Comparison of AMC-dependent CN-conversion formulae

The available antecedent moisture condition (AMC)-dependent runoff curve number (CN) (SCS, National Engineering Handbook, Supplement A, Section 4, Chapter 10, Soil Conservation Service, USDA, Washington, DC, 1956) conversion formulae due to Sobhani (M.S. Thesis, Utah State University, Logan, UT, 1975), Hawkins et al. (J Irrig Drain Eng, ASCE 111:330-340, 1985), Chow et al. (McGraw-Hill, New York, 1988), and Neitsch et al. (Texas Water Resources Institute, College Station, TX, TWR1 Report TR-191, 2002) were compared utilizing the NEH-4 CN-values (SCS, National Engineering Handbook, Supplement A, Section 4, Chapter 10, Soil Conservation Service, USDA, Washington, DC, 1972) as target values. The Sobhani formula was found to perform the best in CNI-conversion, and the Hawkins formula in CNIII-conversion. When evaluated on a large set of Agriculture Research Service (United States) data, a newly proposed formula performed the best of all, and the Neitsch formula the poorest, and therefore, the former was recommended for field use. The poorest performance of the latter is largely attributed to the occurrence of unreasonable negative CNI-values at low CNII-values.