AERODYNAMIC AND SURFACE RESISTANCES OF COMPLETE COVER CROPS: HOW GOOD IS THE “BIG LEAF”?
The Penman-Monteith equation is based on the assumption that the canopy can be reduced to a “big leaf”. 
Given the most commonly used formulation of aerodynamic resistance (ra), this “big leaf” is considered to be, implicitly, 
at the d + zoH level (where d is zero plane displacement height and zoH is roughness length for heat transfer). This can 
lead to negative values of surface resistance (rs) when the leaves of the top of the canopy (between d + zoH and crop 
height hc) are the ones that most contribute to total water loss to the atmosphere. To avoid this, ra should be computed 
from the top of the canopy to the reference height in the atmosphere. Also, one concludes that rs for complete cover crops 
cannot be computed by simply averaging stomatal resistance since the main condition, the driving force being the same in 
all of the elements of the “circuit”, is violated.