The surface resistance in an explicit single big leaf can
be considered as a direct function of parameter values either from a PG file or
from a parameter table, or it may be calculated as a function of leaf area
index, *A** _{l}*, global radiation,

where *g _{l}* is the leaf conductance which is
given by the Lohammar equation (Lohammar et al., 1980; Lindroth, 1985) as:

where *g _{ris}*,

The Lohammar equation can optionally be used only during
the growing season. In this case the maximum conductivity after and before the
growing season (i.e. during winter) is given by the parameter
*g _{maxwin}*. This forth
alternative is only valid for explicit single leaf simulations.