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Soil Profile Water Content Determination

Sensor Accuracy, Axial Response, Calibration, Temperature Dependence, and Precision

USDA-ARS, Conservation & Production Research Laboratory, Soil and Water Management Research Unit, Bushland, TX 79012

View larger version (48K): [in a new window]   Fig. 1. Top and side view cross sections of a soil column showing the placement of access tubes, TDR probes, and thermocouples. Column sides were covered with aluminum foil. For axial sensitivity tests, each sensor was centered above its access tube and at a height (usually 30 cm above the soil surface) such that the soil (either dry or saturated) did not influence the sensor reading. Then the sensor was lowered in 2-cm increments, with a reading taken at each increment, until it passed through the access tube and reached a position below the soil surface that was lower than the depth at which further lowering did not influence the sensor reading (usually 30 cm below the soil surface).

View larger version (24K): [in a new window]   Fig. 2. Example of data for axial sensitivity tests showing axial response of the Delta-T PR1/6 instrument in air-dry soil. Positive x axis numbers are for sensor heights above the soil surface.

View larger version (31K): [in a new window]   Fig. 3. Axial response of the Trime T3 system in air-dry and saturated soils. A photograph of the probe is scaled to match its length with the x-axis scale of the graph. The response in air-dry soil was asymmetrical. The response in saturated soil was also asymmetrical and showed an intermediate peak coincident with the midpoint of the probe where the signal is carried on a flexible conductor between the upper set of plates on the right and the lower set on the left.

View larger version (25K): [in a new window]   Fig. 4. Calibration curves for the EnviroSCAN system for the combined data from Soils A and B and for Soil C, compared with the factory calibration (Factory E) and the calibration of Baumhardt et al. (2000) in the Olton soil, and calibration curves for the Diviner 2000 system for the combined Soils A and B data and for the C soil, compared with the factory calibration (Factory D).

View larger version (16K): [in a new window]   Fig. 5. Calibrations of the Trime T3 tube probe for the combined data from Soils A and B, and for Soil C, compared with the factory calibration.

View larger version (18K): [in a new window]   Fig. 6. (Left) Data of water content from the TDR system in soil B versus output (V) from the PR1/6 for Soil B. Data from the wet end are from warm summer days (Days 207 and 212) plot well to the right of those from cool winter days, indicating that output from the PR1/6 is increasing with soil temperature. In contrast, on the dry end data from warm summer days (Days 199 and 201) plots in the same location as those from cold winter days. Except for Days 199, 201, 207, and 212, all data shown were acquired between Days 2 and 67 of 2001. (Right) Calibrations for Soils A and B combined and for Soil C when Days 207 and 212, 2002 are omitted, compared with the factory calibration.

View larger version (39K): [in a new window]   Fig. 7. (Left) Calibration of PR1/6 in terms of sensor output (V) and soil temperature using combined data of Soils A and B. (Right) Calibration for Soil C.

View larger version (42K): [in a new window]   Fig. 8. Example of significant temperature effects on water content values reported by the Sentek EnviroSCAN in three air-dry soils.

View larger version (24K): [in a new window]   Fig. 9. Waveforms collected from a Trime T3 probe using a Tektronix 1502 C TDR cable tester connected to the coaxial cable at the Trime-FM measuring device. At each depth shown, a waveform was collected with the top end of the probe waveguides placed at that depth in an access tube placed in a uniform smectitic clay soil. Water content as measured with a neutron probe was uniform throughout the profile. Travel time analysis by intersection of tangent lines (red with a vertical red line indicating the intersections) showed that travel time was practically equal at all depths, indicating that conventional TDR would predict equal water contents throughout the profile, although the Trime-FM did not. The relative lengths of horizontal arrows drawn from the left y axis to the waveforms indicate the changes in transit time for different set-point voltages at which a comparator circuit would time the arrival of the reflected pulse. The transit times illustrated in this manner are greater for the waveforms from deeper within the profile even though water contents are the same. The differences in slope of the reflected pulse are due to differences in soil temperature and/or bulk electrical conductivity. Data courtesy of Jean-Paul Laurent, Chargé de Recherches au CNRS, Laboratoire d'étude des Transferts en Hydrologie et Environnement, Grenoble, France.

View larger version (43K): [in a new window]   Fig. 10. Example of variation over time of standard deviation (SD) values for the EnviroSCAN system. Data are for air-dry soils. The SD (top) is calculated using all data values beginning with zero time and continuing until the time at which the data are plotted. The Running SD is the SD for four consecutively acquired values beginning at each time (bottom) at which the data are plotted; it shows a periodic temperature effect.