Eddy Correlation System Research Articles

Evaporation from Lake Kinneret: 2. Estimation of the Horizontal variability using a two‐dimensional numerical mesoscale model

The horizontal distribution of evaporation over a small lake surrounded by heterogeneous terrain is quite complex. It is therefore usually not possible to provide accurate evaporation estimates for the whole lake based upon a single observational site, even if this site is located offshore. In the present study, a numerical two‐dimensional terrain‐following mesometeorological model is applied to Lake Kinneret (area of 170 km2, located in northern Israel) to obtain appropriate spatial and temporal resolution of the climatic parameters needed to estimate evaporation. The model is verified by comparing its calculated evaporation rates with those measured at an offshore site by the eddy correlation system and by its ability to reproduce well‐established characteristics of the microclimate over the lake. Results indicate that, for the typical meteorological conditions characterizing the experimental period, a concave horizontal distribution of evaporation rate is predicted across the lake. Consequently, the assumption of a uniform evaporation rate over the whole lake equal to that at the measuring site could lead to a misestimation of the amount of water lost through the evaporative process.

The unstable surface layer above forest: Regional evaporation and heat flux

Radiosonde measurements above the heterogeneous forest of the Landes region in southwestern France provided vertical profiles of potential temperature and specific humidity in the atmospheric boundary layer. For all of the 62 profiles analyzed under unstable atmospheric conditions a surface sublayer could be identified within which the Monin‐Obukhov similarity was consistent with the regional surface fluxes of sensible heat and latent heat (evaporation). For the potential temperature the vertical extent of this sublayer was found to be 41 (±30) ≤ (z − d0)/Z0 ≤ 130 (±49), where Z0 = 1.2 m is the roughness height and d0 = 6.0 m is the displacement height; for the specific humidity it was 48 (±36) ≤ (z − d0)/z0 ≤ 153 (±63). These results show that irregular forest surfaces are not anomalous, as regards the lower limit of the surface layer in comparison with surfaces with smaller roughness. The surface fluxes derived from the profile measurements were compared with flux measurements obtained by means of an eddy correlation system atop a 29 m mast, some 9 m above a mature section of forest some 4.5 km away from the launching site of the radiosondes; these independent measurements were made by a team from the Institute of Hydrology (Wallingford, England). On average, these two types of estimates were in good agreement. For the sensible heat flux the correlation coefficient was r = 0.75. For the evaporation rate it was r = 0.66. For the evaporation obtained by means of the energy budget from the sensible heat flux it improved to r = 0.82. The profile‐derived fluxes did not compare as favorably with corresponding flux values measured above agricultural crops in clearings. This confirms that the forest was the dominant surface at the regional scale.

Comparisons of surface flux measurement systems used in FIFE 1989

Several types of Bowen ratio energy balance and eddy correlation systems were used to estimate sensible and latent heat fluxes during FIFE. To provide information on the ability of these systems to make representative measurements of sensible and latent heat fluxes at individual sites, several systems were compared by operating them side by side. The different Bowen ratio systems produced estimates that were usually in close agreement; most of the discrepancies were minor and were due to differing estimates of Rn and G. Comparisons between results of Bowen ratio systems and eddy correlation systems at sites 906 (2133‐ECA) and 926 (8739‐ECB), however, were less satisfactory. The discrepancies were occasionally large and seemed to vary in complex terrain with local conditions such as wind direction, upwind slope aspect, and solar zenith angle.

An intercomparison of surface energy flux measurement systems used during FIFE 1987

During FIFE 1987, surface energy fluxes were measured at 22 flux sites by nine groups of scientists using different measuring systems. A rover Bowen ratio station was taken to 20 of the flux stations to serve as a reference for estimating the instrument‐related differences. The rover system was installed within a few meters from the host instrument of a site. Using linear regression analysis, net radiation, Bowen ratio, and latent heat fluxes were compared between the rover measurements and the host measurements. The average differences in net radiation, Bowen ratio, and latent heat flux from different types of instruments can be up to 10, 30, and 20%, respectively. The Didcot net radiometer gave higher net radiation while the Swissteco type showed lower values, as compared to the corrected radiation energy balance system (REBS) model. The four‐way components method and the Thornthwaite type give similar values to the REBS. The surface energy radiation balance systems type Bowen ratio systems exhibit slightly lower Bowen ratios and thus higher latent heat fluxes, compared to the arid zone evapotranspiration systems. Eddy correlation systems showed slightly lower latent heat flux in comparison to the Bowen ratio systems. It is recommended that users of the flux data take these differences into account.

