48 J.L.MONTEITH 



(Penman and Long showed that a crop may gain heat from the soil radia- 

 tively at a rate comparable with C„ : here the term R includes this exchange.) 

 Combination of eqs. 13-15 then gives 



WdlWu=RlG-i (16) 



The theoretical Hmit G=R is observed when the atmosphere is so calm 

 and stable that atmospheric fluxes of heat and vapour are both neghgible. 

 Then lVd=o; I^M = (AR/A(A + y)). With incrcasmg wind speed, turbulent 

 transfer rates increase and soil heat flux decreases. Theoretically, when 



Cd 



CROP 

 CANOPY 



Fig. 2. Heat balance of crop canopy. For symbols, see text. 



G->o, lVu->o and H^<i^(AR/A(A + y)), but this condition is never met in 



nature. 



When Wd and Wu are both fmite, the ratio R/G depends on the relative 

 thermal properties of soil and atmosphere. Relevant parameters for the 

 soil are volume heat capacity p^c^ (cal/cm^) and thermal diffusivity k 

 (cm2/sec) ; and for the air, volume heat capacity p-zc^ and the turbulent heat 

 transfer coefficient Kjy (cm2/sec) . Priestley (1959) showed that the assump- 

 tion of constant k and Kh, though far from reahstic, gives satisfactory heat 

 flux ratios if their values are chosen 'some distance from the surface', and 

 he derived an expression from which 



RIG 



Pl^lVx 



(17) 



