34 M.J. BLACKWELL 



determinations of profiles or gradients will clearly be influenced by eddy 

 patterns which may result from temporary horizontal inhomogeneity. In 

 the mean profiles, for a suitable site with adequate fetch, these cross- 

 correlation terms effectively cancel out. Furthermore, the concept of eddy 

 transfer coefficients is only vahd, at best, for describing mean flow condi- 

 tions. Although, therefore, the means of many computed values of £ and Q 

 are eventually compared with the values derived from the mean profiles, 

 there is no a priori reason for expecting perfect agreement. It is also thought 

 that the periods for which the profiles are obtained should be reduced to 

 about 1 5 minutes to ehminate diurnal effects, and errors caused by averaging 

 non-linear quantities. 



Prehminary results obtained over the last three seasons are still being 

 evaluated but the following tentative trends can be seen. In the cool, wet 

 season of 1958, an energy balance appeared to be estabhshed over some 80 

 hours of data. During the hot, dry season of 1959, an energy balance was 

 often not obtained during the 180 hours of recording but this could not be 

 expected with the much wider range of stabihty encountered, since the 

 original equations are based on near-neutral stabihty conditions. The use of 

 f{Ri) for these results is being investigated. Effects of storage in the surface 

 layers of the soil were of significance for short periods in 1958. Cracked soil 

 and evaporation in depth, mentioned in section 3 , caused difficulties in 1959. 

 A very important point, arising from the 1958 results, was that in many 

 circumstances the energy used in evaporating water is roughly equivalent to 

 the net radiative input (House, PJder and Tug well, i960). This was not 

 substantiated by the 1959 work, and it appears that the whole question 

 will require further investigation. It is too soon to give any definite results 

 from the i960 data, but among the more novel aspects are the first available 

 comparisons of £ (aerodynamic) v. E (soil balance), both evaporation and 

 dewfall being recorded satisfactorily. There are also a few soil flux measure- 

 ments at different depths, from which the storage error can be determined 

 by the method described in section 3 . 



7. CONCLUSIONS 



A rather over-simphfied version of the two standard meteorological 

 approaches to the measurement of evaporation, as followed at Cambridge 

 between the years 1 947-1 961, has been given as an attempt by a physicist 

 to describe the successes and hmitations of current theories and methods, in 

 the hope of giving workers in allied subjects an up-to-date survey of the 

 present state of knowledge. 



