We use a single land-surface layer
where is a land heat capacity; since this is small, on time scales much longer than a day the condition is essentially zero net surface flux.
Evaporation over land is modified as
where is the aerodynamic resistance; is similar to the a combined stomatal and root resistance (Dickinson 1984), and increases as the soil wetness w drops. Evaporation thus becomes less dependent on wind speed and roughness for low soil moisture, akin to biophysical models.
For interactive soil wetness, we use a single layer formulation representing the root zone
with P the precipitation and R the runoff.