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A Variable-Volume TDR Probe for Measuring Water Content in Large Soil Volumes

^a Faculdade de Engenharia Agrícola, Univ. Estadual de Campinas, Campinas, SP, Brazil
^b Univ. of Connecticut, Storrs, CT

View larger version (35K): [in a new window]   Fig. 1. A scheme of the proposed variable volume coaxial probe (VVCP) made of two arrays of 0.26-m stainless steel rods embedded in partially wet soil in a coaxial configuration with a and b as the inner and outer diameters of the coaxial VVCP, respectively. Note that the inner six rods are connected by equal length and flexible conductors to the core of the VVCP primary coaxial cable, whereas the outer six rods are connected to the shield on the cable.

View larger version (25K): [in a new window]   Fig. 2. Two typical time domain reflectometry (TDR) waveforms obtained with the variable volume coaxial probe (VVCP) placed in soil saturated with tap water (electrical conductivity [EC] = 0.01 dS m–1) and with salt solution (resulting in bulk EC of 2 dS m–1). The variables t1 and t2 mark the first and second reflections used for travel time analysis of the waveform. The values V0 and V1 are used for the calculation of EC (see text for detail).

View larger version (19K): [in a new window]   Fig. 3. Variable volume coaxial probe (VVCP) characteristic impedance Z0 as a function of probe geometry (expressed as ln[b/a]). The full symbols are experimental results of Test 1 measurements (with faulty V0 reference), the empty symbols are Test 2 measurements, and the line is the predicted characteristic impedance for a perfect coaxial probe (Eq. [1]).

View larger version (20K): [in a new window]   Fig. 4. Measured soil water content dynamics for uniformly wet soil using variable volume coaxial probe (VVCP) and multiple readings from standard three-rod time domain reflectometry probe (first measurement in saturated soil).

View larger version (30K): [in a new window]   Fig. 5. Measured water dynamics in different soil volumes after application of 1, 2, 3, 4, and 8 L of water.

View larger version (23K): [in a new window]   Fig. 6. Temporal changes in average soil water content in different soil volumes measured by VVCP (square symbols) and standard three-rod probe (circle, average of three measurements) under field conditions during 4 d after application of 20 L of water from a dripper.