Millions of ponds: distribution and significance of small artificial impoundmentsin the conterminous United States. W. H. Renwick, Department of Geography, Miami University; S.V. Smith, Centro de Investigación Científica y de Educación

Superior de Ensenada, Mexico; J.D. Bartley and R.W. Buddemeier, Kansas Geological Survey

How many ponds, and where?We inventoried small water bodies in the conterminous United States, using the USGS National Land Cover Data (NLCD) coverage. This Landsat-derived dataset maps water areas at 30-meter pixel resolution. Our inventory excludes features within 1 km of streams in the National Atlas hydrography layer or within 5 km of major streams in the ESRI coverage as well as any other water features that were clearly identifiable as streams. The resulting data were compared with polygons extracted from the hydrography (blue-line) layer of a sample of 336 1:24,000 USGS topographic quadrangles (DLGs). The DLGs were processed using the same masks applied to the NLCD data. The total number of DLG ponds was estimated by extrapolation from the sampled quadrangles to the conterminous U.S. While the method does not distinguish between natural and artificial water bodies, the distribution makes it clear that the overwhelming majority are of human origin.

Hydrologic significanceSmall ponds tend to have small drainage areas; hence they are located high in the hydrologic cascade. They store water before it enters streams, and as such they may diminish flood pulses. About 25% of all runoff passes through at least one small water body on its way to larger streams.

Because a large number of ponds are located in areas with strong seasonal water deficits, net evaporation from these surfaces can be significant. We define net evaporation from lakes as the added evaporation that takes place as a result of the presence of an open water surface, beyond what would occur from a terrestrial surface. Net evaporation, EN is calculated as follows:

Net Evaporation = Lake Evaporation – (Precipitation – Runoff)

Estimates of total evaporation from all lake and reservoir surfaces are based on areas indicated in the 1992 National Resource Inventory, extrapolated to estimate areas on Federal Lands, is included for comparison.

Impact on biogeochemical cyclingLakes have profound effects on biogeochemical cycling within the hydrologic system. Among these effects are deposition of sediments and substances associated with them.

We selected 10,956 dams from the National Inventory of Dams for with drainage areas exceeding 50 km2, and mapped their drainage divides using a 1-km DEM. We combined these data with information from small ponds to estimate the number of impoundments a drop of runoff would have to pass through to reach the coast. Because there are so many impoundments in the surface-water system, the average drop of water passes through about two impoundments before reaching the coast.

Sedimentation is one example of the biogeochemical impact of lakes for which continent-scale quantitative estimates are possible. We estimated total accumulation of sediment in small ponds by three different methods (0.22 - 1.78 x 106 m3 yr-1). Two of these are based on measured sedimentation rates reported in the RESIS database, which currently includes 3902 sediment surveys from 1771 reservoirs. A third method is based on erosion rates in areas tributary to lakes.

In addition to the millions of small impoundments described here are tens of thousands of larger dams, which themselves trap an estimated 1.67 million m3 of sediment per year.

Dataset Number of water bodies

(thousands)

Total surface

area(1000 km2)

Average area (m2)

Maximum area (m2)

Minimum area (m2)

NLCD 2600 21 7 x 103 2.53 x 107 6.00 x 102

NID* 43 62 1.45 x 107 1.84 x 109 8.00 x 101

USGS DLGs 9000 -- -- -- 2.5 x 101

Parameter Km3 Percent of runoff

Total precipitation 5568 --

Total runoff 1878 --

Total apparent evapotranspiration (P-R) 3690 --

Evaporation from small water bodies 20.5 1.1

Evaporation from NRI water area (Federal lands included by extrapolation) 121

6.4

Net evaporation from small water bodies 8.6 0.46

Net Evaporation from NRI water area (Federal lands included by extrapolation) 56.0

3.0

Method Sedimentation (106m3 yr-1)

Extrapolating from specific sedimentation rates:

Regressions applied to all land using average drainage area = total area / number of impoundments

1.78

Regressions applied to estimated drainage areas on impoundment-by-impoundment basis

0.22

Using erosion occurring on land tributary to impoundments and 80% trap efficiency

0.43

NLCD data: Pontotoc County, Mississippi

NLCD data: Cobb County, Georgia