Reduction of the transpiration due to water stress

When sufficient water is available, the actual transpiration rate is equal to the maximum transpiration rate. The actual transpiration rate is derived from the maximum transpiration rate, taking shortage or excess of water in the root zone into account. Water uptake by the roots depends on the difference in water potential in the plant and the soil and also on the resistance to moisture transport from the soil to the atmosphere (van Keulen & Seligman, 1987).

The actual transpiration rate for the water limited situation is obtained by multiplying the potential transpiration with a reduction factor which is defined with the following equation:

where Rshort is the reduction factor for the calculation of actual transpiration (unitless), SWCt is actual soil water content (m3 m-3), SWCwp is soil water content at wilting point (m3 m-3), SWCws is the critical soil water content (m3 m-3).

The critical soil water content (SWCws), equal to the quantity of stored soil water below which water uptake is blocked, is calculated with the equation:

Where SWCws is critical soil water content (m3 m-3), Wav is the fraction of easily available water (m3 m-3), SWCfc is soil water content at field capacity (m3 m-3), SWCwp is soil water content at wilting point (m3 m-3).

The fraction of easily available water (Wav) can be calculated through the equation:

where Wav is the fraction of easily available water (m3 m-3), α and β are regression constants (α = 0.76; β = 1.5, both unitless), ET0 is potential evapotranspiration rate (cm d-1) and Cropno is crop group number (1 - drought sensitive crops to 5 - drought resistant crops, unitless).

For defining the value of Cropno, the following table is used:

Since the estimate for crop number 1 and 2 is not very accurate, in these cases the following correction is applied:

where  Wav is the fraction of easily available water (m3 m-3), ET0 is potential evapotranspiration rate (cm d-1) and Cropno is crop group number.

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