8 
is one of such common observation that there is no reason to ques¬ 
tion it. Many times land may cease to need water at all, and 
may require drainage. The cause of the lessened need is con¬ 
nected with the change in the level of the ground water, which 
is universally observed. After irrigation the soil gradually becomes 
saturated with water, and the level of water in the wells rises in.the 
course of a few years sometimes forty feet. After the level has ap¬ 
proached the surface, the water which the soil will permit is only that 
needed by the crops and evaporation, and enough to supply the loss 
of the ground water by lateral or downward percolation. In the 
earlier years enough has to be supplied to fill up the sub-soil, and as 
ordinary soil holds a large percentage of its volume of water, the 
duty of later years is materially increased. 
It is evident that a permeable or impermeable sub-soil, and its 
distance from the surface of the ground, will affect the duty. If 
impermeable and close to the surface, there will be little soil to fill, 
there will be a higher duty, and more care will be necessary on 
the part of the irrigator, or he will drown out his crop. A very 
porous sub-soil, as is found in many cases in our river bot¬ 
toms, and near mountain streams, requires frequent and abund¬ 
ant irrigations in general, as the water passes through it like 
a sieve. It is because the sub-soil is of this character that the duty 
in Northern Italy is so small. Lands which naturally sub-irrigate, 
as in the San Luis Valley in this State, and the San Joaquin Valley 
in California, are those where the impermeable sub-soil is close to 
the surface, and lateral percolation may readily take place, because 
of the character of the surface soil. 
The character of the fiow of the supplying stream also affects 
the duty of the water derived from it under the conditions of Colorado 
and most of the Western States. The streams being fed by the melt¬ 
ing snows are high in Ma}^ or June, and low in late summer. In 
consequence, while there may be an excess of water in the former 
month, there may be a deficiency in August and September. In 
many, if not in most cases, there is not sufficient w^ater in late sum¬ 
mer, and the crops do not receive what they should, or what their 
owners would apply if it were to be had. In consequence, it does 
not follow that an increase in water in late summer would give an 
increased acreage, but that the area cropped would give better returns 
and the duty would be less. The cereals which mature early, fre¬ 
quently receive all that would be given them. But alfalfa and 
other forage crops would receive in most cases one or more irriga¬ 
tions in addition to the two which are now generally given them. 
In consequence of this it does not always follow that the duties ob¬ 
tained by dividing the acreage cropped by the water supplied to a 
canal gives a measure of the relative needs of different communities. 
It is partially,, if not entirely, due to this that the newest canals 
will generally give a high duty, for their water supply may be small 
in proportion to the area underneath, and the early canals with 
