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Number, size, distribution, and hydrologic role of small impoundments in Alabama

A small impoundment (SI) inventory was made by county for Alabama by using Landsat 5 TM satellite imagery from the winter of 2007 to enumerate and measure surface areas of water bodies. The result was identification of 278,787 SIs >0.18 ha (0.44 ac) and <2,000 ha (4,972 ac) in surface area with a combined water surface area of 261,880 ha (647,105 ac). The average surface area of SIs was 0.94 ha (2.32 ac)—84.8% were <1 ha (2.5 ac), and 92.7% were <2 ha (4.94 ac) in area. Ground-truthing in one county revealed that the procedure had an accuracy of 80% in identifying SIs. The density of SIs tended to increase slightly in counties of higher population density. Some physiographic provinces had greater density of SIs than others; however, the average surface area of individual ponds did not differ greatly among provinces. The total volume of SIs was estimated to be ≈ 6 km3 (4,900,000 ac ft)—roughly 8.1% of annual runoff for the state. After initial filling by direct precipitation and runoff, SIs are seldom drained, and they overflow after periods of considerable rainfall minimizing retention of runoff. It was estimated that SIs in Alabama lessen annual runoff by about 0.35 cm y−1 (0.14 in yr−1)—a reduction of about 0.6%—mainly because evaporation from their surfaces exceeds evapotranspiration loss for an equal land area. Nevertheless, SIs probably flatten peaks of downstream hydrographs. It should be possible to use water from SIs to supplement existing water supplies in certain localities, and SIs create open-water, shoreline, and wetland habitat as well as detain and improve the quality of surface runoff.