IMPORTANT QUESTIONS RELATING TO IRRIGABLE LANDS. 83 
are some limited localities where a small amount of rainfall in the season 
of growing crops seems to be constant from year to year. In such districts 
irrigation will only be used to supply deficiencies. 
The complicating conditions arising from soil and ‘subsoil are many. 
Experience has already shown that there are occasional conditions of soil 
and subsoil so favorable that the water may be supplied before the growing 
season, and the subsoil will hold it for weeks, or even months, and gradually 
yield the moisture to the overlying soil by slow upward percolation or 
capillary attraction during the season when growing crops require its fer- 
tilizing effect. When such conditions of soil and subsoil obtain, the con- 
struction of reservoirs is unnecessary, and the whole annual supply of the 
streams may be utilized. On the other hand, there are extremely pervious 
soils underlaid by sands and gravels, which speedily carry away the water 
by a natural under drainage. Here a maximum supply by irrigation is 
necessary, as the soils must be kept moist by frequent flowing. Under 
such conditions the amount of water to be supplied is many fold greater 
than under the conditions previously mentioned, and between these 
extremes almost infinite variety prevails. 
Practical agriculture by irrigation has also demonstrated the fact that 
the wants of different crops are exceedingly variable, some requiring many 
fold the amount of others. This is due in part to the length of time 
necessary to the maturing of the crops, in part to the amount of constant 
moisture necessary to their successful growth. But by excluding the varia- 
bility due to rainfall, and considering only that due to differences of soils 
and crops, and by taking advantage of a wide experience, a general 
average may be obtained of sufticient accuracy for the purposes here in 
view. 
In examining the literature of this subject it was found that the 
experience in other countries could not be used as a guide in considering 
our problems. In general, irrigation in Europe and Asia is practiced only 
to supply deficiencies, and the crops there raised are only in part the 
same as with us, and the variation on account of the crops is very great. 
Certain statements of Marsh in his ‘“Man and Nature” have been copied 
into the journals and reports published in the United States, and made to 
