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Although data have been available on the world's largest and most spectacular lakes since the early 1900s, global estimates of the abundance and size distribution have underestimated the abundance and area of small natural lakes. When appropriate GIS

Figure 8 Relationship between the surface area of farm ponds and the annual average precipitation in several political units (after Downing and coworkers). The line is a least squares regression (r2 = 0.80, n = 13) where the area of farm ponds expressed as a percentage of the area of farm land (FP) rises with annual average precipitation (P; mm) as FP = 0.019 e0 0036P.

400 600 800 1000 1200 1400 1600 Annual average precipitation (mm)

Figure 8 Relationship between the surface area of farm ponds and the annual average precipitation in several political units (after Downing and coworkers). The line is a least squares regression (r2 = 0.80, n = 13) where the area of farm ponds expressed as a percentage of the area of farm land (FP) rises with annual average precipitation (P; mm) as FP = 0.019 e0 0036P.

methods and mathematical models are used, the area of land surface covered by lakes has been found to be 2.8% of the terrestrial surface area of the Earth, nearly double some historical estimates. There are 304 million natural lakes in the world that cover 4.2 million km2. More than 207 million lakes are <0.01 km2 in individual area and the area covered by lakes of different sizes is greater for small lake size-classes than for large ones. Inventories of natural lakes consider only lakes with permanent open water, including lakes, ponds, and bogs, but other systems, including flood lakes, temporary water bodies, and wetlands, would add substantially to the area occupied by continental aquatic systems. The land area covered by constructed lakes and impoundments is smaller than that of natural lakes but 77 million world impoundments cover 335 000 km2 or about

0.22% of the land surface. The area covered by small, low-tech farm ponds is about the same as that covered by the world's three largest impoundments. Impoundment area is generally underestimated, however, by poor availability of data on small and moderately sized basins and the number of such low-tech structures is increasing rapidly where agricultural needs are great.

See also: Aquatic Ecosystem Services; Geomorphology of Lake Basins; Mixing Dynamics in Lakes Across Climatic Zones; Origins of Types of Lake Basins.

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