74 THE FUTURE OF ARID LANDS 
pirical formula is being widely used in various water balance 
studies. 
Needed Inventory of Arid Climates 
It has long been recognized that in any adequate program of 
desert research a first step would be an inventory of the dry 
climates of the world. In 1951, at the first session of the Advisory 
Committee on Arid Zone Research in Algiers it was recommended 
that maps of the arid climates be prepared. As a result, Meigs 
drew up for UNESCO a series of homoclimatic maps of the arid 
zones which delineated the basic arid and semi-arid regions by 
means of the moisture indices of my 1948 classification. These 
maps were published in a provisional edition in 1952. They 
represent a starting point in such an arid zone inventory but they 
fail to provide all the information possible from such a mapping 
of arid and semi-arid regions, information which must be made 
available for a complete understanding of arid zone problems (12). 
Water Surplus and Deficiency 
Since rainfall and evapotranspiration are due to different 
things, through the year they are not often the same either in 
amount or in distribution. In some places more rain falls month 
after month than the vegetation can use. The surplus moves 
through the ground and over it to form streams and rivers and 
flows back to the sea. In others, the rainfall is deficient in one 
season and excessive in another, so that a period of drought is 
followed by one with runoff. In still other areas, month after 
month, there is less water in the soil than the vegetation could 
use if it were available. There is no excess of rainfall and no 
runoff, except locally where the soil cannot absorb the rain water 
as it falls. Consequently, there are no permanent rivers and there 
is no drainage to the ocean. 
From a comparison of the monthly march of precipitation 
with potential evapotranspiration at different stations, it 1s pos- 
sible to obtain a clearer picture of the periods of water surplus 
and deficiency and to bring into perspective the nature ot the 
water problems in an area. Figure 1 shows the march of precipita- 
