2 THE FUTURE OF ARID LANDS 
Individual 
annual rainfall 
1000 
500 
Mean 
annual rainfall 
500 1000 
Figure 6. Probabilities of having individual rainfall amounts (ordi- 
nates) in the central plateau of Mexico as a function of mean annual 
rainfall (abscissas). 
idea. In the areas of rather large precipitation, as in the Papa- 
loapan district, local topographical conditions must be decisive 
in explaining the variability, especially in connection with 
fluctuations in the general direction of the rain-giving easterlies. 
Frequency Distribution of Rainfall Amounts 
In studying the frequency distribution of annual rainfall a 
special risk diagram suggested by Landsberg (3) was applied. 
