EVAPORATION IN THE SURFACE ENERGY BALANCE 25 

 itself, since : 



A (i + B) ^"' 



Unfortunately, in arid regions whete accurate knowledge of evaporation 

 is vital, B is no longer small and KhjKw may vary widely from unity. 



In several respects, therefore, we have come to regard the Bowen method 

 as a rough check for E, but of less use for Q, e.g. in studies of eddy conduc- 

 tivity Kh- Considerable discussion has taken place in the hterature, e.g. 

 Rider andRobinson (195 1), on why the diftusivities for heat, vapour and 

 momentum should not be identical when the profiles of the elements have 

 the same form, as this impHes that the ratios of the gradients, and hence of 

 the diffusivities themselves, must be constant with height. It must be 

 remembered however that, with the accuracy until recently possible in 

 measurements of the elements, the test of the identity of the profile shapes 

 is not very critical. Management of wet- and dry-bulb thermocouples has 

 always been difficult, with corresponding scatter being caused in humidity 

 profiles ; recording of temperature is affected by radiation and emergent- 

 stem errors ; while wind profiles measured with sensitive cup anemometers 

 have recently been shown to be much afl'ected by position errors (Rider, 

 i960). There is also the problem of deciding, for a particular site, whether 

 typical profiles have been estabhshed in the available distance upwind or 

 witliin a given time of measurement. Until profdes can be obtained, as a 

 routine, with an accuracy sufficient to determine their second derivatives, 

 controversy about the ratios of the diffusivities is unlikely to be settled by 

 this method and hopes must rest with the eddy fluctuations approach. 



Although there is no simple method for measuring Q, a most promising 

 direct check on E has recently become available following the development 

 of a very large soil-balance by Morris (1959). One of these has been used 

 successfully at Cambridge at intervals during the i960 season, both 

 evaporation and dewfall being recorded satisfactorily. The soil container 

 carries some 2000 Kg of soil which is representative of the site and is 

 covered with a crop of short grass. The latter is accurately coplanar with the 

 natural surface, with as small an air gap as possible. The load is supported 

 on a triangular structure wliich in turn is supported by three levers, one of 

 which receives the load from the other two. In this way the total load can 

 be balanced by a single load from a secondary lever. Having roughly 

 balanced the weight of soil, changes of weight are balanced by the 

 movement of travelling weights, driven by a synchronous motor which 

 is controlled by a self-balancing optical system. 



