882 EXPRKIM KNT STATION 11K( 'OTiD. I Vol. 41 



valleys Is coinpo-sod oT sand and silt derived mainly Inun the granite rocks of 

 the highland area. In general, the water table in the major valleys has the 

 slope and, to some extent, the form of the ground surface, although important 

 differences exist. Observations on 122 wells showed four types of annual 

 fluctuation of water table vaiying from an average of 7 to an average of 1.5 ft. 



Tests of several wells are reported which show in general the dependence of 

 yield on the character and thickness of the water-bearing material. " The 

 speciflc capacity of the wells tested ranged from 6 to 45 gal. per minute per 

 foot of drawdown. The smallest and largest yields were obtained from 

 wells . . . only aI)out two miles apart and which draw their water from 

 gravels that are similar in character and thickness. . . . The tests indicate that 

 wells sunk in the major valleys of San Diego County, penetrating a consider- 

 able depth of coarse sand or from 10 to 15 ft. of open gravel, such as lies at 

 the bottom of the ancient river valleys, if properly perforated or equipped with 

 strainers, may be expected to yield at least 20 gal. per minute per foot of 

 drawdown." 



The minor valleys of the coastal belt seldom exceed in area a few hundred 

 acres. They are underlain and bordered by sedimentary rocks, and contain 

 alluvium and other fill in their lower but not in their upper parts. The water 

 table " lies farther beneath the surface than in the major valleys. In general 

 shape and position, however, it corresponds to that in the major valleys, hav- 

 ing little slope across the valleys but an appreciable slope downstream. Its 

 fluctatious within each year are similar to those in the major valleys of the 

 coastal belt but of \vider range, . . . 



" The quantity of ground water available in the minor valleys of the coastal 

 belt is small, owing to the narrow width and shallow depth of the fill and the 

 small run-off and short period of flow of the streams that traverse the valleys. 

 Supplies adequate for domestic use can, however, be obtained in most of them. 

 The safe yield of most of the wells is very small, because the run-off in dry 

 years is almost negligible. The use of ground water for irrigation in these 

 valleys must as a rule be restricted to lands on the floors of the valleys, but 

 even with this restriction, the supply in many places will be insufficient to serve 

 all the overlying land, particularly in dry years." 



In the highland area the yield of wells in residuum or decomposed granite 

 has been found to range from very small quantities to as much as 150 gal. per 

 minute. " The largest yields are obtained from wells that p^ietrate residuum 

 of considerable depth, that are provided with lateral tunnels and auger holes, 

 and that are situated in valleys irrigated with water from an outside source. 

 In general, it m^ay be said that the specific capacity of the best wells in resi- 

 duum is about 8 gal. a minute per foot of drawdown, that for many wells it is 

 as low as 1 gal. per minute per foot of drawdown, and that for the poorest 

 wells it is much less than 1 gal." 



Analyses of waters from the three different sources in the area show " a 

 wide range in mineral content in the waters within each of the three groups, 

 and the average mineral content in all of them is rather high. . . . Rather 

 highly mineralized sodium-chlorid waters, fairly satisfactory for use for do- 

 mestic supplies and for irrigation, and very bad for use in boilers, are greatly 

 predominant in San Diego County and may be regarded as typical for the area." 



Tests of a number of pumping plants are also reported. " Centrifugal pumps 

 ranged in efficiency from 34 per cent for a SJ-in. pump to 7G.5 per cent for a 

 6-in. pump, the efficiency increasing, other things being (Hiual, with the size of 

 the piunp." riant efficiency ranged from 2G per cent to GS.n per cent, the 

 smaller plants being the less efficient. The annual cost per acre-foot for the 



