 mm. In the moment of achieving full water capacity, the soil can be compared to a sponge fully saturated by water which is not able to retain a millimeter of other precipitation. CONCLUSION Based on the long-time series of experimental studies not only in the Orlickć hory Mts. but also in the Beskids and through the analysis of a number of foreign studies it is possible the following basic findings to be regarded as proved: 1. Surface runoff and subsequent soil erosion are quite negligible in forest stands. This fact refers not only to natural forests but also commercial forests. Even on clear-felled areas, erosion processes (with the exception of boulder localities) are not the result of mere felling the trees but they are always a manifestation of the poorly organised use and movement of heavy means of mechanization and other activities of man. The absence of surface runoff in the forest (the runoff being also eliminated there by the system of water paths created by roots of trees, animals etc. in the forest soil) as against its often occurrence on agricultural land is thus the first and very important condition for the reduction of floods in the landscape. As a textbook example, it is possible to mention a local flood of 15 July 2002 in the Hodoninka river watershed (district of Blansko). In virtually forest-free parts of the cadaster of Crhov and Olešnice, in the course of 2 hours some 100 to 170 mm precipitation fell which immediately flew out on the surface of agricultural land into watercourses causing many-million damage to property and unfortunately even the loss of two human beings. |