EVAPORATION IN THE SURFACE ENERGY BALANCE 27 



dynamic method relies on the eddy viscosity being expressible in terms of 

 relatively easily measured wind profiles. In the simplest case of near-neutral 

 stability, we obtain from eqs. 2 and 4 that : 



Km=k^Z^~ (16) 



Thus from eq. 5, Pasquill (1949) was led to investigate the non-dimensional 

 parameter : 



(17) 



3m 3x ^%^ 



dz dz dz 



If Km = Kw, then values of this parameter should equal k'^ [^^o-ij). By 

 determining £ from small evaporimeters and careful weighing, it was shown 

 that the near-neutral experimental results were grouped closely about o- 1 7. 

 A direct verification of the identity Kw = Km was therefore provided for 

 near-adiabatic conditions. 



By measuring tq with drag plates,Rider (1954a) was further able to show 

 that the non-dimensional parameter : 



,3„ ('«) 



z 



dz 



had virtually identical values to those given by the parameter in eq. 17, 

 not only in neutral conditions but over a wide range of stabihties. This 

 makes the identity Kw = Km of general appHcation. 



BothRider (i954a) and PasquiU (1949) found that such parameters varied 

 systematically with the departure of the wind profiles from the simple 

 logarithmic form of eq. 2, or with the Richardson number Ri. In general, 

 this stabiHty dependence can be expressed : 



= fe'/(^') (19) 



dzdz ^ \dz) 



The major problem now facing us is the specification of the function, 

 f{Ri). First of all it must be remarked that wind profiles tend to approach 

 the simple logarithmic form near the surface, and correspondingly that 

 variations of stabihty or Ri become less for measurements made near the 

 ground. This has dominated most practical apphcations of the method 

 (e.g. Rider, 1954b, 1957) and it seems probable that fluxes based on the 