Estimating forest evapotranspiration at a non-ideal site

Evapotranspiration estimates by a Bowen ratio energy balance system and an eddy correlation energy balance system taken simultaneously at a tall coniferous forest site on sloping terrain with inadequate fetch are analyzed. The analysis of the results of both measurement systems reveals significant differences between the two. For most data points the Bowen ratio technique led to lower rates of evapotranspiration when compared with the eddy correlation results. These differences are attributed to measurement errors, to the violation of theoretical requirements, and to site specifics. Especially fetch requirements were smaller for the Bowen ratio system than for the eddy correlation system suggesting a different source area for the two techniques used. The analysis demonstrates that caution is required, when applying micrometeorological methods to non-ideal sites.

Tests of a robust eddy correlation system for sensible heat flux

Sensible heat flux estimates from a simple, one-propeller eddy correlation system (OPEC) were compared with those from a sonic anemometer eddy correlation system (SEC). In accordance with similarity theory, the performance of the OPEC system improved with increasing height of the sensor above the surface. Flux totals from the two systems at sites with adequate fetch were in excellent agreement after frequency response corrections were applied. The propeller system appears suitable for long periods of unattended measurement. The sensible heat flux measurements can be combined with net radiation and soil heat flux measurements to estimate latent heat as a residual in the surface energy balance.

Applying a simple three-dimensional eddy correlation system for latent and sensible heat flux to contrasting forest canopies

A simple eddy correlation system is presented that allows on-line calculation of latent and sensible heat fluxes. The system is composed of a three dimensional propeller anemometer, a thermocouple and a capacitance relative humidity sensor. Results from two contrasting sites demonstrate the capability of the system to measure turbulent fluxes under varying conditions. A dry mixed (dominantly coniferous) forest in hilly terrain in Austria is compared to a well irrigated, heavily transpiring, deciduous pecan orchard in the Southwest of the US. The US site shows insufficient closure of the energy balance that is attributed to non-turbulent fluxes under advective conditions in a stable boundary layer (Blanford et al., 1991) while the Austrian site exhibits almost perfect closure with the use of the very same instruments when the boundary layer is convective and advection is negligible.

A micrometeorological investigation of surface exchange parameters over heathland

An automated system for measuring the dry deposition fluxes of SO2 on a routine basis based on the micrometeorological gradient technique has been developed for application over grassland and other short vegetation. The ability of the system to determine turbulent exchange parameters such as the friction velocity u* and the heat flux H was investigated by comparison with an automated eddy correlation system.

Modelling and measuring evapotranspiration in a daily drip irrigated cotton field

An eddy correlation system (ECS) was used to estimate evapotranspiration (E) in a daily drip irrigated cotton field. Cotton yield, water use, and their ratio (Water use efficiency; WUE) were compared in four irrigation treatments. Three treatments were irrigated at levels of 85, 100, and 115% of E, while the fourth was irrigated according to grower's usual practice. E data were used to verify a one dimensional numerical model which simulates, in real time, the different energy fluxes existing in the soil-plant-atmosphere system. The model requires input of vegetation parameters (leaf area index, photometric properties, shading factor, root density distribution), soil parameters texture, hydraulic and photo-metric properties, temporal micrometeorological data (solar radiation, wind speed, air temperature and humidity) measured above the field, and irrigation quantities. The verification study was carried out during a cotton growing season in Hula Valley, Northern Israel. Results show that E rates are strongly affected by the intensity and arrival time of the inland penetrating Mediterranean sea breeze. WUE in the treatment which was irrigated according to the ECS was highest. Accurate estimations were also made by the model.

