Potential evaporation

In forests, evaporation of intercepted water may considerably exceed transpiration rates with equivalent local-climatic conditions.

When potential transpiration is used as a driving variable, i.e. for implicit big leaf simulations, a constant relation between wet surface evaporation rate and potential transpiration rate is assumed:

                                                               (3.65)

where e_rat is a parameter.

Otherwise the potential evaporation rate, E_ip, from interception storage is calculated from the Penman combination equation assuming a surface resistance, r_sint, representing the resistance to the single source point of the whole canopy, see eq.(3.12). See viewing function Potential interception evaporation.

The potential interception evaporation rate, E_ip, is decreased if the water on the leaves does not cover the entire leaf, as determined by the parameter, i_fracmin:

                                         (3.66)

where S_i is the interception storage and S_imax is the interception capacity.

When the Penman combination equation is used to calculate E_ip, the e_rat value is calculated with eq. (3.65), and used for example in eq. (3.67).