OCR Text |
Show CONSUMPTIVE USE OF WATER IN IRRIGATION 1369 diversion records are also available for the districts as a whole and for their main laterals. In connection with the engineering studies made by the State Engineer's Office of the Kings River Water Conservation District, Harding made a detailed analysis of the ground-water records. The rainfall at Fresno averages 0.81 ft. annually, and occurs mainly in the winter months. Diversions for irrigation extend from February to October, varying with the available water supply, the period of diversion being longer in the Fresno District due to its earlier priority of right. The method of analysis consisted in determining the seasonal ground-water fluctuation and comparing it with the acre-feet of water conveyed into the area, per acre of cropped land. In all these areas there is extensive pumping for irrigation. Some land was supplied with water entirely by pumping from local ground-water; other land was furnished canal water when available and pumped water during the remainder of the season. The gross area of farms served was used in estimating consumptive use. As the projects are highly developed, the net area cropped is nearly as large as the gross areas herein used. The year was divided into two periods, namely, March 1 to November 30 and December 1 to February 28, because an examination of the ground-water fluctuations in the winter indicated no influence from the delivery of water during the preceding irrigation season. The winter fluctuations vary largely with the winter precipitation, but this is probably an indirect relationship, since with larger rainfall there is less irrigation pumping, and there is also some local tributary run-off. The canal deliveries necessary to maintain the ground-water were taken as the quantities necessary to cause a rise or fall from March 1 to November 30 equal to the fall or rise that would occur from December 1 to February 28 in a year of normal rainfall. The quantities of water delivered to each tract each year, together with the accompanying water-table fluctuations, were plotted as illustrated in Fig. 1, which typifies Harding's method of analyzing consumptive use data. This diagram gives the results obtained, respectively, on the entire area under the Fresno Irrigation District, the Consolidated Irrigation District, and the Alta Irrigation District, The summarized results of canal deliveries and ground-water fluctuations for these three areas for 1922 to 1925 are given in Table 6. The resulting estimated requirements for diversion into each area to supply the consumptive use and maintain the ground-water are also given in Table 6. These estimates were arrived at by plotting the ground-water fluctuation against the deliveries, as shown in Fig. 1, and selecting the intersections of the line drawn through the resulting points with the axis of zero change in level of ground-water, as representing the deliveries that would be required to supply the consumptive use and maintain the ground-water for the periods, March 1 to December 1. The estimated total annual requirements were based on allowing for the ground-water fluctuations from December 1 to March 1 in years of normal rainfall as previously explained. |