27
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 the 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