31
For maximum accuracy, you may
choose to perform a soil
-
specific
calibration rather than use either of
the internal (Standard, High Clay,
or Sand) soil calibrations coded into
the TDR
150’
s firmware. In these
cases, an independent soil moisture content measurement
is required. A relation can then be developed that relates
the meter
’
s period reading (see
Moisture Type
option, p.
12) to actual volumetric water content (VWC). This is
most easily accomplished by doing a regression of one set
of data against another.
VWC data can be obtained with a device such as a neutron
probe, by measuring the weight of a saturated soil column
of known volume as it is gradually dried, or by gradually
wetting a known volume soil with the addition of known
increments of water. In most cases, however, the calibra-
tion will be done with gravimetric sampling. This proce-
dure is briefly described below.
In the field, establish a number of sites to sample. Each
site should be wetted to a different soil moisture content
by adding varying amounts of water. At each site a Field
Scout TDR reading is taken followed by the extraction of
a known volume of soil. Ideally, this would be an undis-
turbed soil core. The wet weight of this soil must be deter-
mined. If the soil cannot be weighed immediately, it
should be stored in a plastic bag to reduce evaporation.
The soil is then oven
-
dried (105
o
C for 48 hours is a com-
mon requirement) and weighed again. The volumetric wa-
ter content is calculated as follows:
Appendix 1
Soil-Specific
Calibration
