Soil Heat Flux Plates Research Articles

Design and Installation of Large Weighing Lysimeters

ABSTRACT THE design, construction, installation, and calibration of two large weighing lysimeters are described. The lysimeters have a surface area of 4 m2 and a net soil profile depth of 2 m. The soil profile was constructed by careful packing. Each lysimeter rests on a sensitive scale capable of measuring the total mass of approximately 20 Mg to the nearest 100 g. Each lysimeter contains four neutron access tubes, 15 soil water electrical conductivity (EC) sensors, 15 soil water matric potential (MP) sensors, and 3 soil heat flux plates. The soil water EC and MP sensors also contain accurate soil temperature measurement devices. The lysimeter data acquisition system computer can send data to another computer at the central laboratory location, which is about 80 km from the lysimeter site by telecommunication. Each lysimeter contains a free water and a vacuum drainage system. The lysimeters were constructed in modular sections to facilitate the transportation to the installation site and to minimize the use of cranes during the installation. They consisted of 5 main components: (a) inner tank, (b) outer tank bottom, (c) outer tank top, (d) access tunnel, and (e) scale. The heaviest component was the outer tank bottom which had a mass slightly in excess of 2 Mg. Close tolerances between the two tanks kept the air gap less than 8 mm. The lysimeters were installed in a 3.6-ha field at the University of California West Side Field Station near Five Points, California. The field had dimensions of 400 m in the N-S direction and 91 m in the E-W direction. The predominant wind direction is from the NW. One lysimeter was installed into a field normally used for row crop research, and the other lysimeter was installed into a grass weather station field. The resulting short-term sensitivity of the lysimeters has been determined to be about 0.02 mm of evaporation (equivalent mass). The scale consists of a commercial flexure-lever action balance with a load cell final output. No counterbalance was used. The lysimeter scale appears to have excellent stability during windy conditions.

Microclimate of Arctic Tree Line 2. Soil Microclimate of Tundra and Forest

Forest and tundra soils display distinctive microclimates for a climatically normal year at Churchill. Forest soils are substantially warmer in the active layer than those of the tundra but the tundra active layer is deeper. Forest soils are much wetter than those of the tundra. This results from the deep winter snow pack, which provides abundant meltwater to already thawed soils. The soils remain wet throughout the year, and the large latent heat release delays the freezing of forest soils until a snow pack is established. As a result, soils stay relatively warm throughout winter and thaw rapidly and deeply before snow pack melting in the spring. The thaw period in the tree rooting zone is about 6 months, compared to 4 months at the same depth in tundra. The magnitude of soil heat storage is large, comprising 18% and 16% of net radiation in tundra and forest, respectively, during the thaw season. During freeze back it is the dominant heat exchange process. Between 80% and 90% of the total soil heat storage is involved in the latent heat exchange accompanying thawing and freezing. Soil heat flux plates strongly underestimate the ground heat exchange and are unreliable in permafrost terrain.

Determination of the mean soil temperature for evaluation of heat flux in soil

A technique for determining surface soil heat flux, based upon measurements of the mean temperature, T , and upon knowledge of the mean moisture content of the soil, is described. A thermocouple probe was developed to measure the mean soil temperature, T , of the upper 50 cm of a soil. The thermocouple probe has 22 copper—constantan thermocouples, placed along its length in such a manner that once inserted vertically into soil it provides an estimate of the average soil temperature down to 50 cm. Measurements of soil heat flux obtained with the probe compared favorably with determinations of the soil heat flux using the temperature integral method. Values of the soil heat flux obtained with the thermocouple probe also agreed with values of soil heat flux obtained with a resistance probe. The probe methods were found to be more reliable for soil heat flux determinations than the soil heat flux plate method, and required fewer measurements and calculations for determination of soil heat flux than with the temperature integral method.

Observations of soil heat flux at Pune using a heat flux plate

Measurements of soil heat flux were made over bare black cotton soil at Pune in the period June 1976 to February 1977, using a soil heat flux plate. Hourly values of soil heat flux from 00 to 24 LAT are presented for selected days typical of the monsoon, post-monsoon and winter seasons. The diurnal variation is generally characterised by a cross-over from negative to positive values at 07 hr, occurrence of maximum around noon and return to negative values between 16 and 18 hr. The effects of rainfall, cloudiness and soil moisture on the soil heat flux are discussed.

