WATER SUPPLIES FOR IRRIGATION. 



CHAPTER VI. STORAGE RESERVOIRS, EVAPORATION AND 



PERCOLATION. 



BY F. C. FINKLE, C. E. 



UCH surveys and exam- 

 inations as will give a 

 complete description of 

 the character and con- 

 dition of the water- 

 shed should be made. 

 The points to be noted 

 are the slope and pitch 

 of the surface at dif- 

 ferent places, the character of the surface 

 formation, the range of elevations and the 

 amount and class of vegetation. All ob- 

 servations of this character furnish data 

 from which the value and capacity of a 

 watershed can be estimated. 



The ability of a watershed to furnish 

 water for storage depends upon the mat- 

 ters which we have already discussed, 

 i. e., the area and character of the water- 

 shed and the amount of annual precipita- 

 tion thereon. The old school of hydraulic 

 engineers were wont to assume that the 

 discharge of water from a watershed, 

 available for storage, was equal to one- 

 half of the total annual rainfall on the 

 watershed. While it is true that this was 

 the general assumption, still there were 

 many who depended more on their own 

 judgment as applied to each individual 

 case. This latter method is not entirely 

 unsatisfactory as the judgment of one well 

 versed in hydrology, when he is acquainted 

 with the geological structure, extent and 

 physical characteristics^of a watershed, is 

 usually able to grasp its capacity for 

 yielding an available water supply, by de- 

 ciding what proportion of the rainfall will 

 be discharged and what proportion will 

 be lost in various ways. But neither of 

 these methods are satisfactory for making 

 close and accurate estimates, and since 

 statistics have been collected from observa- 

 tions made and recorded by government 

 bureaus and hydraulic engineers, the mat- 

 ter has been reduced to more of a science. 

 The observations from all points do not 



exactly correspond, which makes it diffi- 

 cult to produce a formula which will give 

 results for all places. It appears that re- 

 sults obtained from localities where the 

 distribution of the -rainfall is uniform 

 throughout the year, are somewhat differ- 

 ent from those obtained in regions where 

 the year is divided into a rainy and a rain- 

 less season. 



Our investigations being in the interests 

 of irrigation, we will devote ourselves to 

 constructing a formula applicable to 

 regions with the year divided into a wet 

 and a dry season. 



FORMULA FOK STORAGE SUPPLY. 



By plotting such results as have been 

 collected from observations made in irri- 

 gated localities or localities requiring irri- 

 gation, where the distribution of the an- 

 nual rainfall is somewhat irregular, the 

 resultant has been found to follow very 

 closely the following formula: 



Q = A x pcf , in which 



Q = the number of cubic feet of water 

 discharged during the year. 



A = area of watershed in square feet. 



p = percentage due to character of 

 watershed. 



c = a variable factor depending on the 

 annual depth of rainfall. 



f the depth of annual rainfall in feet. 



The percentage due to natural charac- 

 teristics of the watershed and represented 

 in the formula by the factor p varies from 

 .30 to .85. 



The mean values are as follows, and for 

 cases not corresponding exactly to the de- 

 scriptions given, values intermediate to 

 those given can be assigned in actual prac- 

 tice: 



For steep, rocky hills and mountains 

 from .75 to .85. 



For heavily timbered hills or mountains 

 and moist brushy and swampy lands from 

 .65 to. 75. 



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