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A Time-lapse Analysis Of Drought In North America

A time-lapse sequence of daily streamflow maps covering the 48 conterminous United States, Alaska and Southern and Western Canada has been developed on video tape, using mean daily stream discharge for the 1930 through 1988 period. Maps of daily streamflow were generated using 226 Water Resources Subregions defined by the United States Geological Survey's Hydrologic Unit Map series and the Canadian Department of Energy and Mines Drainage Index Map Series. Each subregion was represented by a gaged drainage basin, which was assumed to have the same unit flow as the entire subregion. Flow conditions represented on the daily maps were shown in 3 colors, varying from the darkest blue for high flows, with lighter shades of blue as flows approach normal, which are white, then shading from the lightest to darkest red for low flows. This time-lapse display provides a means for rapidly analyzing extensive spatial and temporal streamflow data and gaining insights into national and regional patterns, which is not possible by traditional methods. For example, study of these sequences may suggest mechanisms that drive large-scale weather phenomena such as droughts. Based on these observations, an analysis was made of North American droughts since 1930, as defined by streamflow in the lowest uartile (hydrological droughts). The drought of 1934 was by Far the most severe annual drought in this 59 year period with just under half of the continent experiencing drought conditions that year. In terms of areal coverage of the North American continent, the drought of 1988 was not as severe as the one that occurred from November 1939 through January 1940. However, the month of June 1988 had the fourth largest drought-affected area out of the 708 months ranked in the 59 year period. The 1988 drought appears to be the the result of the coalescence of two individually formed droughts - one in the Southeast, the other in the upper Midwest.