RIVER OUTFLOW OF THE CONTERMINOUS UNITED-STATES, 1939-1988

A record of 50 years of daily outflows through the boundaries of the c ontinental United States has been assembled based on observations reco rded by U.S. Geological Survey streamflow stations. Only stations with continuous records from 1939 through 1988 were included. These statio ns (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 o f the selected stations covers 77% of the conterminous United States, whereas the existing network of gauging stations covers 83% of the con terminous U.S. area. Station daily data were aggregated over the entir e boundary of the United States and were integrated in monthly and ann ual totals. The 50-year average annual streamflow divergence normalize d 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 mon th-1. The Mississippi-Missouri Basin comprises 50% of the gauged area and dominates the absolute value of the outflow discharge. Spectral an alysis of the monthly outflow anomalies shows an 11-year dominant cycl e. The 1939-1988 period contains four notable droughts. Two droughts a re partially registered in the limits of the records characterized by the negative anomalies extending from 1939 to 1941 and by the 1987-198 8 anomalies for the late 1980s drought. The middle 1950s and early 196 0s droughts are fully included in the dataset. Periods of high outflow s were registered in the middle 1940s, early 1970s, and early 1980s. A nalysis of the spatial coherence of the annual anomalies shows large-s cale features, whereas analysis of the monthly anomalies yields the fr equency and persistence patterns of floods and droughts. An estimate o f the climatological land-surface water budget for the continental Uni ted States was done based on recorded precipitation, panevaporation, a nd outflow. Eigenvector analysis of the monthly outflow residuals per 3-degrees range has been performed to identify the major modes of the spatial correlation structure. The first eight modes explain 66% of th e variance of the system and identify the following regions: Atlantic seaboard, Mississippi-Missouri and Ohio River basins, Northeast, Pacif ic Northwest, Pacific seaboard, Texas Gulf region, North-central, and the Colorado River and Great Basin. Annual and monthly specific outflo w aggregates were used to describe the temporal characteristics of the coherent regions. Both time-domain and spectral analyses of the regio nal outflow anomalies identify the dominant modes of temporal variabil ity.