Agriculture Reference
In-Depth Information
content,
h
is the apparent soil matric potential,
h
e
is the air entry matric potential,
and
a
,
b
,
m
, and
n
are empirical constants.
Electronic sensors that measure soil water content include time domain reflectome-
try (TDR) and capacitance and frequency domain type sensors. Soil dielectric constants
measured with TDR can be used to estimate volumetric water content using a polyno-
mial equation (Topp et al. 1980), which in most cases may also be approximated with a
linear form (Ledieu et al. 1986). During a strawberry field experiment by Wang (2010),
TDR probes were used to measure water content in a peat-perlite substrate irrigated
at different frequencies (Figure 11.3). A linear calibration, similar to that described
by Ledieu et al. (1986) but with new calibration coefficients, was determined for the
substrate. The measured water content values clearly showed rapid responses to each
irrigation event when irrigation was triggered with a 3-mm ET
c
deficit threshold. When
ET
c
threshold was reduced to 1.5 mm, the irrigation frequency increased to two to three
times per day, and the measured water content showed a nearly constant value, indicat-
ing that the water supply nearly met the crop demand without causing large variations.
Capacitance and frequency domain sensors use electronic circuits that are differ-
ent from TDR, but like TDR, none of these sensors directly measure soil water con-
tent. In capacitance sensors, a capacitive element is used in the circuit to measure soil
dielectric properties that are affected by water content. For some of the commercially
available capacitance sensor systems, each capacitor consists of two adjacently placed
cylindrical plates or rings. With these capacitors or capacitor rings located at different
depths within a soil-access tube, water content can be measured for the soil profile.
The frequency domain sensors use a similar mechanism where the frequency of an
oscillating voltage is adjusted until the strongest resonating frequency is detected.
This frequency corresponds closest to the apparent dielectric constant of the soil, thus
12
Water content
Irrigation
0.40
10
0.35
8
6
0.30
4
0.25
2
0.20
0
9/4
9/6
9/8
9/10
9/12
9/14
9/16
9/18
Time
FIGURE 11.3
Volumetric water content measured with time domain reflectometry probes
in a peat-perlite substrate and responses to irrigation events. (Data from Wang, D., Evaluation
of a raised-bed trough (RaBet) system for strawberry production in California,
2009
-
2010
Annual Production Research Report
, California Strawberry Commission, Watsonville, CA,
163-171, 2010. With permission.)
