Plant plus soil resistance to water flow (R(p+s)) was derived, according
to Sands and Theodorou (1948):
El = – (Ψl – Ψm) / R(p+s)
Since the matrix potential is zero in the flooded treatment and close to
zero in the control treatment, plant plus soil resistance can be expressed
(following Savé and Serrano 1986) as:
R(p+s) = – Ψl / El