Root Water Extraction

Subroutine Rootex (curoot.f)

"Root water uptake is estimated for each soil layer from the product of root resistance in that layer (which is a function of root length density) and the potential difference between the xylem and the soil layer of interest. Of course, the root xylem potential is a function of the transpiration rate and a weighted average soil water potential throughout the root zone. Some care must be exercised in using this approach to root water uptake because soil water potentials can become lower than root xylem water potentials, especially at night, implying that water moves from the root to the soil. This may be reasonable behavior in moist soil, but probably is unrealistic for dry soil. To prevent excessive water loss to dry soil layers, any soil layer that becomes dryer than some limiting potential, for example, -15 bars, is removed from consideration in the root uptake equations [WHERE IS THIS CONSTRAINT PLACED?]" (Norman & Campbell 1983, p 164-5)

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