Andrzej Wałęga, Dariusz Młyński and Katarzyna Wachulec
The aim of the study was to assess the applicability of asymptotic functions for determining the value of CN parameter as a function of precipitation depth in mountain and upland catchments. The analyses were carried out in two catchments: the Rudawa, left tributary of the Vistula, and the Kamienica, right tributary of the Dunajec. The input material included data on precipitation and flows for a multi-year period 1980–2012, obtained from IMGW PIB in Warsaw. Two models were used to determine empirical values of CNobs parameter as a function of precipitation depth: standard Hawkins model and 2-CN model allowing for a heterogeneous nature of a catchment area.
The study analyses confirmed that asymptotic functions properly described P-CNobs relationship for the entire range of precipitation variability. In the case of high rainfalls, CNobs remained above or below the commonly accepted average antecedent moisture conditions AMCII. The study calculations indicated that the runoff amount calculated according to the original SCS-CN method might be underestimated, and this could adversely affect the values of design flows required for the design of hydraulic engineering projects. In catchments with heterogeneous land cover, the results of CNobs were more accurate when 2-CN model was used instead of the standard Hawkins model. 2-CN model is more precise in accounting for differences in runoff formation depending on retention capacity of the substrate. It was also demonstrated that the commonly accepted initial abstraction coefficient λ = 0.20 yielded too big initial loss of precipitation in the analyzed catchments and, therefore, the computed direct runoff was underestimated. The best results were obtained for λ = 0.05.
The selected SCS-CN method is used worldwide and adapted to the conditions of Slovakia. GIS environment provides an opportunity to simulate changes in land use, and then to calculate the total volume of water from the river and peak water flow in the river bed of the stream. The simulation was done for two rainfall events, 72 mm and 42.6 mm, which were measured in precipitation station in Jelenec (a village situated next to the area of interest). The calculation was made for 4 possible scenarios - current land use, forest, arable land and grassland and pasture. Culmination discharge and time of outflow from rainfall 72 mm for current land use were calculated using the NRCS method. The calculation of water runoff volume showed that similar values were measured for the rainfall of 72 mm and rainfall 42.6 mm in case of AMC-III. The highest values of water runoff volume were marked in the case of arable land in all calculations, the lowest one for forest. Comparison of designed stream cross section and calculated culmination discharge allowed us to determine the point of outflow from the river bed of the Drevenica stream.
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