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<br />Direct Runoff Measurements
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<br />The ,Use of direct runoff measurements in estimating water supply
<br />should be encouraged. For most places it is easier and probably more
<br />accurate to estimate runoff and obtain its frequency of occurrence by'
<br />correlation with known runoff measurements than to make the prediction
<br />on the basis of precipitation frequency. The natural vadability in runoff
<br />from small stock pond watersheds is illustrated by the records for 25
<br />such basins in the upper Cheyenne River Basin (table 3). These records
<br />show that if 4 acre-feet of of runoff are needed to meet the annual needs
<br />and losses, then almost half of the ponds have 2 or more dry years in 4,
<br />but if only 1 acre-foot is needed, 6 of the p:onds would have been dry for
<br />1 year and 3 for? years or more. A general' regional distr ibution of
<br />annual runoff in Western United States is given in the Select Committee
<br />on Natural Water Resources, United States Senate Report No. 13 (1960).
<br />Fletcher and Rich (1955) have classified watersheds in Arizona, New
<br />Mexico, and northwestern Texas as areas of high, moderate, and low
<br />yield. Somewhat more detailed maps on geometdc mean annual runoff
<br />and year-to-year variation in runoff are cOhtained in U.S. Corps of
<br />Engineers report (1958). These charts give runoff as it occurs from
<br />rather large areas. For compadson it should be noted that the studies
<br />in the upper Cheyenne River Basin indicated generally greater unit runoff
<br />from s mall areas than from large. Other factors als 0 account in part
<br />for the variabil ity in runoff indicated by the data in table 3. Further
<br />study is needed to clarify the area relationships t'? runoff.
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<br />Annual runoff data from several small watersheds in the vicinity
<br />of Safford, Arizona, and Albuquerqu e, New Mexico, are reported in
<br />table 4. The variation from the 21-year average is wide and stock ponds
<br />constructed On channels in this general area to store water might go dry
<br />for several consecutive years but might sfill require large capacity spill-
<br />ways.
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<br />In most of the West longer records of precipitation measurements
<br />are available than of runoff, and precipitation frequenc;:y may therefore
<br />be used as an aid in appraising the expected f!'eq\;ency of runoff. In some
<br />isolated localities estimates of runoff may of necesSity have to be made
<br />on the basis of precipitation. More runoff measurements representative
<br />of areas where stock poncL9 ar:elikely:tobe needed should be encouraged.
<br />These measurem,ents may be made with a range of devices and methods,
<br />including flumes and weirs installed in small watershed channels and by
<br />measu ring volumetrically the runoff to existing stock ponds. Obs ervation
<br />of highwater marks and calculation of a peak flow using cross sectional
<br />ar'eas furnish an idea of the volume of flow from the contributing basin.
<br />Such measurements, if c'orrelated with nearby long-term flow measure-
<br />ments in larger streams or with long-term precipitation measurements,
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