Normalized Antecedent Precipitation Index

Abstract

Antecedent precipitation too often remains a subjectively determined and arbitrarily implemented parameter in rainfall-runoff modeling. Antecedent moisture can account for a several-fold difference in runoff from the same storm in an otherwise identical watershed. The antecedent precipitation index and the Natural Resource Conservation Service's antecedent moisture condition triad are conventional expressions of antecedent precipitation. Unfortunately, neither index consistently characterizes the runoff consequence of watershed moisture preceding a rainfall event. A normalized antecedent precipitation index is proposed that modifies the conventional index in three aspects: inclusion of antecedent precipitation earlier in the day of the event, normalization to the station mean, and normalization to the antecedent series length. The proposed index, based on daily rainfall record, is theoretically simple and mathematically continuous. Initial results show the proposed index to outperform single curve number-based results, even when the curve number is fit to historic rainfall-runoff records. The proposed index enhances delineation of the antecedent moisture condition for estimates based on that parameter and provides a basis for runoff estimates where the initial watershed moisture condition is probabilistic.