Eddy Correlation Instrumentation Research Articles

Regional evapotranspiration estimation based on a two-layer remote-sensing scheme in Shahe River basin

Land surface evapotranspiration (ET) derived from remote sensing data has significant meaning for plant growth monitoring, crop yield assessment, disaster monitoring and understanding energy and water cycle in river basin area and surrounding regions. In the study, we developed a land surface ET remote sensing retrieval system to estimate the daily ET in Shahe river basin using the TM\\ETM+ images. The system is based on the two-layer ET model and includes three parts: inversion of the evaporation ration using two-layer model, calculation of total daily net radiation, and estimation of daily ET based on evaporation fraction method. The results show that the average daily ET is about 2.28mm of the typical days in spring, and 2.97mm in summer, 1.59mm in autumn, and 0.5mm in winter. The ET in upstream areas covered by forest is higher than that in the downstream covered by settlement and farmland. In summer the difference of ET between the upper reaches and lower reaches is smaller compared to the other three seasons. The measurements by large aperture scintillometer and eddy correlation instrument were used for validation. By comparing the observed data with the estimated data, we found the estimation system had a high precision with the relative error between 0 and 16% (mean error of 11.1%), and the variance 0.77mm.

Oxygen optodes as fast sensors for eddy correlation measurements in aquatic systems

The aquatic eddy‐correlation technique can be used to noninvasively determine the oxygen exchange across the sediment‐water interface by analyzing the covariance of vertical flow velocity and oxygen concentration in a small measuring volume above the sea bed. The method requires fast sensors that can follow the rapid changes in flow and the oxygen transported by this flow to calculate the momentary advective flux driven by turbulent motions. In this article, we demonstrate that fast optical oxygen sensors, known as optodes, represent a good alternative to the traditional Clark‐type electrochemical microelectrodes for such measurements. Optodes have the advantage over microelectrodes of being insensitive to flow, less susceptible to signal drift, more durable under typical field conditions, less expensive, and repairable. Comparisons of the response times of optodes and microelectrodes to rapid oxygen changes showed that optimized optodes had the same response time (162 ± 66 ms) as the microelectrodes (160 ± 57 ms) and were fast enough to capture the oxygen fluctuations that are relevant for the eddy correlation flux calculations. Side by side comparisons of benthic O2 flux data collected with microelectrode‐based eddy correlation instruments and optode‐based eddy correlation instruments in freshwater and marine environments showed no significant differences between the measured fluxes. The optodes allow the development of more user‐friendly eddy correlation instruments that combine the advantages of non‐invasive measurements and integration of fluxes over a large footprint area, using a relatively rugged and less expensive sensor.

Simple, robust eddy correlation amplifier for aquatic dissolved oxygen and hydrogen sulfide flux measurements

The aquatic application of the eddy correlation (EC) technique is growing more popular and is gradually becoming a standard method for resolving benthic O2 fluxes. By including the effects of the local hydrodynamics, the EC technique provides greater insight into the nature of benthic O2 exchange than traditional methods (i.e., benthic chambers and lander microprofilers). The growing popularity of the EC technique has led to a greater demand for easily accessible and robust EC instrumentation. Currently, the EC instrumentation is limited to two commercially available systems that are still in the development stage. Here, we present a robust, open source EC picoamplifier that is simple in design and can be easily adapted to both new and existing acoustic Doppler velocimeters (ADV). The picoamplifier has a response time of < 0.1 ms and features galvanic isolation that ensures very low noise contamination of the signal. It can be adjusted to accommodate varying ranges of microelectrode sensitivity as well as other types of amperometric microelectrodes. We show that the extracted flux values are not sensitive to reduced microelectrode operational ranges (i.e., lower resolution) and that no signal loss results from using either a 16‐ or 14‐bit analog‐to‐digital converter. Finally, we demonstrate the capabilities of the picoamplifier with field studies measuring both dissolved O2 and H2S EC fluxes. The picoamplifier presented here consistently acquires high‐quality EC data and provides a simple solution for those who wish to obtain EC instrumentation. The schematic of the amplifier's circuitry is given in the Web Appendix.

