CLIMATOLOGY IN ARID ZONE RESEARCH Ul 
define climatic provinces is not well understood. Recent studies 
have indicated that crop growth is closely related to potential 
evapotranspiration, or the water use of plants (15). Thus, the an- 
nual potential evapotranspiration provides an index of the growth 
potential of an area. A single parameter, annual potential evapo- 
transpiration, because of its dependence on the energy balance, 
can serve both as a moisture and a thermal index. 
The indices are important in defining world climatic regions. 
Their greatest value, however, lies in specifying at every point a 
thermal growth souenclal and the degree of moistness or aridity 
of a climate. In other words, the indices are continuously dis- 
tributed about the earth and do not exist merely along inter- 
provincial boundaries, as do Képpen’s limits. 
As part of a task which the Laboratory of Climatology has 
undertaken for the Office of Naval Research, maps are being pre- 
pared of precipitation, potential evapotranspiration, water sur- 
plus, water deficiency, and the moisture regions for all parts of 
the world on a scale that is consistent with the density of the 
climatic network. As an example, maps of Japan, Korea, and For- 
mosa were made in scale 1:1,000,000. Maps of the entire con- 
tinent of Africa have been completed in color, using the new Amer- 
ican Geographical Society base map on a scale of 1:3,000,000. It 
is anticipated that the mapping of the entire earth will be com- 
pleted within a short time. These large-scale maps give a clear 
picture of the distribution of the arid and semi-arid regions and 
add considerably to an understanding of their water balance 
problems. For instance, from such detailed maps it 1s possible to 
work out exactly the overall irrigation needs and the water sup- 
ply that is available from precipitation. Such information is basic 
in any intelligent evaluation of the water resource potential. 
Variability in Climatic Factors 
The inventory of the arid and semi-arid regions of the world 
should go farther than merely making available the average an- 
nual distribution of the different moisture regions. It has long been 
recognized that variability is one of the main characteristics of 
dry climates. The reliability of precipitation becomes less as the 
