Models in Irrigation andWater Management - Practices of Irrigation and On-farm Water Management - page 399

S = storage of water at soil surface (m)

P = precipitation (m)

E = actual evapotranspiration (m)

R = runoff (m)

Evapotranspiration

In FLOCR, water extraction from the soil profile by evapotranspiration is limited
to the top compartment. When the top compartment dries, evapotranspiration is
reduced in the following way:

For h > a , E r = E p

⎨ ⎫

⎬

h − a

For h < a , E r = E p 1 +

(10.22)

⎩ 1

b +

⎭

a

In which

E p = potential evapotranspiration (cm/d)

E r = reduced evapotranspiration (cm/d)

h = pressure head of the top compartment (cm)

a = pressure head above which E r = E p (cm)

b = soil parameter. The value -1/ b describes the pressure head which below no

water is extracted from the soil profile.

Surface Runoff

Surface runoff is only possible when cracks are closed. When cracks are closed
runoff occurs after a certain threshold value (i.e., maximum surface storage) is
exceeded.

R = S − S max (10.23)

where

R = surface runoff (cm)

S computed storage of water at soil surface (cm)

S max =

=

maximum storage of water at soil surface before runoff begins (cm)

Infiltration into Matrix and Cracks

When rain water reaches the surface of a cracked clay soil, part of the water infil-
trates into the soil matrix and part of it flows into the cracks. In addition, a certain
amount of rain falls directly in the cracks. Surface runoff only occurs when cracks