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<br />ijlll538
<br />may be estimated from maps given by the Corps of Engineers (1958).
<br />Wide differences in annual runoff are seen. In some areas 50 percent
<br />of the years has less than half average runoff. In others, not 1 year in
<br />10 has less than half average runoff. In general, variability in supply
<br />increases as one goes from north to south in the region.
<br />
<br />An inventory of stock ponds, numbers, sizes, and distribution
<br />within river basins and by States is badly needed in apprais ing the pres-
<br />ent impact on the water rights and the probable future development of
<br />stock ponds. Inthe absence of such an inventory, the information here
<br />given must be considered tentative and subject to correction and ampli-
<br />fication.
<br />
<br />Effects of Stock Ponds
<br />
<br />Annual water yield
<br />
<br />A stock pond may have beneficial or detrimental effects on water
<br />yields incidental to its primary purpos es. The effects of stock ponds on
<br />streamflow at any particular downstream location are variable due to the
<br />variability of climatic and hydrologic factors which caus e precipitation
<br />and its attendant runoff. In orderto make an expression of downstream
<br />effects meaningful, a complete description of the particular downstream
<br />location in terms of drainage area and/or downstream distance from a
<br />structure of a system of structures must: be given. Benefits, particularly
<br />on ephemeral streams, may incr~de reduction in peak flows and an increas e
<br />in the length of stream flow season. Detr'imental effects most commonly
<br />ascribed to stock ponds include streamflow depletions due to evaporation,
<br />seepage, and transpiration by riparian vegetation, and changes in the
<br />regimen of flow, each of which may affect established water rights.
<br />
<br />In recent years, considerable attention has been focused on this
<br />latter problem and a number of studies have been made to determine a
<br />quantitative effect of stock ponds on downstream water yield. Results of
<br />studies have shown a wide var iance depending upon the location and size
<br />of the study area. This variance is to be expected since virtually every
<br />study has had a different downstream point of interest at which effects
<br />were determined.
<br />
<br />A cooperative 5-year study, started in 1957 by the Soil Conserva-
<br />tion Service, Agricultural Research Service, and the Bureau of Reclama-
<br />tion, has been attempting to develop methods ofmeasuring the impact of
<br />conservation land treatment (which includes stock ponds) on downstream
<br />water yields. One of the objectives of this study is to develop evaluation
<br />procedures which will have widespread technical acceptance and applica-
<br />tion.
<br />
<br />48
<br />
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