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 canop y can be reduced to a ''big leaf''. Given the most commonly used formu lation of aerodynamic resistance (r(a)), this ''big leaf' is considere d to be, implicitly, at the d + Z(oH) level (where d is zero plane dis placement height and z(oH) is roughness length for heat transfer). Thi s cart lead to negative values of surface resistance (r(s)) when the l eaves of the top of the canopy (between d + z(oH) and crop height h(c) ) are the ones that most contribute to total water loss to the atmosph ere. To avoid this, r(a) should be computed from the top of the canopy to the reference height in the atmosphere. Also, one concludes that r (s) for complete cover crops cannot be computed by simply averaging st omatal resistance since the main condition, the driving force being th e same in all of the elements of the ''circuit'', is violated.