154 THE FUTURE OF ARID LANDS 
Figure 6 shows the probability of having individual annual rainfall 
amounts (ordinates) in the central parts of Mexico as a function 
of mean annual rainfall (abscissas). To show how to use this sort 
of diagram, which seems particularly useful in arid regions of 
high variability, we assume, for instance, that a particular agri- 
cultural plant requires on the latitudes in question at least 750 
millimeters of annual rainfall in 80% of the years in order to be 
profitable on a long-term basis. The diagram shows, from finding 
the intersection between the 750-millimeter value on the ordinate 
and the 20% probability line, that only areas having an average 
annual precipitation of more than 880 millimeters meet the 
requirements. It could be mentioned as a comparison that a 
similar diagram for the Papaloapan district shows that only areas 
having more than 1,040 millimeters annually meet the same 
requirements. 
Diagrams of this kind should be prepared for more limited 
districts in Mexico as soon as enough data are available. 
Agriculture and Variability of Rainfall 
It is evident from the maps of rainfall variability that the areas 
in Mexico with high variability coincide with those where it is 
known that agriculture is difficult. One of the best known examples 
is the northeastern area where, even if precipitation sometimes 
is sufficient, it is so unreliable that agriculture is very hazardous. 
It is also clear that the central plateau area which long has 
been known as the best for agriculture in Mexico is the region of 
lowest variability in all the country. The relation between agri- 
culture and variability is shown from a comparison between the 
maps in Figures 1, 4, and 7. The last one shows the percentage 
relation of cultivated land to the area of states in Mexico in 1940. 
Little relation is found between the amount of rainfall in itself 
and the distribution of cultivated land, but a rather good inter- 
relation exists between cultivation and variability. A good relation 
could also be found between the distribution of cultivated land 
and climatic provinces of Mexico based upon the classification of 
climate according to Thornthwaite, in which evapotranspiration 
is the essential factor. It seems, however, that in applying climatic 
