188 
IOWA ACADEMY OF SCIENCE Vor,. XXVII, 1920 
to note that over the United States this averages about 1500 
cubic miles each year, enough to cover the national domain to a 
depth of thirty inches. In Iowa the rainfall averages a little 
more — nearly thirty-two inches, ranging from thirty-four inches 
in southeastern Iowa to tw'enty-eight in the northwestern por- 
tions. This means that on the average upon each of Iowa’s 
35,575,000 acres there falls annually 116,000 cubic feet of water, 
a total for the state of twenty-eight cubic miles. 
Now what becomes of this vast quantity of water? The experts 
tell us that substantially one-half evaporates and is returned 
quickly to the atmosphere. One-third runs into the rivers and 
back to the sea and hence is known as the run-off. The remain- 
ing one-sixth is consumed by plants or, escaping them, goes 
deep enough to join the permanent body of water beneath the 
surface, the underground reservoir, the ground water. It is this 
latter one-sixth that we are to consider just now. 
Let it be understood that despite popular conception there are 
no extensive underground lakes and very few subterranean 
streams. Such as do exist are gathered chiefly in the relatively 
rare caverns and devious passageways in the upper portion of 
the earth’s crust. The great body of the ground water is dif- 
fused through the pores and crevices and other small openings 
in the rock and soil of the solid earth. The upper surface of 
the ground water the ground water level or water table — - is 
somewhere near the surface of the earth. The lowest extent of 
the ground water is about six miles, below which depth the pres- 
sure of overlying material is so great as to render the subjacent 
rock impervious. Most of the waters in much the greater portion 
of this zone must, however, remain permanently inaccessible to 
man through any means. It is only the water content of the top- 
most few hundreds of feet which can be counted on to be of any 
service to humanity or indeed to life forms of any kind. 
What then is the amount of available ground water, how may 
it be best utilized and by what means shall we seek most intel- 
ligently to conserve it? These are the questions which most con- 
cern us in this inquiry. 
Relation to Rock Strata . — Of the stratified rocks of Iowa the 
Sioux quartzite contains probably less than one per cent of 
voids, the limestones contain one to five per cent and the sand- 
stones contain five to twenty-five per cent. Doctor McGee has 
estimated that the amount of water in the outer one hundred 
feet of the rocks of the United States is sufficient to cover its 
