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.