150 THE FUTURE OF ARID LANDS 
relative interannual variability (la/’)* and the anomalies of the 
relative variability (RY){ in comparison with the Conrad’s 
standard curve showing the interrelation between amount of 
precipitation and variability for the world as a whole (1). Such 
anomalies are much better to study than the (R/) in itself as 
they are no longer influenced by the actual amount of rainfall as 
is the value of (RV). The relative interannual variability is 
shown in Figure 4 and the anomalies in Figure 5. 
The highest variability is found in the areas of low precipitation 
in the northern and northwestern parts of Mexico with a maxi- 
mum of around 50% in the extreme dry areas. A secondary maxi- 
mum of around 40% is found in the northeast, where rainfall is 
greater but the water supply conditions are difficult for agri- 
culture. Third maximas of some 30% are found in the Papaloapan 
district of comparatively high precipitation. The lowest values 
are found on the central plateau and along Sierra Madre Occi- 
dental, reaching 15-20%. 
Looking at the anomalies of Figure 5 we note that the area of 
Baja California shows high positive anomalies, indicating that 
the variability is even larger than normal. The same is true in 
northeastern Mexico, southern Sinaloa, Nayarit, and parts of 
the Gulf Coast area. On the other hand, it is interesting to note 
that the inner part of the dry state of Sonora has variability 
values much /ower than the normal conditions would give. 
It is to be expected that current investigations of the relation- 
ship between the general circulation and precipitation in Mexico 
will explain the reasons for these extreme areas of anomalies, but 
some ideas may be expressed now. 
Thus, it seems likely that the reason for the abnormal variability 
in Baja California, Sinaloa, Tamaulipas, and the Gulf of Tehuan- 
tepec are the hurricanes which frequently hit these coasts in fall. 
Studies of the variability of October precipitation suggest this 
TaV 
x (Ia) ef (1 — pr») at (pe — ps) tee. tb (Pri — Pn) \/ Pro; (laV) rei = a . ) 
p = mean annual precipitation; 7 = number of years on record. 
T{[RY] = 100° where ¢ = Bay where d; = pi — ?p. 
n 
