WATER STORAGE ON SWEETWATER AND SAN JACINTO RIVERS. 



357 



or in part, if their true value wei-e outlined by such preliminary studies. For this 

 reason the experience of the Sweetwater sj^stem is of value as a precedent. 



The supplj' of water required in irrigation on the sj-stem has been assumed to be 

 about 1 acre-foot per acre per annum, or what is equivalent to 12 inches of rainfall, 

 and this is the volume apportioned by the company as the annual allowance granted 

 in their water-right contracts. The actual use, including the consumption for 

 domestic suppl}- to the inhabitants of National Citj- and Chula Vista, has averaged 

 about 1.5 acre-feet per acre. A rough check on this estimate, which has been arrived 

 at from independent observation and data, is obtained in the following manner: The 

 total run off in thirteen years was 180,070 acre-feet. all of which was used in twelve 

 years; the total amount passing over the top of the dam as waste during this period was 

 approximately 80,070 acre-feet, leaving for utilization and to supply evaporation, 

 100,000 acre-feet. This 100,000 acre-feet was equivalent to a mean of 8,333 acre-feet 

 per annum over the twelve-year period. Taking this volume as the assumed mean 

 contents of the dam, the mean area of surface exposed to evaporation was 436 acres. 

 As the evaporation loss was measured for several jears its mean was ascertained to be 

 about 4. 5 feet in depth per annum; and this depth in twelve j'ears would have amounted 

 to 23,544 acre-feet, or 23.5 per cent of the total salvage from the flow of the stream. 

 The remaining 76,454 acre-feet would represent the volume actualh' consumed in 

 irrigation, after deducting domestic service and leakages. 



As a check upon this computation of the total loss by evaporation I have taken 

 the weekly record of gage heights kept without interruption from April 30, 1888, to 

 January 1, 1898, kindlj^ furnished by Mr. Savage; and from the table of areas and 

 contents of the reservoir at different levels I have placed opposite each weeklj' gage 

 reading the corresponding area of reservoir surface exposed to evaporation. The 

 sum of these areas divided b\' the number taken each year gives the yearly mean 

 area of surface exposure. From this data I have made the following interesting table: 



Reserroir surface exposed to eraporaiion, 1889-1897. 



This figure of mean surface exposure closelv corresponds with that derived by a 

 totally distinct method and affords a gi-atifying check upon the computation, although 

 a more accui-ate and more laborious process of reaching the result would be to 

 multiply the measured evapoi-ation each week by the area corresponding and sum up 

 the total instead of taking a mean evaporation loss for a period of years. 



