A national floodplain map

TO show graphically what proportion of land area is in floodplain might be useful in studying land use relative to the flood hazard or in comparing experiences among areas with flood problems. Data on land subject to inundation commonly are aggregated by states or large drainage basins. Until now there has been no map showing the spatial distribution of land in floodplain. The map herewith represents a first approximation of these spatial differences. Obtaining floodplain data The few estimates for the United States of the land area in floodplain differ by as much as 25 percent. There are several reasons for the discrepancy among estimates: the difficulty of determining the area actually subject to inundation; double counting of some areas while omitting others; use of inconsistent criteria, such as a two percent chance of occurrence (50-year) versus a one percent (100-year) flood definition; and the natural variability associated with the methods of statistical sampling on which the estimates are based. As part of the U.S. Department of Agriculture's National Inventory of Soil and Water Conservation Needs, which was conducted from 1959 through 1962, estimates of land area in floodplain were made for 12,711 watersheds …