8 BULLETIX 367, U. S. DEPAETMEXT OF AGKICULTUEE. 



covering small areas. This general truth may be recogiiized 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 daiW 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 mind. 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, 30-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 normal 

 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 I, figure 1. 



