RUNOFF CURVE NUMBERS DETERMINED FOR A SMALL GAUGED WATERSHED WITH MODERATELY PERMEABLE ARGIUDOLL SOILS

Abstract

The runoff associated with water erosion often determines an environmental cost due to the loss of water, soil, nutrients, and organic matter from agricultural land as well as the transport of pollutants to water bodies. A widely used procedure for estimating surface runoff in small watersheds is the curve number method (CN) (USDA) which is used in various hydrological models, water erosion estimation and contaminant generation and transport models. Due to its empirical nature, it is necessary to have calibrated values for the local conditions in which it is applied. We registered 20 continuous months of runoff measurements in a small agricultural gauged watershed dominated by the Arroyo Dulce series under continuous no tillage of which there was no prior hydrological information. The period analyzed was characterized by the occurrence of 83 rains some of them very intense with a return period of 10 years and there were 13 runoff events that allowed to contrast the hypotheses raised. On the one hand, a trend of standard hydrological behavior was corroborated as mentioned by Hawkins and other researchers, associated with a "hortonian" runoff response. Moreover, the CNs obtained were consistent with the intrinsic properties of the dominant soil, the Arroyo Dulce series, a deep typical Argiudoll with moderate permeability. CN tended to values between 60 and 70 for extraordinary rainfall and the median was located at CN-71 with extreme values between 63 (dry soil) and 87 (wet soil). Comparison with the results previously obtained in a small, gauged watershed with vertic Argiudoles soils of the Ramallo Series (median CN 82), made it possible to clearly separate both series.