Runoff Curve Number (CN model) Evaluation Under Tropical Conditions

In tropical countries the Curve Number method (CN) of the Soil Conservation Service (SCS) is widely used in civil engineering (to calculate drainage infrastructure) as well as in agricultural and environmental studies. However, little is known about the accuracy of CN method in tropical watersheds. To reveal the CN method accuracy, this study compares the CN method results with field data measured in an Andean micro watershed. For this, CN values for the tropical micro watershed "La Vega" were identified using the tables proposed by the SCS and its respective moisture and slope corrections (CNAMC2S); subsequently CN values were derived from 55 events (CNQ) in which runoff and rainfall were measured. It was observed a different of 27% between CNQ and CNAMC2S (CNQ = 80, CNAMC2S 58.1). According to that, the data shows that in the tropical micro watershed evaluated in this study, the SCS method overestimated runoff. The CN model of the SCS is strongly influenced by the antecedent humidity and its impact becomes stronger after high rainfalls records; however, the antecedent humidity influence was not observed in measured runoff data. Despite the CN – SCS model did not present a relationship between CN and precipitation, this relationship was observed in measure data. This study shows that CN method has some inaccuracies and it requires further studies to know its applicability in tropical conditions.