CLIMATOLOGY IN ARID ZONE RESEARCH 81 
The irrigation scheduling procedure which was developed for 
use in the humid climate of New Jersey will also apply in arid 
climates; but here the problem is even simpler than in the moister 
climates because there 1s little or no need to be concerned over the 
possibility of rainfall affecting the program (13, 16). Thus, it 
would not be necessary to provide a “‘safety factor,’ and one 
could schedule irrigation to bring soil moisture just back to field 
capacity in the desired root zone each time. 
Estimating Water Needs 
In the arid and semi-arid regions of the world, agriculture 1s 
not feasible without recourse to supplemental irrigation. It is 
therefore of primary importance that reliable information on the 
magnitude of the water need in these areas be made available. 
Computations of the water need for the various climatological sta- 
tions in the states of Utah, Colorado, Arizona, and New Mexico 
are plotted in Figure 4. It varies from a little over 60 inches in the 
hot, dry regions of southwestern Arizona to less than 15 inches in 
the mountainous regions of Colorado. Irrigation engineers have 
attempted to arrive at figures for water need from the few avail- 
able measured observations of evaporation from pans. Meyer has 
presented a map of the evaporation from shallow lakes and reser- 
voirs which indicates values of over 100 inches a year in the south- 
western part of Arizona and up to 80 inches a year in New Mexico 
(7). Horton, analyzing records from Weather Bureau Class A 
evaporation pans, found values of evaporation of over 120 and go 
inches a year respectively in these same areas (2). Horton’s map 
of evaporation from pans has been reprinted, but it is reprinted 
as a map of potential evapotranspiration (8). 
The high values of evaporation which have been found by 
Meyer and Horton without due consideration of the effect of the 
size of the evaporating area on the rate of evaporation have been 
used to arrive at figures for water need. They have led to false 
conclusions concerning the water requirements in these arid areas 
and, in certain cases, have resulted in the excessive use of water 
with damaging results to the soil. It is time for irrigation engi- 
neers and others who are attempting to determine the water needs 
and requirements of these dry regions to reevaluate the available 
