7 
whole. It is disadvantageous, for the loss in the large number 
of channels and in being spread in shallow depths is greater than 
when kept in more compact masses. Besides, after water has once 
been used, it can afterward be applied only to a more restricted area ; 
to the land which is lower down the valley. Its reappearance, if it 
disappears, is only after the lapse of some time, so that it cannot be 
applied at will. 
GENERAL CONSIDERATIONS AFFECTING DUTY. 
The amount of water actually needed, as every irrigator knows, 
varies according to many conditions. The method of irrigation, the 
slope of the ground, character of the soil, kind and character of sub¬ 
soil, the crop, amount of rainfall, the use of water in large or small 
heads, preparation of ground, the skill and knowledge of the irri¬ 
gator, thorough cultivation. 
In general, the more rain the less irrigation needed. This is 
true for crops of the same character and in the same community It 
is not necessarily true of different communities widely separated, nor 
of different crops where irrigation is carried on not from necessity of 
drouth, but as a means of furnishing nutriment to the plant. The 
amount used may be very excessive, as in the hay lands of the 
Vosges in France, which use over 200 feet in depth per year. 
Certain methods will be best adapted to certain slopes and 
crops. With a given method there is a slope of the ground 
at which a given amount of water will do the most work. The ob¬ 
ject being to reach the roots of the plants, unnecessary slowness in 
the w^ater permits increased evaporation, and perhaps unnecessary 
absorption. Much more water is needed for a thorough irriga¬ 
tion than one unaccustomed to irrigation would think necessary, but 
the experience of all countries finds it practically impossible to make 
an irrigation with a depth of less than three inches of water on sod 
ground, and from four to six on cultivated crops. 
Different crops require different amounts of water and at differ¬ 
ent periods. Grasses being grown for forage, an increase of water 
usually means an increase in product. With the cereals, as well as 
with grasses when grown for seed, there is a limit beyond which 
irrigation may be detrimental. Different cereals, as well as different 
vegetables, have different powers of withstanding excessive moisure 
on the one hand or drouth on the other. Hence irrigation is ap¬ 
plied with greater care, and perhaps more frequently in case of 
scarcity, to the one crop than to the other, and the duties obtained 
under the conditions of ordinary practice will vary in consequence. 
A soil retentive of moisture will need fewer irrigations than a 
sandy soil, and if the irrigations in the two cases can be made with 
the same depths of water, will furnish a higher duty. 
It is a common observation throughout the irrigated valleys 
that land requires less water after it has been irrigated a series of 
years. Though w^e have no definite measures on this point, the fact 
