Acta agriculturae Slovenica, 85 - 2, november 2005
362
buried at any orientation to the surface (Campbell Scientific, 2004). The measurement
frequency for the water content reflectometer (WCR) varies with VWC and is generally
between 15 and 45 MHz (Seyfried and Murdock, 2001).
Laboratory experiment was set at the Center for agricultural land management and
agrohydrology, Biotechnical Faculty, University of Ljubljana, Slovenia. Three different soils
were used, clay loam, silt loam soil and sand. Each soil was moistened on four different soil
Volumetric Water Contents (VWC) and packed in a box. Rods were carefully inserted
vertically in the soil 5 cm apart. For each VWC six consecutive readings were made.
3 RESULTS AND DISCUSSION
TDR 100 Reflectometer VWC measurements from 10 cm and 20 cm rods were
evaluated, original and replicated, and compared the results with gravimetrically
determined VWC (gVWC). Gravimetric method is a standard method for soil water
content determination (Topp and Ferré, 2002). Tables 1 – 3 show results of soil VWC
measurements in the three soils used in the experiment for the four different water
contents (average VWC of six repetitive readings for every individual soil VWC,
coefficient of variation (CV), standard deviation, min and max as well as range are
given for each waveguide pair, with TDR 100 10a and 20a being labels for original
rods; TDR 100 10b and 20b for homemade rods).
For interpretation of the TDR 100-10 waveguide for Clay Loamy Soil (Figure 2),
broader measurement interval than manufacturer’s recommendation was taken.
Measurement interval determination for individual soil type is extremely important
(Evett, 2000) and the TDR 100 sensor construction enables manual manipulation. For
the measurement interval determination tangent crossings on the waveguide, best
representing the beginning and final pulse reflection, were used.
10b gave the best results for the first measurement (15.9% gVWC ±0.84). Original
20a rods showed the biggest deviation from the gVWC (±4.4%). All the rods 10 and
20 cm underestimated VWC in the second measurement (21.1% gVWC ±1.74) as
well as in the third measurement (22.3% gVWC ±1.07), thou 10a and 10b gave better
results. For the fourth measurement, the best agreement with (37.1% gVWC ±1.87)
showed 10a rods (Table 1).
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