8 BULLETIN 367, U. S. DEPARTMENT OF AGRICULTURE. 
covering small areas. This general truth may be recognized any 
summer, since it is often possible to see several separate showers 
falling at the same time in different directions from the, observer. 
Another fact brought out by a study of the records is that, while 
the showers do not occur at the same time at the three stations, rain 
at one station is usually preceded or followed by rain at the others. 
In other words, while each shower is small in extent of area covered, 
the single shower is only one of many occurring in a period of time 
extending over several days, which ultimately fall on most of the 
area. The daily differences disappear when the monthly and annual 
totals are made up, and other facts appear when these are plotted 
as curves. By this method the seasonal character of the distribution 
for each station is shown, and the averages for a period of years 
bring out the difference due to elevation. 
One further consideration must be kept in ined The stations at 
which rainfall records have been obtained are all in the edge of the 
mountains, at elevations of 4,000 to 5,000 feet. The records obtained 
at MacBeath’s are about typical for the upper edge of the pastured 
areas; those from McCleary’s for the lower edge, which is at the 
same time the upper edge of the recovery pasture. The lowest part 
of the recovery pasture is about 8 miles from the mountains, toward 
the middle of a wide bolson, or basin, and 1,000 feet lower in alti- 
tude. It therefore does not get the same amount of rainfall as that 
received at McCleary’s, the nearest station. The only other station 
from which we have records bearing upon the problem is that of 
Tucson, 80-odd miles to the north and 600 feet still lower down. 
What may be called the normal annual precipitation at McCleary’s 
is about 17 or 18 inches. This amount falls upon about 16 sections 
(28 per cent) of the reserve. About 10 sections (17 per cent) of the 
reserve, most of which is pastured, gets about a 20-inch normai 
rainfall. Assuming that it is fair to interpolate between the normals 
for McCleary’s and Tucson on the basis of altitude, we have 32 sec- 
tions (55 per cent) of the reserve receiving a normal of something 
like 12 to 14 inches. Besides these general differences in precipita- 
tion, we have an increasing degree of annual fluctuation in amount 
of precipitation; a greater amount of evaporation, due to increased 
temperature; poorer,soil protection by vegetation; and longer peri- 
ods of desiccation as we go from the mountains toward the middle 
of the basin. All these factors are registered in the vegetation, both 
in its character and its quantity, and the summation of these dif- 
ferences affects most profoundly the carrying capacity of this region 
for stock. Snow in quantity, depending largely upon the elevation, 
occurs at rather rare intervals in the winter, but lies on the ground for 
only a short time. One of the heaviest snows for a number of years is 
shown in Plate J, figure 1. 
