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3 <br />(~ excessively drained sands or gravels." The antecedent moisture condition <br />T~ <br />(At1C) selected o-ras At•1C II corresponding to "an average of the conditions <br />that have preceded the occurrence of the maximum annual flood." A pasture <br />or range in "good hydrologic" condition under the above circumstances <br />o-rill have a curve number (CN) equal to 39. For a curve number of 39, the <br />corresponding maximum retention is 15.6 inches and using [a = 0.25 yields <br />3.1 inches for the initial abstraction. <br />The design storm is that which corresponds to a 24 hr precipitation <br />event with a 10 yr return period. The 24 hr, 10 yr precipitation event <br />for the study area was estimated to be 3.inches from Rainfall Frequency <br />Atlas of the U. S. for Durations from 30 ;tinutes to 24 Hours and Return <br />Periods from 1 to 100 Years, Technical Paper tlo. 40, U. 5. Department <br />• of Commerce, flay, 1961. <br />~.. For the above conditions, the 24 hr, 10 yr precipitation is less than <br />the initial abstraction indicating no surface runoff from such a storm. <br />This computation agrees with the on-site observation of no evidence of <br />surface runoff, even in wheel tracks or other areas particularly prone <br />to show evidence of overland flow. Tlws, it is believed to be a reliable <br />estimate. <br />If the hydrologic condition of the watershed is regarded as being <br />"fair" rather than "good", the value of CN is 49, and the computations <br />indicate 0.07 inches of runoff. The total area of the catchment shown <br />~n figure 1 is 1675 acres and, therefore, the runoff volume is O.a acre- <br />~'1 <br />feet under this assumption. It is believed that a runoff depth of 0.07 ft <br />is small relative to the accuracy and reliability of the method employed <br />• and cannot be regarded as significantly different from zero. <br />r~ <br />