ABSTRACT 



Basic theory of water potential in the soil-plant-atmos- 

 phere continuum is considered, together with a review of 

 the use of thermocouple psychrometers. The construction, 

 methods of water potential measurement, and calibration of 

 thermocouple psychrometers are discussed in detail. The 

 effects of protective ceramic cups on the rate of water vapor 

 exchange between the soil mass and the internal psychrom- 

 eter cavity are compared with the effects of fine screen cups. 

 The ceramic cup imposes a much greater resistance to vapor 

 exchange than the screen cups; the magnitudes of possible 

 errors resulting from this resistance in estimates of water 

 potential are discussed. Possible uses of thermocouple psy- 

 chrometers for measuring water potentials of soils and plants 

 under intensive forest management practices are considered. 



The use of trade, firm, or corporation names in this publication is for the information and con- 

 venience of the reader. Such use does not constitute an official endorsement or approval by the 

 U.S. Department of Agriculture of any product or service to the exclusion of others which may 

 be suitable. 



