River Outflow of the Conterminous United States, 1939–1988

Abstract

A record of 50 years of daily outflows through the boundaries of the continental United States has been assembled based on observations recorded by U.S. Geological Survey streamflow stations. Only stations with continuous records from 1939 through 1988 were included. These stations (197 total) are near the outlets of rivers located at the vicinity of the Canadian, Mexican, Atlantic (including the Gulf of Mexico), and Pacific borders of the continental United States. The drainage area of the selected stations covers 77% of the conterminous United States, whereas the existing network of gauging stations covers 83% of the conterminous U.S. area. Station daily data were aggregated over the entire boundary of the United States and were integrated in monthly and annual totals. The 50-year average annual streamflow divergence normalized by the aggregated drainage area is 210.2 mm yr−1 reaching a peak in April with 27.3 mm month−1 and a minimum in September with 8.7 mm month−1. The Mississippi–Missouri Basin comprises 50% of the gauged area and dominates the absolute value of the outflow discharge. Spectral analysis of the monthly outflow anomalies shows an 11-year dominant cycle. The 1939–1988 period contains four notable droughts. Two droughts are partially registered in the limits of the records characterized by the negative anomalies extending from 1939 to 1941 and by the 1987–1988 anomalies for the late 1980s drought. The middle 1950s and early 1960s droughts are fully included in the dataset. Periods of high outflows were registered in the middle 1940s, early 1970s, and early 1980s. Analysis of the spatial coherence of the annual anomalies shows large-scale features, whereas analysis of the monthly anomalies yields the frequency and persistence patterns of floods and droughts. An estimate of the climatological land-surface water budget for the continental United States was done based on recorded precipitation, panevaporation, and outflow. Eigenvector analysis of the monthly outflow residuals per 3° range has been performed to identify the major modes of the spatial correlation structure. The first eight modes explain 66% of the variance of the system and identify the following regions: Atlantic seaboard, Mississippi–Missouri and Ohio River basins, Northeast, Pacific Northwest, Pacific seaboard, Texas Gulf region, North-central, and the Colorado River and Great Basin. Annual and monthly specific outflow aggregates were used to describe the temporal characteristics of the coherent regions. Both time-domain and spectral analyses of the regional outflow anomalies identify the dominant modes of temporal variability.