Evaluation and validity of the antecedent moisture condition (AMC) of Natural Resources Conservation Service-Curve Number (NRCS-CN) procedure in undeveloped arid basins

The available conversion formulae for AMC-CN are estimated in an environment that is not arid. The validity of these formulae in arid regions is questionable. Therefore, in the current study, a focus is made on studying the validity of the NRCS-CN theory, its AMC, formulas, and derivation of conversion formulae of CN for some arid basins in the southwest Saudi Arabia (SA). One hundred sixty-one rainfall-runoff events have been collected over a period of 4 years on 19 sub-basins in the study area. The results show that in arid regions the AMCI antecedent moisture condition at (dry) conditions is dominant. It has been shown that the abstraction ratio, λ = 0.01, is the best to fit the NRCS-CN theory when compared with λ = 0.2 or 0.3. The observed and estimated runoff show that the root mean square error (RMSE) is the lowest for λ = 0.01 (1.8 mm, 3.69 mm, and 1.36 mm) when compared with λ = 0.2 (4.21 mm, 12.13 mm, and 3.15 mm) and λ = 0.3 (5.74 mm, 13.92 mm, and 3.1 mm) for CNI (CN at dry condition), CNII (CN at normal condition), and CNIII (CN at wet condition) respectively. The upper and lower limits of λ, in SA, are 0.02 (90% probability) and 0.00001 (10% probability) respectively. The runoff coefficient is less than 0.4. This is due to high transmission losses which is typical in arid regions. The study recommends using λ = 0.01 and the developed AMC-CN formulae for enhanced runoff predictions in SA.