THE ROLE OF EVAPORATION IN THE SURFACE 



ENERGY BALANCE 



M.J.Blackwell 



Meteorological Office, Agricultural Research Unit, School of 

 Agriculture, University of Cambridge 



I. INTRODUCTION 



During the past fifteen years, a small research group of the Meteorological 

 Office has been working in Cambridge on the development of methods for 

 the evaluation of natural evaporation from land surfaces. For much of tliis 

 time, the subject has been viewed as a meteorological problem in turbulent 

 transfer and several observational programmes have been designed to test 

 the physical basis of an aerodynamic equation first suggested by Thorn- 

 thwaite and Holzman. Although the method is vahd only for occasions 

 when the lower layers of the atmosphere are in the near-adiabatic state, 

 Hmited practical use can be made of the method if certain conditions are 

 observed in the siting of apparatus. 



Other theoretically inspired or empirically derived modifications of this 

 equation have been shown to be capable of providing accurate results in 

 conditions other than adiabatic, but further refinement entails more exacting 

 observations requiring highly specialised techniques which tend to become 

 impracticable for routine use. Nevertheless a fundamental insight into the 

 nature of the physical mechanisms seems necessary if the more promising 

 of the simpler field techniques are to be modified to yield acceptable results 

 over a wide range of conditions. 



In order to introduce an independent and continuous check on the degree 

 of success achieved by the aerodynamic method, techniques have recently 

 been developed for measuring the four components of the energy balance 

 at the surface. To handle such a vast amount of data, modern methods of 

 data-handUng have been introduced. The main purpose of these is to 

 measure gradients or profiles by digitisation on to counters, but there is an 

 additional facihty for computing the energy balance components directly 

 by means of an analogue computer. 



The combination of the aerodynamic approach and the surface energy 

 balance control is beginning to enable us to assess the role of evaporation 

 under a wide range of conditions. It is hoped that the shortcomings in the 

 theories of turbulent transfer, which result from over-simpHfication of our 



