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Dear Raffaele,
Whether you have to disturb the probes by repeatedly connecting the cables
depends on your probe design. I use probes with cable permanently
attached (solder joints between cable and probe rods are embedded in an
epoxy handle). I've had good success using two probes; one horizontal and
near the surface; the other vertical but with the handle buried just under
the soil surface. The horizontal probe gives the water content for the
surface layer and the vertical probe gives the water content for the next
layer which is typically much deeper. For example, digging a shallow hole
to 5 cm allows a vertical probe to be inserted so that the bottom of the
handle (where the stainless steel rods emerge from the handle) is at 5 cm
depth. A horizontal probe can be inserted into the side of the hole at
2.5 cm depth to measure the zero to 5-cm deep layer. Depending on the
length of the vertical probe the layer it measures will differ; but say
the length was 30 cm - then the vertical probe would measure the layer
from 5 cm to 30 cm.
Cables from both probes should be buried in a shallow trench leading to
some point away from the measurement area. I typically have 3 m cables
and I bring them in a shallow trench to a common point for connection to a
multiplexer or directly to a cable tester. I place a small piece of
plastic pipe, with a slot cut in one side, vertically in the trench at the
end of the trench farthest from the probes. The cable can be slipped into
the slot in the pipe and the slot covered with tape. An end cap covers
the top of the pipe, effectively protecting the cable ends (BNC
connectors). When I want to take a reading I remove the end cap, connect
the probes to the cable tester using a short extension cable, and take the
readings.
This method has the advantage of completely burying the probes and cables
so that they are protected from disturbance. It also lessens the
possibility of water moving preferentially along the rods. Just about any
probe has this problem to some extent, especially those probes used in
access tubes that extend above the surface (e.g. neutron scattering probes
and capacitance type probes used in a tube). A problem that you didn't
mention, but that worries me for vertically installed probes, is the
possibility that the probe handle will somehow shadow the rods below it,
causing water to flow outside the zone containing the rods. However, I
have done side by side comparisons of installations, done as I've
described above, to installations done to the same depth using a suite of
probes installed horizontally only; and there doesn't seem to be a
difference in profile water contents observed.
As far as long term measurements go, I've had some of these probes
installed for over two years with no problems. Currently, I have an
installation under alfalfa that is a year old and will continue for two
more years.
Good luck and happy holidays, Steve
Steve Evett srevett@ag.gov
USDA-ARS, P.O. Drawer 10, Bushland, Texas 79012 U.S.A.
(1/2 mile W., Interstate-40 S. access road)
Tel. 806-356-5775, FAX: 806-356-5750