Campbell Scientific Sentek TriSCAN® Manual v 1.2a - Manual (Page 40)

TriSCAN Manual Version 1.2a
Copyright © 1991 – 2004 Sentek Pty Ltd All rights reserved Page 33
Benchmarking Soil Salinity – TriSCAN Calibration
The TriSCAN sensor currently provides an output in VIC (Volumetric Ion Content). This is sufficient for the
monitoring of trends in changing soil salinity. In many instances, however, it is important to be able to relate
VIC to soil Electrical Conductivity (EC). The following procedure outlines a simple methodology for
benchmarking different levels of VIC in relation to EC. It is not a site-specific calibration as such, as it
assumes a linear relationship between VIC and EC, which may not be the case in your soil. Therefore it will
not give an absolute value of soil EC, but it will provide an approximate value of soil EC.
The procedure involves setting up 2 access tubes in close proximity to the monitoring system. One access
tube is bathed in low EC irrigation water (ideally less than 0.3dSm
-1
), and the other in high EC water
(recommended 7dSm
-1
). After 24 hours (48 hours on low infiltration rate sites), the monitoring probe is
temporarily repositioned into each of these. Soil samples are immediately collected from relevant depths for
EC determination. The VIC signal from the TriSCAN probe is then related to actual soil EC by a simple linear
mathematical equation.
This benchmarking procedure may be performed at any stage of the growing season while the monitoring
probe is in place, but is most useful if done early in the cycle.
Sampling Method
For each macro-zone, for which probes are planned undertake the following steps:
Step 1
Insert two access tubes (P1 and P2) at a distance of 5 metres apart, into a representative site (micro-zone)
to the depth of the monitoring system.
Step 2
Temporarily reposition a probe from the monitoring system into one access tube and reconnect the data
logger. Collect data from each access tube location for at least 10 minutes. Download the data and compare
soil water content and salinity readings. Ensure that data sets from locations are similar. Data similarity willl
provide a good baseline for benchmarking. If data are very different (e.g. if soil water content varies by
20mm and salinity varies by 2000 VIC’s at comparable depth levels), then reinstall the access tube at a
different location. Possible reasons for these differences are:
•
Soil disturbance (old backhoe pit)
•
Air gaps around the access tubes during installation
•
Site polluted with fertilizer spill