20 



THE IRRIGATION AGE. 



For rolling grassy brush or timber land 

 from .50 to .65. 



For tolerably flat or gently sloping al- 

 luvial plains, with little grass and brush 

 from .40 to .50. 



For flat or gently sloping cultivated 

 lands from .30 to .40. 



The variable factor c has the following 

 values: 



For an annual rainfall of 40 inches or 

 upwards, \ ; for 30 inches, f ; for 20 

 inches, J; for 10 inches, ^; for less than 

 10 inches, 



The values of c intermediate to those 

 given can be found by simple proportion, 

 and all values when used in the formula 

 should be written in per cent decimals, 

 which are better adapted to logarithmic 

 computation. 



From the recorded observations and the 

 formula deduced therefrom, it is patent 

 that a large portion of the water falling 

 from the clouds does not flow off on the 

 surface of the ground. This water be- 

 comes lost in various ways, or in other 

 words goes where it is not visible in chan- 

 nels on the surface, nor can it be collected 

 to fill storage reservoirs. A portion of it 

 is evaporated, and returns to the air to 

 again fall as rain; a portion is consumed 

 by the growth of vegetation, and the re- 

 mainder, which is by far the greatest por- 

 tion, sinks into the ground and goes to 

 constitute underflow of streams, subsoil 

 water, spring water, and the water con- 

 tained in artesian strata. 



When it is possible to do so it is always 

 advisable to measure the daily discharge 

 of water from a watershed, in addition to 

 making a survey of it and gaugings of the 

 rainfall. By doing this it is possible to 

 determine accurately what is the actual 

 quantity of water running off on the sur- 

 face without the inaccuracies and uncer- 

 tainties which are always involved in theo- 

 retical formula and calculations. Usually 

 the cost of measuring discharges from 

 streams is quite great in comparison with 

 the cost of gauging the rainfall, and for 

 this reason it can not be carried on so 

 easily nor for any great length of time, 

 unless in exceptional cases. In all cases, 

 where it is possible to do so, the discharge 

 should be measured and the rainfall 

 gauged as well for at least one year. In 

 this way the variable factors in the formula 

 can be given fixed values applicable to the 

 watershed in question, and the . formula 



can be so expressed as to eliminate all ele- 

 ments of uncertainty, so that perfect re- 

 sults for the watershed in question can be 

 obtained for subsequent years, although 

 observations of the rainfall alone are 

 made. 



EVAPORATION AND PERCOLATION IN STORAGE 

 RESERVOIRS. 



All the water accumulated in storage 

 reservoirs can not be applied to useful 

 purposes. There are certain losses caus- 

 ing shrinkage in the volume of water after 

 it is impounded, which must be deducted 

 before the amount available for use can be 

 ascertained. The two principal means by 

 which water is abstracted from a storage 

 reservoir are evaporation and percolation. 

 The former of these can, in ordinary cases 

 within the region where irrigation is prac- 

 ticed, be considered to equal the amount 

 of rainfall on the surface of the reservoir. 

 At all events it is perfectly safe to make 

 this assumption, as the rainfall on the 

 surface of the reservoir is more likely to 

 exceed the evaporation than not to equal 

 it. Of course, when evaporation is ac- 

 counted for in this way, the area of the im- 

 pounding basin must be subtracted from 

 the total tributary watershed above the 

 dam site, in making the surveys and esti- 

 mates of available watershed for filling the 

 reservoir. 



The amount of percolation is a more 

 difficult matter to arrive at, as the soil 

 comprising the basins of storage reservoirs 

 may be of any conceivable nature. Very 

 often ledges or veins in the sides of the 

 basin may convey a considerable amount 

 of the water impounded away to points 

 below the dam. In cases of this kind, 

 where the loss of water is great, it can 

 readily be noticed and remedied. If the 

 water entering the reservoir is heavily 

 charged'with silt and sediment, the diffi- 

 culty may remedy itself. The most dan- 

 gerous species of percolation, however, is 

 that which finds its way into entirely un- 

 derground channels and can not be noticed 

 except for its effect in diminishing the 

 water in the reservoir, as its reappearance 

 takes place so far away that it can not be 

 easily detected in this way. The only 

 way of determining its amount is by ac- 

 curately measuring the water drawn from 

 the reservoir, while observing the exact 

 quantity disappearing from the reservoir 

 during the same time, as well as the quan- 

 tity entering it if any. 



