EVAPORATION IN THE SURFACE ENERGY BALANCE 29 



dz 





f^^HS 



which implies, over the range of vaHdity of eq. 22, that 



2/9-2 



Because of the failure of eq. 22 in stable conditions, attributed mainly to 

 the fact that z^ is no longer constant in these conditions, eq. 24 cannot be 

 applied for Ri>o. In neutral and unstable conditions, however, there is 

 excellent agreement between the parameters given in eq. 19 and 



'■©'"■ 



It is not intended to make more than a passing mention of the suggested 

 forms for f{Ri) under stable conditions. The best known are due toRossby 

 and Montgomery (1935) : 



and to Holzman (1943), who gave: 



f{Ri)^i-aRi (26) 



This latter relation has often been used as a basis for discussing experimental 

 results, even for unstable conditions, for example, by Priestley (i959)- 

 Suggested values of ct are given in the next section. 



5. PRACTICAL ASPECTS 



The first modification of the formulae given in the previous section arises 

 when measurements are required over a crop which is tall compared with 

 the heights of the apparatus above ground level. This may be allowed for 

 by replacing z by (z-d), where J is known as the zero-plane displacement. 

 Rider (1954b) used the simple eq. 14, with this modification, throughout 

 the growing season of a field of oats. For tall crops which bend appreciably 

 with wind, d can change with wind speed by a factor of two as well as 

 varying progressively with crop height. It is determined by plotting Uz 

 against loge{z-d) for suitable values of (/ until a linear relation is found. 

 Daily evaporation from similar cropped surfaces can be obtained with an 

 accuracy of about 15%, there being a systematic tendency to underestimate 

 the water loss. 



