HISTORY AND PROBLEMS ) 
TABLE 2 
Area of Arid Lands Based on Climate 
Semi-arid 8,202,000 
Arid 8,418,000 
Extreme arid 2,244,000 
Total 18,864,000 
ercent 530 (Land area 51,970,000) 
Drought due to lack of moisture is characteristic of the arid 
zone. There are areas at high elevations and at high latitudes with 
low precipitation where drought is not a characteristic. High 
temperatures are generally characteristic of the arid zone but they 
are not limited to this zone. The extremely high temperatures in 
the arid zone are largely due to the absence of water for evapora- 
tion or transpiration to cool the air. In the arid zone there is often 
a marked inverse ratio between evaporation rate as measured and 
the actual loss of water from the region. In an area with no 
water to transpire or evaporate the measurements of loss from a 
water surface or a wet bulb or any other measurement of dew 
point will be very high although no water may be lost from the 
land. Absence of precipitation has no damaging effect on vegeta- 
tion if the soil has a supply of available moisture. This is evident 
in any irrigated region where drought can occur only when the 
source of irrigation water is cut off. 
The vegetation cover is a measure of aridity, but here also there 
are difficulties in application. An example will serve to illustrate 
this difficulty and to emphasize the danger of assuming that 
measurements based on a single species can be applied to all 
plants, as is so generally done. The open sand blowouts in eastern 
Colorado were being invaded and partly held down in 1915 by 
Redfieldia flexuosa, Psoralea lanceolata, and the Andropogons 
gerardi, hallit, and scoparius, and an occasional plant of Muhlen- 
bergia pungens. During the dry hot years of the thirties these 
plants were killed out, with the exception of the Muhlenbergia. One 
would have expected the open sand areas to be greatly extended, 
but just the opposite occurred for the sand areas were rapidly 
covered by Muhlenbergia pungens, a grass more at home on the 
hot arid sands of Arizona and New Mexico and before the hot dry 
years only occasionally present in eastern Colorado. Had the 
measurement of the years been made on the basis of the amount of 
