SEEPAGE OR RETURN WATERS FROM IRRIGATION. 
51 
measurement of the Poudre by CoL Nettleton, in 1887, the increase 
amounted to eighty-seven cubic feet per second. 
§ 47. Where there is abundant rainfall, there is no question 
but that it furnishes a supply to the streams through underground 
passages, with effects similar to those noticed in the measurements of 
these streams. The amount which thus percolates through the 
ground is the portion of rainfall remaining after the run-off and 
the evaporation have been supplied. We have no direct observa¬ 
tions under our conditions to determine positively how much, if 
any, of the rainfall remains to supply the underground water of 
the soil. Lawes & Gilbert, of Pothamstead, have maintained a 
series of drainage gages for a number of years. In the twenty-two 
years, from 1871 to 1892, fifteen to sixteen inches of the rainfall 
passed through forty to sixty inches of soil, and joined the subsoil 
water. This would be available for springs, and doubtless largely 
increased the volume of the streams draining the country. This 
was out of a total rainfall averaging 29.95 inches. Hence it fol- 
low^s that some thirteen inches in the humid atmosphere of England 
was required for evaporation from the surface of the soil, which was 
left uncropped and free from vegetation. 
§ 48. With a smaller rainfall, it is not probable that the 
evaporation would be less. The greater dryness of our climate, the 
greater amount and intensity of the sunshine, which heats the sur¬ 
face of the soil intensely, are conditions which favor evaporation. 
The uniform dry condition of the soil shows that there is none too 
much for the evaporation alone. Our average rainfall is but little 
more than the amount which was evaporated from the soil in Eng¬ 
land, and some of this runs directly to the streams. It does not 
seem probable that there can be any left for percolation into the sub¬ 
soil, except under unusual circumstances. In 1895, when eighteen 
inches of rain fell, not much more than usual was available for 
evaporation and percolation, since with the heavier showers a larger 
proportion runs off. 
That the inflow comes almost entirely from irrigation is shown 
indirectly by the well-known effect of irrigation upon the height 
of water in the ground. Before irrigation, the distance to water 
is generally great, and the quantity frequently scanty. The appli¬ 
cation of water in large quantities to the surface, as in irrigation, 
fills the subsoil when porous, and raises the level of the ground 
water as much as forty to sixty feet in some cases. This establishes 
a steeper grade to the surface of the water in the soil, and gives the 
conditions which causes the water to pass through the ground with 
greater rapidity, and also with larger cross-section, thus increasing 
the [amount of flow from both causes. The great distance to the 
ground water before irrigation, the scanty supply, the low grade of its 
surface, would in itself show that the amount received from the nat- 
