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<br />EM 11111-2,1405 <br />31 Aug 59 <br /> <br />putation procedures alone. Even though it were practical to analyze the runoff phenomenon with <br />scientific accuracy, the limitations of basic data would generally preelude such studies. The reliability <br />of estimates of runoff rates and quantities is dependent as much upon th~ soundness of judgment uspd in <br />the interpretation of data as in the detail of the computation procedures applied. A thorough stud,' <br />of the characteristics and meteorological influl:"IleeS affecting runoff from a specifie basin eonstitutes an <br />essential basis for sound judgment. <br />b, The reliability of a rational analysis of recorded hydrographs, and the accuracy of a hypothetical <br />hydrograph designed to represent runoff under specified conditions, are dependent upon the ability to <br />evaluate the predominating influences of the following factors on runoff: <br />Rainfall- <br />Intensity, duration, sequence. <br />Areal distribution during successive time intervals. <br />J nfiltration- <br />Initial loss, or loss before appreciable runoff begins. <br />~linjmum avprage capacity, or in BomC' eases, the relation of capacity to field moisturp, <br />conditions. <br />Re[limm of Runoff-- <br />Effects of basin configuration, stream dC'nsities, and arra.ngPffif'nt of tributaries. <br />Effects of natural storage- <br />Tn tributaries, lakes, swamps, etc. <br />I n principal stream channels and valleys. <br />Effects of artificial strueturcs- <br />Reservoirs. <br />Channel improvements. <br />Land use practices. <br />Effects of slopes- <br />I n principal stream channels and flood plains. <br />Tn drainage areas tributary to principal runoff rhannrls. <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 />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 bhsin involves the construction of mass rainfall curves <br />similar to those shown in plate Ko. 2. The mass curve is primarily a means of recording the conclusions <br />reached by correlating such miscellaneous data as may be available regarding rainfall intensit.\' and time <br />of occurrence.22 * The following routine is ordinarily used in their construction. <br />a. A preliminary total,storm isohyetal map similar to plate Ko, 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 arc prepared for eaeh prineipal rainfall'period. <br />b. ~lass curves for recording precipitation stations are plotted on a composite shert, in orurr to <br />facilitate a study of progressive variations in rainfall intensities within the storm area. If properly <br /> <br />.:Xumbl'l~ shown in this mannrf frfef to nUmhl'fl'd rl'frff'nreS listrd in App('ndix II. $('1' 1I]"O pan;!!:mph 2/). <br /> <br />2 <br /> <br />e <br /> <br />, <br /> <br />. <br /> <br />e <br /> <br />. <br /> <br />e <br />