182 CLASSIFICATION OF THE PUBLIC LANDS. 



The study of available water supply therefore involves an investiga- 

 tion of the possibilities of storage. Unless information is available 

 from private surveys, a reconnaissance «xamination is made of the 

 drainage area for the purpose of locating storage sites. The methods 

 followed are merely approximate and do not differ from those used 

 in reconnaissance power surveys. 



If no actual records are available showing the discharge of the 

 stream, it is necessary to establish gaging stations at once and make 

 a careful study of the drainage area and records of precipitation. 

 If reliable maps are available from which the area of the drainage 

 basin can be measured, and records are available showing the average 

 precipitation on the basin, it is then possible to figure roughly the 

 probable run-off. This method of determining the available water 

 supply is unsafe and is used only where no records of discharge are 

 available. 



After determining the gross run-off from the basin the unutilized 

 flow or the water available for the reclamation of new lands is deter- 

 mined by deducting the amount of water required to satisfy prior 

 appropriators. 



PKESENT XjnXJZA.nON OF WATER POB tRRTaATIOHr. 



Mention has already been made of the necessity for allowing for 

 prior water rights in determining the amount of land that can be 

 successfully irrigated from any stream. Irrigation systems already 

 installed at the time of the examination furnish one of the most 

 troublesome problems the engineer has to investigate. Under the law, 

 as usually interpreted in the West, the water is public property, 

 permission for its use being granted by the State, and wherever 

 lands are classified for irrigation in the West it is nearly always 

 true that the State has granted prior rights to the use of the 

 water. In the earlier days of irrigation these rights were granted 

 with the utmost liberality, without much regard for the amount 

 of water actually flowing in a stream. It is therefore common to 

 find that these prior rights exceed in aggregate amount the volume 

 of water carried by the stream during irrigation seasons. This 

 condition has been recognized by persons desiring to use water 

 from such streams, and many of them have secured rights to use the 

 flood waters which flow in the stream channels at periods of the year 

 when irrigation is not practiced. Such utilization is dependent, of 

 course, on the construction of storage reservoirs. Thus it may occur 

 that, however well adapted for agriculture a piece of public land may 

 be, on superficial examination there appears to be no available water for 

 its reclamation. It is necessary for the engineer to bear in mind the 

 liberality with which prior rights have been conferred and to examine 



