24
Relative water
Content Mode
In addition to displaying volumetric water content (VWC), the
meter can also display the relative water content (RWC) and
Water Deficit (see MODE button, p. 8). RWC is an index
value calculated with respect to upper (wet) and lower (dry)
VWC set points. The set points are configured with the soft-
ware (refer to Meter Settings, p. 16). An RWC of 0 indicates
the soil is at the dry set point while an RWC of 100 indicates
the soil has reached the wet set point. (Example: Assume the
dry set point is VWC=25% and the wet set point is
VWC=40%. If the meter measured a VWC of 35%, this
would translate to a RWC of 67 because 35% is 2/3 between
25% and 40%.) If the soil’s volumetric water content is out-
side the range of the set points, it is possible to get a negative
RWC or an RWC greater than 100.
If the volumetric water contents for field capacity and perma-
nent wilting point are the wet and dry set points respectively,
the RWC value will be equivalent to Plant Available Water
(PAW). A general rule of thumb is to recommend irrigation
when the soil has reached 50% of the PAW.
Also included on the first line is the Water Deficit. The Water
Deficit is the amount of rain or irrigation water necessary to
raise the soil water content to the wet set point. This calcula-
tion applies to a soil depth equal to the probe rod length. The
water deficit can be extrapolated further into the profile if the
porosity and water-holding characteristics are similar to the
volume of soil sampled by the probe.
RWC=25.5 D=3.17in
A=23.4 N=06 Asnte