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ŠUMARSKI LIST 13/2005 str. 229 <-- 229 --> PDF |
P. Kovär: RISK ASSESSMENT OF EXTREME HYDROLOGICAL SITUATIONS CASE STUDY ... Šumarski list SUPLEMENT (2005). 2I9-22S portancc in water resources management and flood control (Falkenmark, 1999). A case study based on water balance computation has been carried out using data from two small catchments Vseminka and Drevnice in Moravia. The aim of EXPERIMENTALBoth Moravian experimental catchments covering similar areas have suffered at least twice from floods during recent years (1997, 2001). Fig. 1 gives a general view of the situation of the catchments. The shape of the Vseminka catchment is longitudinal, the upper sides of the central valley are forested down from the water divide, and land use on the other parts is diversified. The Drevnice catchment is fan-shaped, highly forested and only its lower part has a more structured land use. The soil characteristics of the catchments are given in Table 1 a, b. The soil groups have been selected in accordance with methods widely used (U.S. SCS, 1986). The land use and physiographic factors are presented in Table 2. The WBCM model that was implemented with the aim of quantifying the soil moisture dynamics of the experimental catchments is a lumped model with probability parameter distribution over the area. It is based on the integrated storage approach. Each storage element represents the natural storage interception, soil surface and active zone. In this version not all the unsaturated and ground water zones were used, however only a recharge to them was considered as an output from the active zone in the form of deep infiltration. The mo- RESULTS ANDThe model has been implemented annually since 1992 to compute the water balance components. Thus the model parameters had already been calibrated. The aim of this paper is to study two pre-flooding periods in July 1997 with reference to soil moisture content in the active zones of both experimental catchments. There were two flood waves over the territory of Eastern Bohemia and Moravia (4/7 to 10/7 and 18/7 to 27/7/ 1997). The major data on these floods is shown in Table 3. Then, the WBCM-5 model was implemented to compute soil moisture content fluctuation in the form of daily moisture deficit values (SMD), as well as the daily antecedent precipitation index (API). The period of June 1 to September 3, 1997, shows how dangerous flooding situation appears when the SMD values drop below 10 mm and even lower and the catchments have no more retention capacity to store next rainfall. In particular, the period 5/7 to 7/8/ 1997 on the Vseminka and Drevnice was the most hazardous. The result was a flood close to 100-year return period, which caused major loss in human life and in property. The second this study, in a broader sense, is to investigate the effect of soil, physiographical factors and land use on water retention capacities in the active zones of the catchments. CATCHMENTS del with daily step input/output rates simulates the following processes (Kovar 1999, Kovar 2000): potential evapotranspiration, interception and throughfall direct runoff active soil moisture zone dynamics Note: Soil moisture content of the whole unsaturated zone, ground water dynamics, base flow and total flow were computed only for the process of parameter calibration. The modified Monteith-Penman method was used for computing potential evapotranspiration. A linear distribution of local interception capacities was in principle used for actual interception and throughfall. For quantifying direct runoff, the US Soil Conservation Service, SCS method based on runoff curve number (CN) assessment was used. The recharge of the active zone and its depletion depend mostly on soil parameters (field capacity FC, porosity POR, hydraulic conductivities, KS), on soil moisture content and on potential evapotranspiration conditions. For this computation, the one-dimensional Richards equation is used in the finite difference form (Kovar, 2001). DISCUSSION part of this research was to select two extreme periods: dry and wet ones of about the same duration. As a dry year with the least precipitation and the highest air temperature in vegetation period (April - October) in the last 20 years the year 1992 was selected in the data record. Similarly, as a wet year was selected the year 1997 with the highest precipitation determined from the data record from the same stations located appropriately for both experimental catchments. However, the next aim of this paper was to select two periods, one typically dry and the other typically wet, both of the duration about 6 to 7 weeks when there was either no rain in dry season or substantial torrential storms combined with regional rainfalls with their high periodicity (p< 0.01, N > 100 years) in the wet season. Thus these daily rain and corresponding daily runoff records were analysed in both characteristic years 1992 and 1997 and for the Vseminka and Drevnice catchments the following periods were adequately selected: Dry period 1992: July 16 to August 31,1992 (47 days) Wet period 1997: June 26 to August 14,1997 (50 days) |