52 
SEEPAGE OR RETURN WATERS FROM IRRIGATION. 
ural rainfall is small, and if this furnishes any inflow at all to the 
streams, it must be but a small proportion of the amount at present 
furnished under the conditions introduced by irrigation. 
§ 49. From the mountain water-shed of the Poudre river our 
observations show that from four to six inches of water runs off 
from the whole area during the course of the year.* From the 
plains included in the measurements reliable observations are lack¬ 
ing. From the curve shown, by F. H. Newell, in the report of the U. 
S. Geological Survey for 1892-3, the amount of run-off may be 
estimated as from two to four inches. The amount varies with the 
soil, the slope of the ground, and the character of the rainfall. 
When the precipitation is in slight showers, nearly all the rain¬ 
fall evaporates within a short time, without penetrating more than 
the surface of the soil. It requires a heavy rain to saturate more 
than the surface, and furnish some water for percolation. In the 
ordinary condition, a rainfall of two inches will penetrate not over 
ten or twelve inches. Heavier rainfalls within a short time are 
needed before there can be any percolation from the rain. On beds 
of pure sand most of the water immediately soaks in, and very lit¬ 
tle is lost either by evaporation or by run-off, hence it is that water 
is generally found at moderate distances from the surface in the 
sand hills. There have been but twentv-two months in eleven 
« 
years of observation at Fort Collins in which the total rainfall in 
one month has exceeded two inches, and in only eleven cases has 
as much as this fallen in one week. If the rain falls rapidly a 
larger proportion runs off than when there is time to soak into the 
ground. The case most favorable to percolation which our records 
show is in 1895, when two rainfalls, each of 2.5 inches, followed each 
other with only a few days interval. The first one nearly all soaked 
into the ground. The second fell on a ground already saturated 
and nearly all ran off, causing unusually high water in the streams 
in consequence. There are only one or two other cases in which as 
much as three inches fell within a few days. But even here, the 
most favorable of the cases, if the ground is dry, which is its ordin¬ 
ary condition, there cannot be much percolation, and it is very 
doubtful if there is any. 
If, however, the ground is already wet, as may be the case with 
the lands which have been irrigated, and the surface is loose and 
porous so as to absorb the rain as it falls, as is the case with culti¬ 
vated lands, the<re is reason to expect that the rain will cause an 
increase in the underground flow. The rainfall alone, without the 
irrigation, would not cause it, and it is a consequence of the arti¬ 
ficial conditions introduced by irrigation, and may properly be con¬ 
sidered as due to irrigation. A portion of the unusual increase 
^‘Annual reports, 1890, 1891, etc. 
