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<br />EM Il1l1-2-1~05 <br />31 Aug 59 <br /> <br />putation procedures alone. Even though it were practical to analyze the runoff phenomenon with e <br />scientific accuracy, the limitations of basic data would generally p......lude such studies. The reliability <br />of estimates of runoff rates and quantities is dependent as much upon the soundness of judgment used in <br />the interpretation of data as in the detail of the computation procedures applied. A thorough study <br />of the characteristics and meteorological influences affecting runoff from a specific basin constitutes an <br />essential basis for sound judgment. <br />b. The reliability of a rational analysis of recorded bydrographs, and the accul'lU'~. of a hypothetical <br />hydrograph designed to represent runoff under specified conditions, are dependent, upon t,he ability to <br />evaluate the predominating influences of the following factors on runoff: <br />Rainjall- <br />Intensity, duration, sequence. <br />Areal distrihution during successive time intfiVals. <br />lnfiltration- <br />Initial loss, or loss before app......iable runoff begins. <br />:\-linimum &Vf"r&ge capacity, or in som~ cases, the relation of capacity to field moisture <br />condit.ions. <br />Rellimen of llurwff- <br />Eff~ts of basin configuration, stream densities, and 8JT8.ngt"'mpnt. of t,ribut-arit's. <br />Effects of natural storage-- <br />1 n trihutaric-s, lakes, swamps, etc. <br />I n principal stream ebannels and valleys. <br />Effects of artificial strudures- <br />Reservoirs, <br />Channel improvements. <br />Land use practiees. <br />Effects of s1opes- <br />I n principal stream ..hannels and ftood plains. <br />In drainage areas tributary to principal runoff channels. <br />Land slopes. <br />Effects of land coverage- <br />Forested areas. <br />Cultivated areas. <br />Pasture lands and barren areas. <br />Ability of subsurface soil to transmit infiltrated water to surface channels within the period <br />required for direct runoff to pass through the channel storage phase of runoff. <br /> <br />e <br /> <br />RAINFALL DETERMINATIONS RELATED TO NATURAL HYDROGRAPH <br />ANALYSIS <br /> <br />6. MASS RAINFALL CURVES. One of the most convenient methods of estimating the intensity and <br />chronology of rainfall at various points in a bfLSin involves ..he construction of mass rainfall curv('S <br />similar to those shown in plate Ko. 2. The mass curve is primarily a means of recording the conelusioll~ <br />reached by correlating such miscellaneous data lIS may be available regarding rainfall intensity and time <br />of occurrence,u * The following routine is ordinarily used in their construction. <br />a. A preliminary total-storm isohyetal map similar to plate No. 1 is prepared from published <br />precipitation records to delineate the general rainfall pattern. If the storm is characterized by two or <br />more distinct intervals of heavy rainfall, isohyetal maps are prepared for each principal rainfall 'period. <br />b. .\1ass curves for recording precipitation stations are plotted on a composite sheet, in order to <br />facilitate a study of progressive variations in rainfall int..nsities within the storm area. If properl,\' <br /> <br />.Xumbl"ls shoa"n in tbis IIUUlDft'" rPfer to numhPred Tt"ren-n~ listPd in Appplldis. II, 5f'f" al<<o panl!mlph 2'1. <br /> <br />2 <br /> <br />e <br />