CLIMATOLOGY IN ARID ZONE RESEARCH 79 
arid climates the moisture deficiency is the most significant value 
than can be determined from the water balance bookkeeping pro- 
cedure since it gives an indication of the severity of the water 
problem and reveals how much water must be supplied in other 
ways. It is thus necessary to carry the bookkeeping procedure one 
step farther to determine the year-to-year fluctuations in the 
moisture deficiency and to derive the probabilities of occurrence 
of different levels of deficiency. Figure 3 gives the magnitude of 
the water deficiency in the Missouri Valley area of the United 
States for the period 1920-44. This region is one of critical impor- 
tance in any research on arid and semi-arid zone problems as it 
marks the transition zone between regions where agriculture is 
feasible without irrigation and where it is not. Average annual 
values have little meaning in this region. The limit of economi- 
cally feasible agricultural production can vary so greatly over the 
region from year to year that probabilities based on a number of 
years of observations must be studied in any intelligent approach 
to the problem. 
Daily Water Balance Bookkeeping 
It is possible to compare the precipitation with the evapotrans- 
piration or water need on a daily as well as a monthly basis. If 
this is done and a regular account is kept of the day-to-day addi- 
tions and withdrawals from the soil moisture bank, it is possible 
to determine when moisture is lacking and replace it through ir- 
rigation. This insures that there is never any deficit of water in 
the soil and also that there is no overirrigation and wasteful misuse 
of the water resource. An irrigation schedule can be derived as a 
natural result of the daily bookkeeping of precipitation and water 
use. 
Until recently the most common practice of farmers had been 
to watch the crops for signs of moisture deficiency and to irrigate 
an undetermined amount when such signs were recognized. This 
practice was far from satisfactory, for when the crops showed 
signs of distress, a considerable reduction in yield had already oc- 
curred. The irrigation bookkeeping procedure eliminates this prob- 
lem by telling when and how much to irrigate before any defi- 
ciency exists in the soil. 
