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 canlead to negative values of surface resistance (rs) when the leaves of the top of the canopy (between d + zoH and cropheight hc) are the ones that most contribute to total water loss to the atmosphere. To avoid this, ra should be computedfrom the top of the canopy to the reference height in the atmosphere. Also, one concludes that rs for complete cover cropscannot be computed by simply averaging stomatal resistance since the main condition, the driving force being the same inall of the elements of the circuit, is violated.