Application of Oxygen Eddy Correlation in Aquatic Systems

Abstract The eddy correlation technique is rapidly becoming an established method for resolving dissolved oxygen fluxes in natural aquatic systems. This direct and noninvasive determination of oxygen fluxes close to the sediment by simultaneously measuring the velocity and the dissolved oxygen fluctuations has considerable advantages compared to traditional methods. This paper describes the measurement principle and analyzes the spatial and temporal scales of those fluctuations as a function of turbulence levels. The magnitudes and spectral structure of the expected fluctuations provide the required sensor specifications and define practical boundary conditions for the eddy correlation instrumentation and its deployment. In addition, data analysis and spectral corrections are proposed for the usual nonideal conditions, such as the time shift between the sensor pair and the limited frequency response of the oxygen sensor. The consistency of the eddy correlation measurements in a riverine reservoir has been confirmed—observing a night–day transition from oxygen respiration to net oxygen production, ranging from −20 to +5 mmol m−2 day−1—by comparing two physically independent, eddy correlation instruments deployed side by side. The natural variability of the fluctuations calls for at least ∼1 h of flux data record to achieve a relative accuracy of better than ∼20%. Although various aspects still need improvement, eddy correlation is seen as a promising and soon-to-be widely applied method in natural waters.

Early Spring Oceanic Heat Fluxes and Mixing Observed from Drift Stations North of Svalbard*

Abstract From several drifting ice stations north of Svalbard, Norway, observations were made in early spring of the ocean turbulent characteristics in the upper 150 m using a microstructure profiler and close to the under-ice surface using eddy correlation instrumentation. The dataset is used to obtain average heat fluxes at the ice–water interface, in the mixed layer, across the main pycnocline, as well over different water masses in the region. The results are contrasted with proximity to the branches of the warm and saline Atlantic water current, the West Spitsbergen Current (WSC), which is the main oceanic heat and salinity source both to the region and to the Arctic Ocean. Hydrographic properties show that the surface water mass modification is typically due to atmospheric cooling with relatively less influence of ice melting. Surface heat fluxes of O(100) W m−2 are found within the branches of the WSC and over shelf areas with elevated levels of mixing due to strong tides. Away from the shelves and WSC, however, ocean-to-ice turbulent heat fluxes are typical of the central Arctic. Deeper in the water column, entrainment from below together with equally important horizontal advection and diffusion increase the heat content of the mixed layer and contribute to the heat flux maximum in the upper layers. The results in this study emphasize the importance of mixing along the boundaries, over shelves, and topography for the cooling of the Atlantic water layer in the Arctic in general, and for the regional heat budget, hence the ice cover and cooling of the WSC north of Svalbard, in particular.

Eddy correlation measurements of oxygen uptake in deep ocean sediments

We present and compare small sediment‐water fluxes of O2 determined with the eddy correlation technique, with in situ chambers, and from vertical sediment microprofiles at a 1450 m deep‐ocean site in Sagami Bay, Japan. The average O2 uptake for the three approaches, respectively, was 1.62 ± 0.23 (SE, n = 7), 1.65 ± 0.33 (n = 2), and 1.43 ± 0.15 (n = 25) mmol m−2 d−1. The very good agreement between the eddy correlation flux and the chamber flux serves as a new, important validation of the eddy correlation technique. It demonstrates that the eddy correlation instrumentation available today is precise and can resolve accurately even very small benthic O2 fluxes. The correlated fluctuations in vertical velocity and O2 concentration that give the eddy flux had average values of 0.074 cm s−1 and 0.049 µM. The latter represents only 0.08% of the 59 µM mean O2 concentration of the bottom water. Note that these specific fluctuations are average values, and that even smaller variations were recorded and contributed to the eddy flux. Our findings demonstrate that the eddy correlation technique is a highly attractive alternative to traditional flux methods for measuring even very small benthic O2 fluxes.

Methodological Considerations Regarding the Measurement of Turbulent Fluxes in the Urban Roughness Sublayer: The Role of Scintillometery

We address some of the methodological challenges associated with the measurement of turbulence and use of scintillometers in the urban roughness sublayer (RSL). Two small-aperture scintillometers were located near the roof interface in a densely urbanized part of Basel, Switzerland, as part of the Basel Urban Boundary-Layer Experiment (BUBBLE) in the summer of 2002. Eddy correlation instruments were co-located near the mid-point of each scintillometer path for data verification purposes. The study presents the first values of the inner length scale of turbulence (l 0) and the refractive index structure parameter of air $$(C_{n}^{2})$$ for a city and demonstrates the influence of mechanical driven turbulence on dissipation. Comparison of dissipation values determined from the two approaches show large scatter that is possibly due to the spatial inhomogeneity of the turbulence statistics within the RSL. Velocity and temperature spectra display a −2/3 slope in the inertial subrange, although the spectral ratio is less than the theoretical prediction of 4/3 expected for isotropy. Conventional Monin–Obukhov equations used to calculate fluxes from the scintillometer were replaced with urban forms of the equations. The results suggest that the scintillometer may be an appropriate tool for the measurement of sensible heat flux (Q H ) above the rooftops given a suitable determination of the effective measurement height.

