20 BULLETIN 1207. U. S. DEPARTMENT OE AGRICULTURE. 
The ground-water table is by no moans horizontal, but is more 
nearly parallel to the ground surface throughout the irrigated area. 
As a result of this condition two important phenomena occur: First, 
there is a well-defined lateral movement down slopes; second, the 
application of water to higher land results almost immediately in a 
rise of the ground water down the slope, often for considerable dis- 
tances, due to hydrostatic readjustment. 
A rise of the ground-water table docs not come alone from the 
application of irrigation water to the particular tract involved but 
also from seepage from canals and ditches, both within and without 
flu 1 area, and seepage from the irrigation of higher lands. 
The injury from a high water table is not confined to low-lying 
lands but in most cases it becomes more or less general throughout 
a given district. The higher lands often show injury first, and in 
many instances injury is most pronounced on the higher lands, par- 
ticularly if they are situated at the foot of sharper slopes on which 
are leaky canals, or are adjacent to still higher irrigated lands. 
Tt is manifest that the need for drainage is brought about by the 
artificial introduction of water into the area involved, and since this 
introduction is effected for the common good the factor of common 
responsibility is introduced. Relief from such responsibility is one 
of the factors of special benefit in an irrigated district and may be 
considered in the assessment of benefits to lands and to irrigation 
canals passing through or by the district and sharing in the re- 
sponsibility. 
Another important difference between humid and arid projects is 
that in the case of the former conditions are stable, so that with a 
given precipitation or given river stage certain lands require definite 
protection or improvement; while in the case of the latter, condi- 
tions are variable and injury is more or less progressive, involving 
more acreage as the years pass and becoming more and more serious. 
Protection against possible future injury is therefore an important 
consideration in the assessment of benefits in an irrigated drainage 
district. 
The need for actual reclamation of the lands rendered more or 
less unproductive because of water-logging or the accumulation of 
salts is. of course, the prime and by far the most important factor 
of special benefit. 
The amount of drainage furnished does not have so important a 
bearing as in humid districts where either outlet systems or com- 
plete systems may be installed. Under conditions obtaining in the 
arid section outlet drainage systems are not feasible, and it is the 
business of drainage districts to install complete systems. As a rule 
all lands in a district receive full drainage, the usual exceptions being 
lands lying too low to be afforded full drainage depth or lands al- 
ready naturally or artificially drained or partly drained. 
Measure of special benefits. — The courts have almost universally 
held that the measure of the benefit received by agricultural land 
from drainage improvements is the increased value of the land. 
This principle has been announced by many courts. Judge Cooley 
on Taxation, page l w 2r»f, third edition, says: 
It has been said that, in assessing benefits, the only safe and practicable 
course, and the one which will do equal justice to all parties, is to consider 
what will he the Influence of the proposed Improvement on the market value of 
