72 



THE IRRIGATION AGE. 



the depths shown on the measuring rod 

 on the tube will be ten times the rainfall 

 in inches. 



In places where snow falls, a round dish 

 of equal diameter at top and bottom and 

 of sufficient depth to receive all the snow 

 which may fall in any one storm is used. 

 This dish should be securely set in an 

 open, level place so as to receive all the 

 falling snow due to its area and so as not 

 to allow any snow to drift into it. After 

 each storm the snow should be melted and 

 the obtained depth of water measured, 

 which together with the depth of the snow 

 before melting should be carefully record- 

 ed. For accurate gauging the practice of 

 placing the instrument on the roofs of 

 buildings or on other objects elevated 

 above the surface of the ground is to be 

 condemned, as, at any considerable eleva- 

 tion above the ground, the drops of rain 

 in their downward course are deflected in 

 their vertical descent by the force of the 

 wind and less water will enter the rain- 

 gauge than the proportion due to its sec- 

 tional area. Fornulse which have been 

 suggested for making corrections for dif- 

 ferent elevations are practically valueless, 

 as so much depends on the relative force 

 of the rain and wind, factors which it is 

 impossible to determine accurately. 



In making observations of the rainfall 

 on a watershed, rain-gauges should be 

 placed so as to cover all points where a 

 material difference in the precipitation is 

 likely to occur. The location of the 

 gauges should be correctly determined, so 

 that, when the watershed has been sur- 

 veyed, the exact points of all observations 

 can be noted on the plat as stations and 

 numbered. A record of each station 

 should be kept showing the date and 

 amount of rainfall each day. 



The character of the gathering ground 

 upon which the rain falls has considerable 

 to do with flood discharges from a water- 

 shed. The maximum may in some cases 

 reach two- thirds of the amount which 

 actually falls; but this is only to be 

 depended upon when the watershed is 

 small and the surface impervious and bar- 

 ren, and the ground frozen and free from 

 snow. It is impossible to give the maxi- 

 mum discharge with any claim of reliabil- 

 ity in the case of large watersheds with 

 loose soil, except where data have been 

 collected in regard to that particular 



watershed by actual gauging of the rain- 

 fall and measurement of the discharge. 

 Some cases are of record, where the dis- 

 charge from a large basin with loose 

 ground and slight descent has been so 

 insignificant as to amount to practically 

 nothing. 



With zero as a minimum and two-thirds 

 of the total rainfall as the maximum, re- 

 sults vary so much that no ratio can be 

 confidently named even with a good map 

 and description of the watershed at hand. 

 Absolute safety in making estimates of the 

 ratio of discharge for a given rainfall lies 

 in having made a sufficient number of 

 measurements of the flow in the outlet 

 channel to establish a rule applicable to 

 the watershed in question. 



In calculating the volume of flood water 

 falling on a given watershed there are 

 three factors to be taken into considera- 

 tion, i. e. (1) Area of watershed. (2) 

 Depth of rainfall and (3) duration of pre- 

 cipitation. By means of these factors we 

 can find the number of cubic feet which 

 have actually fallen from the sky during a 

 flood. But in hydraulics the time as well 

 as quantity are considered so that water is 

 always measured and estimated by the 

 rate of flow in a given space of time, and 

 the units adopted are the cubic foot of 

 water and the second of time, so that the 

 rate of flow is expressed in cubic feet per 

 second. 



We now have everything necessary for 

 establishing a formula by which can be 

 determined the rates of a volume of water 

 falling on a known area exposed to an 

 observed precipitation for a given length 

 of time in cubic feet per second. 



This formula is expressed as follows: 



Q 



in which 



Q = Flow in cubic feet per second re- 

 sulting if all the water falling were dis- 

 charged uniformly in the same length of 

 time which it has taken to fall. 



A =Area of watershed or basin exposed 

 to the rainfall in square feet. 



D = Observed depth of rainfall by the 

 pluviometer or rain-guage in feet. 



S = Time in seconds during which the 

 observed depth has fallen. 



The following table may be of use in 

 making approximate estimates without em- 

 ploying the formula: 