Measurements of the Energy Fluxes Involved in the Energy Budget of a Snow Cover

Measurements of the various energy components involved in an energy balance of a snow cover were made at the Elora Research Station, University of Guelph during the winter of 1976. A pressure sphere anemometer and a sonic anemometer-thermometer in conjunction with fast response, fine wire, resistance thermometers and Lyman-alpha humidiometers were used to measure the fluxes of sensible and latent heat by eddy correlation techniques. A net radiometer and soil heat flux plates measured the radiative and soil heat fluxes. The data allowed a complete energy budget to be calculated including the energy stored in the snowpack and/or utilized in the fusion process, determined as a residual. The data indicated the absence of any close relationship between the net radiation and sensible and latent heat fluxes during the diurnal cycle. The flux of heat into the snowpack was found to be a major component of the energy balance. Maximum evaporation from the snow surface was observed to occur following the replacement of warm moist air masses by cold dry air masses. During these periods the sensible and latent heat fluxes were greater than the net radiation and resulted in a rapid loss of energy from the snowpack. The determination of evaporation from the snowpack using a Bowen ratio energy balance approach was found to be impractical because the large energy flux into the snowpack could not be independently determined with sufficient accuracy.

Conduction of heat from sheep to ground

The conductive heat loss from a sheep sitting on grass was measured with soil heat flux plates and an electrical model was built to study the heat loss to other substrates. The thermal resistance of the fleece on the model was estimated as 2.2 s cm −1 per cm fleece depth. The resistance found for the live sheep was 50% less, probably due to preferential heat flow from the parts of the body with little fleece covering, together with disturbance of the fleece due to breathing. The resistance of thin layers of several dry insulation materials were, per unit depth, similar in magnitude to the fleece resistance, but a six-fold reduction in resistance was observed when sawdust was wetted. The nature of the substrate and the duration of each sitting period had only a secondary effect on the rate of conduction. The measurements indicate that heat loss by conduction from a sheep living on cold, poorly-insulated ground may approach 30% of its minimum heat production.

An apparatus for calibrating soil heat flux plates

The design and construction of an apparatus for calibrating soil heat flux plates is described. The apparatus produces a known constant heat flux through a layer of an appropriate porous medium contained in a small, well-insulated box. A secondary heater, positioned below the primary heater, operates automatically to ensure that all the heat generated by the primary heater must flow upward through the box. The thermal conductivity of dry sand was calculated using this apparatus and the results agreed with previously published values. Soil heat fluxes in a wheat field measured using heat flux plates calibrated in the apparatus agreed with those calculated using an energy balance method.

Calibration of soil heat flux plates by a radiation technique

In a recent paper in this journal (Idso, 1971) I described a simple technique for the calibration of long-wave radiation probes. It consisted essentially of placing a cooled or heated (about 15°C below or above ambient temperature) blackened metallic plate beneath or over the radiation sensor in a constant light and temperature environment room, and then monitoring the change of plate temperature by thermocouple and the concomitant change in sensor output as the plate warmed or cooled to ambient room temperature. The slope of the plot of sensor output versus calculated radiation received from the plate at various plate temperatures then yielded the long-wave calibration factor in terms of mv/(cal.cm −2 min −1). It is the purpose of this short communication to demonstrate that this identical procedure may be used to accurately calibrate soil heat flux plates.

Application of Lettau's Theoretical Model of thermal diffusion to soil profiles of temperature and heat flux

Diurnal cycles of soil temperature and heat flux measured on the Pampa de La Joya, Peru are analyzed harmonically. A theory of thermal diffusion put forth by H. H. Lettau (1962) is used to determine the relative calibration of the soil heat flux plates and the relative depth intervals between the flux plates in comparison with the soil temperature profile. The absolute value of the soil heat flux is based on an allowed range of values for the density of the soil.

Calibration and Field Test of Soil Heat Flux Plates

AbstractSoil heat flux plates were calibrated in the laboratory in steady‐state conditions and checked in the field vs. the soil temperature gradient thermal conductivity measurements of heat flux density. Good agreement was obtained after correcting for the flux divergence in the layer of soil where the temperature gradient was measured. Soil heat flux density measurements in a bare soil indicated that the heat storage in a 5‐cm layer of soil above the flux plates contributes significantly to the diurnal soil heat flux wave.