ABLE Soil Moisture and Temperature


General Purpose

Four instrument systems at two ABLE sites provide in situ half-hour averages of soil measurements. The systems and the measurements made by each one are:

Surface Soil Heat Flow:

Whitewater: The EBBR station includes a set of 5 soil sensors (0-5 cm depths) to measure surface soil heat flow. See the ABLE EBBR web page for a detailed description of these sensors and the measurement technique.

Smileyberg: The AWS system includes a set of 5 soil sensors (0-5 cm depths) to the southeast of the AWS tower to measure surface soil heat flow. This is exactly the same set of sensors using the same technique as the Whitewater EBBR station. See the ABLE EBBR web page for a detailed description of these sensors and the measurement technique.

Soil Profiles:

Whitewater: A Time Domain Reflectometry (TDR) and soil temperature profile system is installed in the middle of the field (and just northwest of the EBBR station). This system includes four TDR probes for measuring soil moisture and 8 thermistor profiles.

Smileyberg: A high frequency (50 MHz) soil dielectric constant measurement system allows the estimation of soil water content, soil temperature, and soil salinity. Two arrays northwest of the AWS tower make measurements at seven depths down to 75 cm.


Primary Quantities Measured with System

All measurements are made at 1 Hz frequency (except for the Smileyberg AWS system; 2 Hz) and averaged to 30 minute values. The measurements are considered to be representative of the grassland areas at their respective locations.

Detailed Description

List of Components and System Configuration

Surface Soil Heat Flow:

Whitewater and Smileyberg:

Soil Temperature: Platinum Resistance Temperature Detector, MINCO Products, Inc., REBS Model # STP-1, MINCO Model # XS11PA40T260X36(D), Detection Limits -30 to 40 deg C, Accuracy +/- 0.5 deg C.

Soil Moisture: Soil Moisture Probe (fiberglass and stainless steel screen mesh sandwich), Soiltest, Inc., REBS Model # SMP-2, Soiltest Model # MC-300, Accuracy not specified by manufacturer (varies significantly depending on soil moisture and soil type). Detection limits for this sensor are limited by the ability to fit a polynomial to the calibration data; the detection limits are approximately 1% to 50% by volume, but are restricted by the calibration and soil type (in the datalogger programming) to a smaller range than that.

Soil Heat Flow: Soil Heat Flow Probes, Radiation & Energy Balance Systems, Inc., Model # HFT3.1, Accuracy not specified by manufacturer but it is approximately +/- 10 % of the measured reading.

Soil Profiles:

Whitewater: Time Domain Reflectometry (TDR): TDR probes; E.S.I. Moisture Point Model # PRB-A, 5 segments (0-15 cm, 15-30 cm, 30-60 cm, 60-90 cm, 90-120 cm); probes located 14.3 m N, E, S, and W of the TDR boxes. Accuracy is primarily dependent on soil type; estimated at +/- 10% of the measurement.

TDR Measurement System; E.S.I. MoisturePoint MP-917.

Datalogger; Campbell Scientific, Inc. CR10X with multiplexers; Detection Limits vary by voltage range selected, Accuracy +/- 0.1% of full scale reading.

Soil Temperature Profiles:

Thermistors at 14.3 and 57.2 m N, E, S, and W of the TDR boxes; depths: 7.5, 22.5, 37.5, 60, 90, and 120 cm. Accuracy: 0.05 deg C.

Smileyberg: High frequency (50 MHz) soil dielectric constant measurement system:

Arrays: Two (North and South), about 10 m northwest of the AWS tower; depths: 5, 10, 20, 30, 45, 60, 75 cm. Accuracy is primarily dependent on soil type; estimated at +/- 10% of the measurement.

Datalogger; Campbell Scientific, Inc. Model CR10X with multiplexers; Detection Limits vary by voltage range selected, Accuracy +/- 0.1% of full scale reading.


Description of Measurement Methods

Surface Soil Heat Flow:

Average soil heat flow (ave_shf) is measured with five sets of soil heat flow, soil temperature, and soil moisture probes; Soil heat flow plates at 5 cm measure the vertical flow of energy through the soil. This is adjusted for the conductivity of the soil surrounding the heat flow plates with measurements from the soil moisture probes and by knowing the soil type.

The change in energy storage with time in the soil above the plate is determined from the 0-5 cm change in temperature with time multiplied by the soil specific heat (determined from the soil type and the measured soil water content).

The change in energy storage (ces1, ces2, ces3, ces4, ces5)is added to the soil heat flow plate measurement (shf1, shf2, shf3, shf4, shf5) to give the surface soil heat flow (g1, g2, g3, g4, g5) for each soil sensor set. These soil heat flows are averaged to produce the average soil heat flow (ave_shf), as follows:

g1 = shf1 + ces1,

etc.

The expressions for g2, g3, g4, and g5 are similar.

ave_shf = (g1 + g2 + g3 + g4 + g5)/5.

When data from one or more soil set(s) is incorrect, that(those) soil set(s) can be eliminated and the average soil heat flow determined from the remaining sets. Logic for doing this in the datalogger was added in late 2003.

Soil Profiles:

Whitewater: Time Domain Reflectometry (TDR):

An electrical signal is sent between a pair of diodes along the TDR probe. The propagation time, in combination with the probe calibration and affects of soil type, are used to determine soil water content.

Soil Temperature Profiles:

Thermistors are used to measure soil temperature with standard instructions in the datalogger.

Smileyberg: High frequency (50 MHz) soil dielectric constant measurement system:

A complex dielectric constant measurement resolves the capacitive (reflecting soil water content) and conductive (reflecting primarily soil salinity) parts of a soil's response to a 50 MHz signal. Temperature is determined from a thermistor in the probe; this temperature is used to remove the temperature effects on the capacitance and conductance measurements.

As soil water content increases, the capacitive response of the soil increases, and the greater the amount of salt in the soil, the greater the conductivity the soil/water will have. Using these measurements, salinity and soil water content can be resolved.


Theory of Operations

All soil measurements are performed and logged by Campbell Scientific, Inc. CR10X dataloggers, using various peripherals manufactured by either Campbell Scientific, Inc., E.S.I., or Tektronix. Quality cabling from the loggers or peripherals to the soil instruments is a crucial aspect of the measurements.

The soil profiling system at Smileyberg, the thermistor array at Whietwater, and the soil surface heat flow measurement systems that are part of the Whitewater EBBR and Smileyberg AWS have been virtually unchanged since they were installed (a second multiplexer was added to the Smileyberg AWS at one point to increase measurement flexibility). The measurements from these systems have been very reliable.

The original TDR probes at Whitewater did not seem to produce reasonable data and the probes died suddenly only a few months after intallation because of a lightning strike; they are very susceptible to damage from lightning energy. Subsequent installations of replacement probes did not seem to produce reasonable data either. Approximately 2 years later an inspection of the wiring of the probes showed that they were incorrectly wired to the data collection system. In the intervening period, all but a couple of the probes experienced corrosion of their connectors or damage from lightning. The system was rewired properly and four new probes were installed in 2003.

Soil water content measurements are needed, and are being made, to allow proper calibrations of the Whitewater TDR probes and the Smileyberg soil profiling system to be determined. It is expected that this activity will be completed by late Spring 2004.


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