The curve number - CN Method (SCS-USDA) is widely applied for small watersheds in Brazil, although its precision can be questionable, specially for small watersheds of ephemeral flood that are different from watersheds of perennial flood. The objective of this work is to study the initial abstraction of watershed of ephemeral flood and suggest a mathematical model to calculate this parameter which is an important variable of CN-Method. In all 25 events of rainfall-runoff evaluated during the hydrologic year 2002/2003 in order to determine the initial abstraction values for each event and then, the potential storage of watershed soils was evaluated by the CN-Method. The initial abstraction (depth and time) was correlated to hydrologic variables and their influence on this parameter was studied. The mathematical model was adjusted by linear multiple regression with the hydrologic variables as independent variables. The relationship between depth of initial abstraction and potential storage produced results which are quite different from value suggested for CN-Method. The mean value for the constant was 0.0155 against the 0.20 suggested, with the variability being 0 to 0.081, which did not allow a good estimation of the initial abstraction based upon just potential storage.
hydrologic model; CN-method; ephemeral watershed; runoff
