ABLE Energy Balance Bowen Ratio (EBBR) Station


General Purpose

The ABLE Energy Balance Bowen Ratio (EBBR) Station provides in situ half-hour averages of the surface vertical fluxes of sensible heat, latent heat, net radiation, and soil surface heat flux. The fluxes are obtained by the energy balance Bowen ratio technique, a gradient method that uses vertical gradients of temperature and relative humidity in combination with point measurements of net radiation and soil heat flow from 5 sets of soil sensors. The EBBR station is installed at the Whitewater, KS grassland site.

Primary Quantities Measured with System

The primary quantities measured are 30-minute averages of the energy flux densities (watts per meter squared) of sensible and latent heat, net radiation, and soil heat flow. The measurements are considered to be representative of the grassland area within about 50 meters of the EBBR station. Estimates of sensible and latent heat flux have an accuracy of +/- 10 % of the measured value; offset and calibration drift errors in the measurements of the temperature and relative humidity gradients are significantly reduced by the AEM, which switches the positions of the upper and lower sensors every 15 minutes.

Secondary quantities include air temperature, reference temperature, relative humidity, near-surface soil moisture, near-surface soil heat flow, near-surface soil temperature, atmospheric barometric pressure, wind direction, wind speed, and battery voltage. The secondary quantities are not designed to be used as primary quantities or replacements for the same quantities measured by the co-located AWS systems. The units and heights (or depths) of secondary quantities vary (units vary depending on averaging time).

All measurements are made at 1 Hz frequency and averaged to 5, 15, or 30 minute values.

Detailed Description

List of Components

The accuracies cited below are generally those stated by the manufacturer. They are sensor absolute accuracies and do not include the effects of system (i.e., datalogger) accuracies.

The detection limit is normally restricted to the range (sometimes called Calibrated Operating Range) over which the accuracy applies. In the case of the EBBR, some of the detection limits are those determined by the vendor (REBS) who performed the calibration, not by the manufacturer of the sensor. Some manufacturers also specify an Operating Temperature Range in which the sensor will physically and electronically function, even though the calibration may not be appropriate for use throughout that range. When no detection limits have been listed by the manufacturer or the calibrating vendor, none are stated below.

Air temperature: Chromel-constantan thermocouple, Omega Engineering Inc., REBS Model # ATP-1, Detection Limits -30 to 40 deg C, Accuracy +/- 0.5 deg C.

Temperature/Relative Humidity Probe: Operating Temperature Range -20 to 60 deg C. Temperature: Platinum Resistance Temperature Detector (PRTD); Detection Limits -30 to 40 deg C, Accuracy +/- 0.2 deg C Relative Humidity: Capacitive element, Vaisala Inc., Model # HMP 45D; Detection Limits 0% to 100% RH, Accuracy +/- 2% (0-90% RH), +/- 3% (90-100%), uncertainty of RH calibration +/- 1.2%.

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.

Barometric Pressure: Barometric Pressure Sensor, Met One Instruments, Model # 090D-26/32-1, Detection Limits 26 to 32 kPa; Accuracy +/- 0.14 kPa.

Net Radiation: Net Radiometer, Radiation & Energy Balance Systems, Inc., Model Q*7.1, Accuracy +/- 5% of the measured reading when ventilated.

Wind Direction: Wind Direction Sensor, Met One Instruments, Model # 020C, Detection Limits 0 to 360 deg physical (for greater than 0.3 ms-1 wind speed), 0 to 356 deg electrical, Accuracy +/- 3 deg.

Wind Speed: Wind Speed Sensor, Met One Instruments, Model # 010C, Operating Temperature Range -50 to 85 deg C, Detection Limits 0.27 to 50 ms-1, Accuracy +/- 1% of the reading. Operational Limit on speed 60 ms-1.

Datalogger: Campbell Scientific, Inc., Model CR10, Detection Limits vary by voltage range selected, Accuracy +/- 0.1% of full scale reading.


Description of System Configuration and Measurement Methods

The meteorological observations and the heights at which they are made are:

Air temperature at two heights (1 m separation)
Relative humidity at two heights (1 m separation)
Net radiation (at 2 m typical)
Soil moisture at 2.5 cm depth
Soil heat flow at 5 cm depth
Soil temperature, integrated 0 to 5 cm
Barometric pressure
Wind direction at 2.5 m
Wind speed at 2.5 m
Reference temperature of control box

The EBBR sensors (except for soil probes) are mounted on two tripods that sit on the soil surface. The ventilated net radiometer mount extends from the south side of the tripod that holds the automatic exchange mechanism (AEM). The AEM helps to reduce errors from instrument offset drift. The AEM extends from the north side of the tripod. Aspirated radiation shields (which house the air temperature and relative humidity probes) are attached to the AEM. The openings of the aspirated radiation shields face north to reduce radiation error from direct sunlight.

The soil probes are buried just outside the view of and in an arc to the south of the net radiometer.

The reference temperature sensor, barometric pressure sensor, datalogger, storage module, and communication equipment are located in the control box, which is attached to the other tripod. This tripod also has a vertically extended pipe on which the wind speed and direction sensors are mounted and is located northeast of the tripod that holds the net radiometer and AEM.

The EBBR station is located southeast of the TDR boxes.


Theory of Operations

The EBBR station uses a standard Bowen ratio approach that has been described in textbooks and articles. A general description can be found in the book titled Evaporation in the Atmosphere by W. H. Brutsaert (D. Reidel Publishing Company, Dordrecht, Holland, 1982, pp. 210-212). For a recent article, see p. 18,549 of the special FIFE issue of the Journal of Geophysical Research (Vol. 97, pp. 18,343-19,110).

The surface energy balance equation is used:

q + ave_shf + h + e = 0,

where q is net radiation, ave_shf is the ground surface heat flow, h is sensible heat flux, and e is latent heat flux. The units for the terms in the equation above are Watts per meter squared.

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 plates 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.

The Bowen ratio is measured as the ratio of the gradients of temperature and vapor pressure (the latter calculated from relative humidity and temperature) across two fixed heights within 3 meters of the surface.

The Bowen ratio (B = h/e) is computed on the basis of the gradients and the following computations are performed:

e = -(q + ave_shf)/(1 + B)

h = B*e


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