24 CONSERVATION OF GROUND WATER 



tial evapotranspiration" as a climatic factor in the following 

 description: 2 



The vegetation of the desert is sparse and uses little water be- 

 cause water is deficient. If more water were available, the vegeta- 

 tion would be less sparse and would use more water. There is a 

 distinction, then, between the amount of water that actually tran- 

 spires and evaporates and that which would transpire and evapo- 

 rate if it were available. When water supply increases, as in a 

 desert irrigation project, evapotranspiration rises to a maximum 

 that depends only on the climate. This we may call "potential 

 evapotranspiration," as distinct from actual evapotranspiration. 



We know very little about either actual evapotranspiration or 

 potential evapotranspiration. We shall be able to measure actual 

 evapotranspiration as soon as existing methods are perfected. But 

 to determine potential evapotranspiration is very difficult. Since it 

 does not represent actual transfer of water to the atmosphere but 

 rather the transfer that would be possible under ideal conditions 

 of soil moisture and vegetation, it usually cannot be measured 

 directly but must be determined experimentally. Like actual 

 evapotranspiration, potential evapotranspiration is clearly a cli- 

 matic element of great importance. By comparing it with precipi- 

 tation we can obtain a rational definition of the moisture factor. 



It has been found that when adjustments are made for variation 

 in day length, there is a close relation between mean monthly 

 temperature and potential evapotranspiration. Study of all avail- 

 able data has resulted in a formula that permits the computation 

 of potential evapotranspiration of a place if its latitude is known 

 and if temperature records are available. 



Where precipitation is exactly the same as potential evapotran- 

 spiration all the time and water is available just as needed, there 

 is neither water deficiency nor water excess, and the climate is 

 neither moist nor dry. As water deficiency becomes larger with 

 respect to potential evapotranspiration, the climate becomes arid; 

 as water surplus becomes larger, the climate becomes more humid. 



Thornthwaite's areas of dry climates — in which the poten- 

 tial evapotranspiration exceeds the average annual precipita- 



2 Thornthwaite, C. W., An Approach toward a Rational Classification of 

 Climate, Geog. Rev., vol. 38, pp. 56, 63, 75, 1948. 



