Assessment of antecedent moisture condition on flood frequency: An experimental study in Napa River Basin, CA

Study region: This study region is the Napa River basin in California whose antecedent soil moisture states and precipitation magnitudes are primary drivers to occur extreme floods. Study focus: This study assessed the influence of antecedent moisture condition on flood frequency, based on an experimental application scheme and pre-processing. For this purpose, T-year flood simulations were conducted using a distributed hydrologic model. Distributed precipitation patterns which have an amount of precipitation corresponding to a specific T-year return period were generated by representative radar-based precipitation fields and precipitation frequency analysis. Dry, normal, and wet of antecedent moisture condition were applied to each T-year flood simulation to reflect variable initial soil moisture states. New hydrological insights for the region: The relationship among flood frequency, antecedent moisture condition, and precipitation frequency was derived for a specific target storm event. For normal antecedent moisture states, the relation showed that T-year precipitation could generate floods having return intervals nearly identical to those derived using gage records. For saturated soil conditions, a 7-year precipitation event could trigger a 100-year flood. Conversely, a 200-year precipitation event with dry soil conditions might generate only a 15-year flood event. The results emphasize the importance of soil moisture to flood runoff and suggest that soil-moisture monitoring could aid in improving flood forecasting.