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Soil Water Retention Measurements Using a Combined Tensiometer-Coiled Time Domain Reflectometry Probe

^a Embrapa, Agricultural Instrumentation Center, P.O. Box 741, 13560 970 Sao Carlos, Brazil
^b Dep. of Land, Air and Water Resources, Hydrology, Univ. of California, Davis, CA 95616
^c Embrapa, Semi-Arid Center, P.O. Box 23, 56300-000, Petrolina-PE, Brazil
^d Technical Univ. of Denmark, Bldg. 115, DK-2800 Lyngby, Denmark

View larger version (27K): [in a new window]   Fig. 1. Detailed diagram of the tensiometer-coiled TDR probe.

View larger version (22K): [in a new window]   Fig. 2. Waveforms of the tensiometer-coiled TDR probe in water, saturated and dry glass beads (particle size = 150–300 µm, bulk density [] = 1.55 g cm-3).

View larger version (37K): [in a new window]   Fig. 3. Experimental design for determining soil water retention curves with the combined tensiometer-coiled TDR probe.

View larger version (22K): [in a new window]   Fig. 4. Dielectric constant measured with the tensiometer-coiled TDR probe (coil) as a function of the soil water content.

View larger version (17K): [in a new window]   Fig. 5. Comparison of soil water retention curves for Oso Flaco and Ottawa (a) and SRI, Columbia and Lincoln (b) with water content measured by drainage outflow (open symbols) and after calibration of the combined tensiometer-coiled TDR probe (solid symbols).

View larger version (23K): [in a new window]   Fig. 6. Bulk soil dielectric constant (soil) as measured with a conventional TRD probe (two-rod 5-cm long) as a function of water content as measured by drainage outflow measurements.

View larger version (20K): [in a new window]   Fig. 7. Dielectric constant of the tensiometer-coiled TDR probe in air, as a function of the porous cup water potential (or drainage curve).

View larger version (22K): [in a new window]   Fig. 8. Comparison of measured soil dielectric constant measured using the combined tensiometer-coiled TDR probe with estimated values using the mixing model approach of Eq. [2].