Eddy Correlation Systems for Measuring EVAPOTRANSPIRATION

ABSTRACT A portable eddy correlation system was used to make direct, real-time mfeasurements of evapotranspiration over irrigated alfalfa. T^e surface energy balance equation was used to evaluate system performance, with the closure ratio defined as the ratio of the sum of latent and sensible heat fluxes to the sum of net radiation and soil heat flux. For six 24-hour periods, the mean uncorrected closure ratio was 0.85. After corrections were made for sensor separation, the mean closure ratio improved to 0.95. Measurements were made during several other periods, however, sensor malfunctions resulted in erroneous data during these periods. Care was required in deploying and maintaining the instruments, but the system proved to be a viable alternative for making real-time evapotranspiration measurements with portable equipment. KEYWORDS. Evapotranspiration, Eddy correlation. Irrigation, Measurement, Sensors.

Hourly ET Model Calibration with Eddy Flux and Energy Balance Data

The Penman equation for estimating hourly evapotranspiration (ET) of an alfalfa reference crop was calibrated for both daytime and nighttime conditions. Hourly net radiation, sensible heat flux, and soil heat flux were measured in the field for 16 24‐hr periods, and evaporative flux was determined as the residual of the surface energy balance equation. The sensible heat flux measurements were made using a portable eddy correlation system that included a sonic anemometer and a fine‐wire thermocouple. Following calibration, the hourly Penman model was applied to independent weather data from two growing seasons. Since only conventional weather parameters were measured during these two seasons, empirical equations were first developed to relate net radiation to solar radiation, and soil heat flux to solar radiation and air temperature. The resulting hourly ET estimates were summed over 2–4 day intervals and compared to ET data obtained from soil water balance measurements. Good agreement was found between es...

An integrated micrometeorological system for evaporation measurement

An integrated eddy correlation system is described. This system comprises a multi-headed sensor, a real-time processing unit and software for analysing and storing the data on a microcomputer. The instrument, known as the Hydra, was first used for research in 1981. Since then, experience gained in using the instrument on four separate research projects has been applied to the redesign and production of an improved instrument designated the Mark 2. This instrument is described together with two applications, the first over a millet crop in the Sahel region of West Africa and the second over a pine forest in south-west France during the HAPEX experiment.

Corrections for the effect of background concentration change and sensor drift in real-time eddy correlation systems

This paper investigates errors in the measurements of flux and variance of an atmospheric entity given by an eddy correlation system which employs real-time analysis, when there is real or apparent change in its measured concentration. Theoretical correction formulae are derived and their application illustrated with reference to field data.

Frequency response corrections for eddy correlation systems

Simplified expressions describing the frequency response of eddy correlation systems due to sensor response, path-length averaging, sensor separation and signal processing are presented. A routine procedure for estimating and correcting for the frequency response loss in flux and variance measurements is discussed and illustrated by application to the Institute of Hydrology's ‘Hydra’ eddy correlation system. The results show that flux loss from such a system is typically 5 to 10% for sensible and latent heat flux, but can be much larger for momentum flux and variance measurements in certain conditions. A microcomputer program is included which, with little modification, can be used for estimating flux loss from other eddy correlation systems with different or additional sensors.

A microprocessor system for Eddy-correlation

The application of a microprocessor-controlled data acquisition and processing system is described. The system, which is battery powered, takes data from six sensors at a frequency of 10 Hz and computes the fluxes of heat, water vapour and momentum in real time, using the eddy correlation method. From results obtained at a field site, the microprocessor system is separately compared with a proven off-line eddy-correlation system and with measured net radiation. These tests indicate that, provided care is taken in interpreting the data in terms of the performance and exposure of the sensors, the system described is capable of adequately measuring energy fluxes at unattended sites.

Eddy-correlation measurements of sensible heat fluxes over a grass surface

Direct measurements of sensible heat fluxes were conducted over a grass surface at Ladner, British Columbia, using yaw-sphere-thermometer eddy-correlation systems. The results show that for half-hour averaging periods, there is no phase-lag between sensible heat and net radiation flux densities. Field comparison of two yaw-sphere-thermometer systems gave good and consistent agreement. At a height of 2 m above ground and a horizontal crosswind separation of 1.5 m, less than 5% variability was noted in the measured heat fluxes. For a 19-m horizontal separation, the variability was less than 20%. The aridity index (α) advanced by Priestley and Taylor (1972), is shown to be a potentially useful climatic indicator.