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The Effects of Salinity on the Accuracy and Uncertainty of Water Content Measurement

^a Dep. of Biology, Univ. of Victoria, Victoria, BC V8W 3N5, Canada
^b Dep. of Hydrology and Water Resources, Univ. of Arizona, Tucson, AZ 85721-0011

View larger version (35K): [in a new window]   Fig. 1. (a) Schematic diagram of the ANA in transmission mode. A(f) and (f) are the amplitude and phase measured at each frequency. The primed variables represent digital samples of these results. A'(t) is the simulated TDR response. (b) Transmission line model of a TDR probe of length L.

View larger version (15K): [in a new window]   Fig. 2. Incident and transmitted sine waves with period, T. The time difference t between the phase of the incident wave and the phase of the transmitted wave is shown, as well as the linear amplitudes, Ainc and Atrans.

View larger version (12K): [in a new window]   Fig. 3. A typical transmission mode waveform collected with 0.28-m long rods in sand saturated with 10-dS m–1 NaCl solution. The fitted tangent lines used to determine the one-way pulse arrival time through the medium and cables (tn = 20.4 ns) are shown. The rise time, tr = 7 ns, is the time delay between tn and the time at which the waveform amplitude is half of its eventual maximum, t0.5 = 27.4 ns. The one-way travel time through the cables and an air-filled container is tn-air.

View larger version (23K): [in a new window]   Fig. 4. Automatic network analyzer (ANA) results collected for a sand saturated with a 10-dS m–1 NaCl solution. (a) Relative amplitude as a function of frequency. The dashed line represents the minimum amplitude for reliable measurements based on the manufacturer's reported feed-through cutoff. (b) Linearized phase as a function of frequency for the 10-dS m–1 and air cases and the difference between these two measurements.

View larger version (21K): [in a new window]   Fig. 5. (a) Time domain reflectometry (TDR) traces collected from the air-filled container, and in sand saturated with distilled water (0 dS m–1) and three different NaCl solutions with electrical conductivities ranging from 10 to 40 dS m–1. (b) Amplitude normalized to the amplitude measured at 75 ns as a function of time for the solutions shown in Fig. 5a.

View larger version (12K): [in a new window]   Fig. 6. Travel time difference (t) as a function of the pore-water electrical conductivity (EC) in saturated sand. The travel time difference is converted to a water content measurement error on the second y axis.

View larger version (9K): [in a new window]   Fig. 7. Time domain transmission (TDT) waveforms collected with 0.28-m rods in four repackings of sand saturated with 40-dS m–1 NaCl solution. Each waveform is normalized to the amplitude measured at 75 ns.

View larger version (9K): [in a new window]   Fig. 8. Water content measurement error as a function of rise time.

View larger version (34K): [in a new window]   Fig. 9. (a) Magnitude as a function of frequency for sand saturated with distilled water (0 dS m–1) and NaCl solutions of three electrical conductivities (EC) ranging from 10 to 40 dS m–1. The dashed line represents the manufacturer reported feed-through cutoff. The 25- and 40-dS m–1 cases have –40-dB cutoff frequencies of 22.8 and 64 MHz, respectively. (b) Linearized phase difference as a function of frequency for sand saturated with distilled water and the three NaCl solutions shown in (a). For clarity, the results are only shown for the lowest 200 MHz. The cutoff frequencies for the higher EC cases are shown as dashed lines.