This model postulates three stages of surface layer soil drying:

1. higher than wilting point;

2. between wilting point to the soil air-dried water content of the top soil layer, estimated as 1/3 of the permanent wilting point;

3. lower than soil air dried water content.

This model postulates three stages of soil drying in the surface layer:

1. soil water content higher than the wilting point;

2. soil water content ranges from wilting point to the soil air-dried water content of the top soil layer, estimated as 1/3 of the permanent wilting point; and

3. soil water content higher than soil air-dried water content.

When the soil water content is higher than the wilting point, the actual evaporation is equal to potential evaporation (mm d-1). When the soil water content value ranges from wilting point to the soil air-dried water content, the actual evaporation is calculated according to Campbell and Diaz (1988):

where EvapSoilAct (mm d-1) is the actual soil evaporation; EvapSoilPot (mm d-1) is the potential soil evaporation; SWC (m3 m-3) is the water content of soil layer; WP (m3 m-3) is the permanent wilting point water content of layer; and ADWC (m3 m-3) is the air-dried soil water content of the top soil layer, estimated as 1/3 of the permanent wilting point.

When the soil water value is lower than the air-dried soil water content, the actual evaporation is equal to 0.

The computation of evaporation is done with an internal time step of one hour to avoid over weighting the initial soil water content, to allow for the progressive decrease in water content and to obtain an appropriate reference to the drying stage. The potential evaporation is split into hourly values simply by dividing the daily input value by 24. This is a suitable approach because prediction of evaporation at hourly time step is not of importance in the model: the hourly time-step is used only to obtain consistent simulation of daily evaporation amount.

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