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PRESENTATION AT THE INTERNATIONAL SYMPOSIUM Sumarsk i list - SUPLEMENT (2005). 219-228 RISK ASSESSMENT OF EXTREME HYDROLOGICAL SITUATIONS CASE STUDY OF THE VSEMINKA AND DREVNICE CATCHMENTS, CZECH REPUBLIC Pavel KOVÄR* SUMMARY: Use of water balance models gives an opportunity to identify extremes in the rainfall-runoff processes, namely soil moisture content in the upper soil zone of small head-water catchments can indicate potential floods or drought. These upper catchment zones (0.20-0.60 m) are usually called "active zones" and strongly influence the formation of direct runoff, particularly when their soil moisture content is close to saturation, or lead to drought when the water content is exhausted. Also forest management and good forestry practices play a significant role in rainfall-runoff processes, namely in the water-holding processes of upper catchment layers. This role is even more important on small catchments with highly fluctuating water discharges. The paper shows how a simulation of these hydrological processes can be performed using water balance modelling. The WBCM-5 model (Water Balance Conceptual Model, version 5) running in one-day steps on several experimental catchments in the Czech Republic was implemented for this purpose. The two preflooding periods just before the catastrophic floods in July 1997 are clearly shown. Similarly, periods of drought can also be well identified from the historical records. The trends of "drying" can be found by implementing the model, and they show how sensitive a catchment is in rainless periods. Conceptual soil moisture content is related to the Antecedent Precipitation Index (API). A quantitative study is also made of the impact of land use and management on direct runoff formation and on subsurface storage replenishment. The paper gives the model results for the flood period of July 1997 on two Moravian experimental catchments, Vseminka (48 %forested) and Drevnice (81 % forested), and compares them with those from the Rokytenka catchment (24 % forested) in Eastern Bohemia. It is evident that the depth of the active zone on a catchment has a predominant influence on the degree of flood and/or drought risk. These hydrological extremes can be reduced partially and only to a small extent. However, rational catchment management and well organised land use and forest management based more on the protection function than on production is prerequisite for dangerous hydrological events with catastrophical consequences. Key words: water balance model, antecedent moisture conditions, hydrological extremes, soil moisture deficit, scenario simulation. INTRODUCTION Water regimes of a catchment can be well quantified particular, the upper soil layer in a catchment is extreby an analysis of water balance components with refemely important, as its moisture content predetermines rence to direct runoff and subsurface water recharge. In possible floods or droughts according to its soil moistu re deficit value (Maidment , 1993). Therefore, this * Pavel Koväf, Czech University of Agriculture Prague, layer (usually 0.20 m to 0.50 m in depth) is known as Faculty of Forestry and Environment, Kamycka 129, the active zone. The impact of land use on the water 165 21 Prague 6-Suchdol, Czech Republic E-mail: kovar@nc.czu.cz balance in an active zone is also a matter of great im |