Transpiration Parameters

Note: All parameters are defined at the beginning of an evapotranspiration period and are assumed to vary linearly from the beginning of one period to the beginning of the next period.

Potential transpiration (PTVAL): [L]/[T]. Typically computed from climatic data, using an equation such as the Penman or Jensen-Haise equations.

Rooting depth (RD) : [L]. Maximum depth below land surface in which root extraction is allowed.

Activity at root base (RA base): 1/[L2]. Defined as the length of roots in a given volume of soil divided by that volume. Values range from 0 to about 3 cm-2, depending on the plant community and its stage of development.

Activity at root top (RA top): 1/[L2]. Defined in a similar manner as above.

Pressure head in root (HROOT): [L]. Usually set to the permanent wilting point, defined as the pressure head in the soil at which the plant wilts and dies. For most agricultural crops, the permanent wilting point is equivalent to about -150 m of water.

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