REW stands for "Relative Extractable Water" and it refers to the amount of soil water available to the plant.
Absorption of soil water through the roots is essential to keep up the biological activity in plants. When the amount of water in the soil decreases it gets increasingly difficult for the roots to provide water to the plant and compensate for the simultaneous loss of water from the leaves. In order to decrease this demand, a similar effect as that of high VPD is produced: stomatal pores close and CO2 flow and photosynthesis decrease.
Field capacity (FC) is maximum amount of water that a soil can hold.
Wilting point (WP) is defined as the minimal point of soil moisture the plant requires not to wilt. A plant wilts and can no longer recover its turgidity if moisture decreases to any lower point.
REW is calculated using measured soil moisture (SM, m3 m–3), wilting point and field capacity:
REW = ( SM – WP ) / ( FC – WP ).