Flood-generating mechanisms and trends in the Western United States

Reliably predicting how flood frequency and intensity will change in the coming decades is essential to prevent loss of life, property damage, and future water resource management. Increases in annual maximum precipitation is predicted by climate models, and has been confirmed in observations. However, increases in precipitation extremes has not been shown to be leading to increased flood frequency and intensity, suggesting complexities in the transformation of extreme precipitation to floods that are not well understood. We investigate flood mechanisms in 120 carefully quality-controlled streamflow records that are only minimally affected by upstream anthropogenic activities for the Western United States west of the Continental Divide for the period 1960-2015. We identified six possible flood generating mechanisms consisting of combinations of four atmospheric and two terrestrial mechanisms. The atmospheric mechanisms are winter frontal storms, convective storms, atmospheric rivers, and monsoonal and tropical storms, while the terrestrial mechanisms are radiation-driven snowmelt and rain-on-snow driven snowmelt. We also investigate the role of antecedent soil moisture, and its influence on flooding across the catchments. We address the questions (1) how do these flood mechanisms vary spatially across the Western United States, (2) have there been any shifts in contributing flood generating mechanisms on a catchment by catchment basis over our period of analysis, and finally (3) what is the link between flood generating mechanism shifts and flood trends? By analyzing the spatial-temporal change in flood-generating mechanisms, we are better able to understand the influence that these mechanisms have on both the frequency and intensity of floods in order to improve the prediction of flood trends.