It is necessary to calculate the maximum upward flux from the water table to the root zone.
Dry stress is calculated from the ratio of the maximum water supplied to the plant.
In the Drainmod program, water is removed from the root zone until the root zone is dried to the wilting point. The result of this procedure is that there is a drastic reduction in ET at the point that the root zone dries, and this calculated phenomenon does not match experimental data. Unavailability of soil water characteristic curves from most agricultural fields as well as model instability with the Richard's equation have resulted in a simpler and less accurate although faster and more stable approach in the Drainmod model. In the previous lecture, we derived the Richard s equation and calculated one-dimensional vertical flow between soil layers. Darcy's law was used to calculate the flow velocity between cells.