Campbell Hydrosense, Instruction Manual

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HydroSense
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rocky soil, the measured water content can differ by several percent if a large
rock occupies part of the sensitive volume of one measurement but not the
other.
6.3 Rod Length and Insertion Angle
Rods for the HydroSense probe are available in 12 and 20 cm length. There is
no significant difference in performance between rod lengths and the choice of
length depends on the application. Threaded inserts on the probe body allow
easy replacement of damaged rods (see Section 8.2) or use of different length
rods. Bent rods should be replaced immediately since unparallel rods can
introduce error and more bending can lead to breaking. Spare/ replacement
rods are available from Campbell Scientific. See Table 1 of Section 8.2.
Since the water content is averaged over the length of the rod, the reading from a
probe with 20 cm rods inserted vertically into the soil surface will be the average
value for the surface 20 cm of soil. Probe rods can be inserted at an angle. For
example, 20 cm rods inserted at a 30 degree angle to the surface will provide
average water content in the top 10 cm.
6.4 Interpreting Measurement Results
It is important to recognize some intrinsic soil properties and understand how
these properties affect water content distribution and the measurement of water
content. One such property is the spatial variability of the water content, i.e.,
the difference in actual water content in proximal locations which are
apparently similar. The water contents in two locations which are only tens of
centimeters apart can differ by several percent.
The scale of the measurement using a device such as the HydroSense must be
considered in relation to the scale of the field or plot being monitored. The
sensitive volume of the HydroSense measurement covers the length of the rods
and extends out radially from the rod surface about 3 cm. The shape is
approximately elliptical extended over the rod length. The total volume is
approximately 650 cubic centimeters for 12 cm rods and 1100 cubic
centimeters for 20 cm rods.
Soil is not necessarily a homogeneous medium on the scale of most soil water
content measurement methods. Soil features such as cracks, rocks, pore size
distribution, plant roots and texture layers are not always distributed evenly
throughout a measured profile. If the water content over a large area such as a
cropped field is to be determined, either a measurement method must be used
which is sensitive on a very large scale or several measurements can be taken
with a device such as the HydroSense and an average value used. See
Appendix B for a discussion of statistical methods that can be applied to
optimize measurement quality.
7. Description of HydroSense Measurement Method
The HydroSense uses the soil physical property dielectric permittivity to
estimate the volumetric water content. The dielectric permittivity of liquid
water is more than one order of magnitude greater than other soil constituents