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<br />G01537
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<br />The number of stock ponds estimated for future 'construction in
<br />the Great Basin is, believed to be excessive for the same reasons as those
<br />mentioned for the'Colorado Basin. Les s is known concerning potential
<br />needs in the Central Pacific and South Pacific areas.
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<br />The above report also gives est imates of the evaporative loss es
<br />from ponds in the Southwest -- excess of evaporation over precipitation
<br />in the ponds on an average annual basis (p. 46). Such losses are believed
<br />to be the major use resulting from stock ponds (Committee on Hydrology,
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<br />ASCE, 1949), probably exceeding by a factor of three or four the water
<br />consumed by livestock in most areas. Nevertheless seepage losses from
<br />ponds may also increase consumption of water, in that seepage may increase
<br />evaporative losses from the channel below the pond. The extent of such
<br />increases, if they occur, is not knowp, but can be minimized by careful
<br />pond construction to reduce seepage losses.
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<br />Present and ultimate water use
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<br />Estimates of present and ultimate water use for stock ponds in
<br />the Pacific Southwest basins can be projected from evaporation data and
<br />estimated present and future surface areas of such ponds as shown in
<br />table 6. These estimates are based on data taken again .from Committee
<br />Report No.. 13 (supra). according to this analysis some three-fourths
<br />of the est imat,ed 75,000 acr e- feet addit ional water needed for the new
<br />79,000 stock ponds that might be built by the year 2000 is needed in the
<br />Colorado River Basin. The use of water for stock-pond development
<br />might have an important effect on small local areas, even though the
<br />estimated total ,need of only 75,000 acre-feet is certainlya small part
<br />of the total water supply of the area.
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<br />Reg ional supply
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<br />A regional supply and distribution of runoff waters for stock
<br />ponds and other uses may be estimated from available data. Rather
<br />detailed maps of average annual runoff'for the Pacific Southwest are
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<br />given in Committee Print No. 13 (supra) and, Corps of Engineers (1958).
<br />Because stock ponds are usually in the headwater tributaries of various
<br />drainages, thes e runoff amounts, at '.least in a phys ical s ens e, are fully
<br />available for stock-pond supply in proportion to the area of their water-
<br />sheds. As mentioned under the "supply for individual stock ponds,"
<br />the runoff per unit of area from small tributaries tends to be much
<br />higher than from major streams from which supply figures are given.
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<br />, Year-to-yea;r variation in runoff may be equally important to
<br />that of the average annual amount in stock pond supply. The frequency
<br />with which deficient and excessive runoff occurs in the Pacific Southwest
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