Relationship between Curve Number and φ-Index

The curve number (CN) model was originally developed to estimate the total runoff resulting from a given rainfall event. However, the CN model has a limited ability to estimate realistic incremental rainfall excesses within rainfall events. This limitation is problematic because incremental rainfall excesses during a rainfall event are usually the input data required for calculating runoff hydrographs. In contrast to the CN model, the phi-index model provides more realistic estimates of incremental rainfall excesses during a rainfall event. This study investigated the relationship between the curve number and the phi-index at 10 different catchments in the United States. The results show that, for any given rainfall event, the phi-index can be directly related to the curve number, and there is a fair degree of consistency in the CN-phi relationship between catchments.