23
To improve accuracy, correlate TDR period
readings with a soil
-
specific sample set.
VWC data can be correlated by measuring
the weight of a known volume of saturated
soil as it is gradually dried, by gradually
wetting a known volume soil with measured
increments of water, or by using a neutron probe. In most cases,
gravimetric sampling is performed. This procedure is briefly de-
scribed below.
1. Establish a number of field sites to sample.
2. Wet each site with varying amounts of water.
3. Obtain FieldScout TDR period reading at each sample site.
4. Extract a known volume of soil at each sample site. Ideally, an
undisturbed soil core. Reduce evaporation
-
store samples in a
sealed plastic container.
5. Weigh the wet soil samples.
6. Dry the samples (105
o
C for 48 hours) and weigh again.
7. Plot sample measurements against FieldScout TDR readings.
Regression analysis is used to develop a formula to correlate TDR
readings to the sample data.
Volumetric water content calculations:
VWC
= 100*(M
wet
-
M
dry
)/
(
w
*V
tot
)
Gravimetric water content calculations:
VWC
=
GWC *
(
b
w
)
GWC
=
100*(M
wet
-
M
dry
)/M
dry
b
=
M
dry
/V
tot
Where:
M
wet,
M
dry
=
mass (g) of wet and dry soil respectively
V
tot
=
total soil volume (ml)
w
=
density of water (1g/ml)
Appendix 1: Soil-Specific
Reading Correlation
