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P- Kovär: RISK ASSESSMENT OF EXTREME HYDROLOG1CAL SITUATIONS CASE STUDY . Šumarski list SUPLEMENT (2005), 219-228 These periods were further analysed and their daily water balance components reconstructed. For the reconstruction of the selected dry and wet periods were used the WBCM model parameters calibrated earlier for the whole vegetation periods of the years 1992 and 1997 resp. (Kovar etal, 2001). Next to the previous analysis of the situation in 1997 the situation in 1992 was completely different and the active zone was closed to the wilting point. Daily SMD (mm) values are reciprocally to daily antecedent precipitation index values, API (mm). When we compare both experimental catchments, it is evident that the Vseminka catchment is more "water keeping" than the Drevnice catchment in spite of larger forested area on the latter case. The final aim of this work was to simulate the hypothetical situations how both experimental catchments respond to significant rainfall in dry and wet conditions. These rainfalls were those of the design character with the recuiTcnce time N (years): 2, 5, 10, 20, 50, and 100 years.These rainfalls were derived for the Velikovä station from the publication (Samaj, et al,1983): For the sake of brevity, only the 100-year design rain results are given here, in the paper. This scenario- design rain was supposed to fall at the end of dry period, on August 20, 1992 as well as at the end of wet period on July 27, 1997 uniformly distributed over both catchment areas of Vseminka and Drevnice. In dry pe riod 1992 on the Vseminka catchment, the impact of 100-year rain has increased direct runoff from 0.5 mm to 7.3 mm, and on the Drevnice catchment in the same period it varies from 4.0 mm to 32.0 mm. On both catchments this direct runoff increase has no catastrophical character, on the Vseminka catchment it was +6.8 mm, on the Drevnice catchment + 28.0 mm only. However, during the wet period this increase was more significant. On Vseminka was +45. lmm and on Drevnice even +81.3 mm. It means that on the latter case on a shallow-soil catchment with fully saturated active zone, almost 93 % of rainfall can leave the catchment in the form of direct runoff. This could undoubtedly bring a catastrophical event. The increase of subsidy to subsurface storage is also remarkable in all four scenario results. There the existing situation as well as the situation when the design rainfall of 87.1 mm has fallen are presented. Low values soil moisture deficit, SMD (and correspondingly high values of antecedent precipitation index, API) represent the situation when there is no more retention to replenish subsurface storages on the catchment. The situation on the Drevnice catchment is similar, even more sensitive for faster fluctuation because of the shallower active zone. Tables 4 and 5 and also Figures 2 and 3 give results of the comparative water balance component values. Figures representing the Drevnice catchment are similar but not attached to this paper because of its limited extent. CONCLUSIONS The data on hydrometeorological observations, soil, land use and on watershed management were checked for reliability and then spatially processed by ARC/INFO on the catchments of Vseminka and Drevnice. Then, after implementing the WBCM-5 model on that data, the resulting outputs were analysed and the following conclusions can be made: Comparing rainfall-runoff events in July 1997 on the experimental catchments, in spite of the relatively greater percentage of forestation in the Drevnice catchment (81.0 % against 48.2 % on Vseminka), the depth of rainfall (Drevnice about 30 % higher than those on Vseminka) and its intensity are more important factors than land use in a flood consideration. Water balance modelling such as implementing only "upper layers" (i.e. active zones) on catchments can easily quantify daily soil moisture deficit (SMD) values which signalize a degree of water saturation In dry period of the year 1992 was direct runoff from the scenario-design rainfall of 87.1 mm on the experimental catchment Vseminka 13 % and on Drevnice 32 %. In wet period of the year 1997 was direct runoff from this rainfall on Vseminka 52 % and on Drevnice as much as 93 %. A degree of situation expressed either by API or SMD is, besides the design rainfall characteristics its depth, duration and intensity, very substantial in a direct runoff formation process. Changes in subsurface water subsidy are logical, the great recharge improvements in dry periods or both catchments are remarkable, their excess in wet periods is not dangerous as groundwater flow from their subsurface storages is supposed as not too rapid. Small changes of actual evapotranspiration depend on an active zone and whole unsaturated soil water content. These changes are not essential. The WBCM model can be used only for individual water balance components quantification but also for an identification of the potential harm of floods and/or droughts. ACKNOWLEDGEMENT Support from the Scientific Grant Agency of the Mi- lity, retention and accumulation capacities of water in nistry of Education, Czech Republic, Project No. MSM the landscape " is gratefully acknowledged. 414100008 "Possibilities to increase ecological stabi |