SUBSOII/ ABSOKPTION. 29 



second. Eice culture requires a supply equal to one 

 cubic foot per second, for each 30 to 80 acres. These 

 examples will serve as a basis for calculations, needed to 

 meet the widely different circumstances which exist in 

 the United States, where possibly the variations of soil 

 and climate, over our extensive territory, are unparalleled 

 in any other single country in the world. 



As to the Subsoil. This point needs very little elucida- 

 tion. From the preceding remarks, the effects of loose 

 or compact subsoils will be seen to exert a considerable 

 influence upon the requisite water supply. There are 

 soils which rest upon open, coarse, gravelly subsoils, 

 which may be compared to a sieve. Other soils, with re- 

 tentive clay subsoils, furnish examples exactly the reverse. 

 These are unusual cases, but as they may occur, they 

 ought to be considered. It does not seem necessary to 

 discuss this point further than to call attention to the 

 importance of ascertaining the character of the subsoil 

 of any tract which it is proposed to irrigate, as a serious 

 element in calculating the needed supply of water. The 

 minimum direct loss through absorption by the subsoil 

 should not be estimated at less than 15 per cent of the 

 supply, and a much larger allowance should be made 

 when, upon examination, the subsoil is found to consist of 

 coarse sand or gravel. 



When we consider the quantity of water needed for 

 irrigation, it is clearly seen that springs are rarely of suf- 

 ficient volume to be of material value, excepting for 

 meadows, and then only for small surfaces or partial 

 watering. They may be made, however, to serve an im- 

 portant purpose in such cases as these where the area to 

 be irrigated is small. Storage reservoirs, in which is 

 collected the water of those temporary courses, which 

 flow only when the rain-fall is largest and the amount of 

 evaporation is least, may be made important sources of 

 supply. It is by means of these that a large portion of 



