Abstract
This paper investigates a possibility of using asymptotic functions to determine the value of curve number (CN) parameter as a function of rainfall in small agricultural watersheds. It also compares the actually calculated CN with its values provided in the Soil Conservation Service (SCS) National Engineering Handbook Section 4: Hydrology (NEH-4) and Technical Release 20 (TR-20). The analysis showed that empirical CN values presented in the National Engineering Handbook tables differed from the actually observed values. Calculations revealed a strong correlation between the observed CN and precipitation (P). In three of the analyzed watersheds, a typical pattern of the observed CN stabilization during abundant precipitation was perceived. It was found that Model 2, based on a kinetics equation, most effectively described the P-CN relationship. In most cases, the observed CN in the investigated watersheds was similar to the empirical CN, corresponding to average moisture conditions set out by NEH-4. Model 2 also provided the greatest stability of CN at 90% sampled event rainfall.
Highlights
The runoff amount from an ungauged watershed is one of the basic hydrological parameters used in hydraulic engineering design, flood protection, or in the process of modeling of watershed water balance components [1,2,3,4,5,6,7,8]
The lowest precipitation was recorded in the watershed G (51.6 mm), where it accounted for only 5.7% of an average annual precipitation (51.6 mm/898 mm = 5.7%)
The greatest precipitation variability during runoff triggering episodes (Pmax-Pmin) was found in the watershed G and it was 299.6 mm, while the smallest variability was noticed in the watersheds A and D—258.0 mm
Summary
The runoff amount from an ungauged watershed is one of the basic hydrological parameters used in hydraulic engineering design, flood protection, or in the process of modeling of watershed water balance components [1,2,3,4,5,6,7,8]. Storm water runoff from small agricultural watersheds was estimated using the Soil Conservation Service (SCS) curve number (CN) method, developed by the United State Department of Agriculture (USDA). This method is currently known as the Natural. Its scope has expanded beyond the evaluation of storm runoff and it has become an integral part of more complex, long-term simulation models [9] This method represents an event-based lumped conceptual approach [10,11,12,13,14,15,16]. A primary reason for its wide applicability and acceptability is the fact that it accounts for major runoff generating watershed characteristics, namely soil type, land use/treatment, surface conditions and antecedent moisture conditions (AMC)[4,25,26,27,28,29]
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