Seasonality of monthly runoff over the continental United States: Causality and relations to mean annual and mean monthly distributions of moisture and energy

Abstract

Summary Observed monthly climatology of streamflow over the continental United States showed significant differences from the monthly precipitation climatology. The objective of this study is to provide an overview of the seasonality of streamflow over the continental US and also to understand the processes that control the streamflow seasonality. For this purpose, we employ principal component regression on five predictors – both climatic and land-surface characteristics – that explain the spatial variability in the streamflow seasonality. While the distribution of mean monthly precipitation is uniform throughout the year over most of the eastern United States (except peninsular Florida), mean monthly streamflow exhibits pronounced seasonality with peak runoff occurring during the winter (early spring) over the Southeast (Mid-Atlantic and Northeast) regions. The spatial variability in the seasonality index – the ratio of peak mean monthly value to the annual total – of runoff over the eastern US primarily depends on the covariability between monthly moisture and energy cycles. As the coherence between these two cycles change from negative to positive over the eastern US, increased moisture availability during the summer results in decreased seasonality index. In contrast, over the western US, both precipitation and streamflow exhibit strong seasonality with respective monthly peaks occurring in early and late winter months. Given that the moisture and energy cycles over the west exhibit significant negative correlation, limited energy availability during peak months of precipitation results in peak monthly runoff occurring in the same season as that of precipitation. Thus, the spatial variability in runoff seasonality over the western United States is strongly dependent on the basin aridity and the seasonality index in precipitation. For catchments over the Midwest and peninsular Florida, given the significant positive correlations in moisture and energy cycles, mean monthly runoff peaks occur in the spring and early summer season with the magnitude of streamflow seasonality being dependent on the aridity index and soil moisture holding capacity of the basin.