EVAPORATION IN THE SURFACE ENERGY BALANCE 31 



scatter of the results of the three programmes, shown in Fig. i, would be 

 fitted to a first approximation by two straight hnes, implying that the 

 corresponding extrapolation of the fluxes would also be linear. 

 Rider (1957) made further measurements over bare soil and over three 



k^f(Ri) 



0-9- 



o-e- 



0-7. 



0-6- 



0'5. 



0-4- 



0'3- 



0-2' 



oo«. 



-400 



unstable 



neutral 



-300 



-200 



t 



-100 



stable 



k^(l - l5Ri) for Ri <0 

 k^{l - 7Ri) for Ri> O 



• • • • results of three observational 

 programmes 



o 



100 



200 



300 



Ri X lO-^ 



, 400 



Fig. I. Showing the variation of eddy diffusivity with stabiHty of the atmosphere. 

 The simple aerodynamic formula gives ky{Ri) — const, (i.e. o- 17) • Possible modifications 

 are indicated by the straight lines k^{i-aRi). 



different crops which were said to be transpiring at their potential rates, 

 and concluded that the water losses could be significantly different for the 

 three crops. It was found that the humidity gradients over neighbouring 

 crops could vary over fairly wide hmits. This work suggested re-examina- 

 tion of the assumption, that potential transpiration is the same for all green 