The impact of broadleaved woodland on water resources in lowland UK: II. Evaporation estimates from sensible heat flux measurements over beech woodland and grass on chalk sites in Hampshire

Abstract. The impact on recharge to the Chalk aquifer of substitution of broadleaved woodland for pasture is a matter of concern in the UK. Hence, measurements of energy balance components were made above beech woodland and above pasture, both growing on shallow soils over chalk in Hampshire. Latent heat flux (evaporation) was calculated as the residual from these measurements of energy balances in which sensible heat flux was measured with an eddy correlation instrument that determined fast response vertical wind speeds and associated temperature changes. Assessment of wind turbulence statistics confirmed that the eddy correlation device performed satisfactorily in both wet and dry conditions. There was excellent agreement between forest transpiration measurements made by eddy correlation and stand level tree transpiration measured with sap flow devices. Over the period of the measurements, from March 1999 to late summer 2000, changes in soil water content were small and grassland evaporation and transpiration estimated from energy balance-eddy flux measurements were in excellent agreement with Penman estimates of potential evaporation. Over the 18-month measurement period, the cumulative difference between broadleaved woodland and grassland was small but evaporation from the grassland was 3% higher than that from the woodland. In the springs of 1999 and 2000, evaporation from the grassland was greater than that from the woodland. However, following leaf emergence in the woodland, the difference in cumulative evaporation diminished until the following spring.

The energy budget of a spruce forest: field measurements and comparison with the forest–land–atmosphere model (FLAME)

The components of the energy budget (net radiation Rn, soil heat flux G, canopy heat storage S, turbulent fluxes of sensible and latent heat ( H, λE) were determined above and inside a 30 m tall spruce forest in central Germany in summer 1994 using micrometeorological equipment and eddy correlation instruments. Data of net radiation, turbulent fluxes and profiles of temperature and wind speed are compared with results obtained with the Forest–Land–Atmosphere-ModEl (FLAME), which is initialized with temperature, humidity and wind speed profiles within and above the canopy and boundary conditions in the soil and at the top of the atmospheric boundary layer. The model is able to quantify the contribution of large eddies transporting momentum and scalar quantities from above the canopy into the trunk space.

A comparison of surface fluxes at the HAPEX-Sahel fallow bush sites

The variability between surface flux measurements at the fallow sites of the three HAPEX-Sahel supersites is examined over periods of three or four consecutive days. A roving eddy correlation instrument provided a common base for comparison at each supersite. The inhomogeneity of the surface and the instrumental layout did not provide the conditions to allow the separation of the effects of instrument error from those due to the spatial variability of vegetation cover and soil moisture. Surface fluxes of sensible and latent heat and energy balance terms were intercompared at each supersite over summation timescales of 1 hour and 3 days. It is shown that, generally, HAPEX-Sahel hourly sensible heat flux and latent heat values have confidence limits of 15% and 20% respectively. The three-day period energy balance shows the combined sensible and latent heat fluxes to have a confidence limit of 3%. It is concluded that, due to the averaging effect of longer time periods and larger flux footprints on spatial inhomogeneity, confidence in the surface flux measurements increases with longer summation periods and with neutral atmospheric surface layers which characterise the rainy period of the Intensive Observation Period.

Production of a filtered and standardized surface flux data set for FIFE 1987

A filtered and standardized surface heat flux data set, which includes net radiation (Rn), latent heat flux (λE), sensible heat flux (H), and ground heat flux (G), was produced from an analysis of field measurements taken by a network of 16 Bowen ratio and 6 eddy correlation instruments deployed during the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment in 1987 (FIFE 1987). The objective was to produce time series of flux data that could be used for studies involving surface‐vegetation‐atmosphere‐transfer (SVAT) models operating in the inverse mode (see Sellers et al. 1992b). To be useful for this work, erroneous flux data had to be identified and systematic instrument‐related differences between the net radiation values reported by flux stations had to be accounted for. The following procedures were carried out on the original data sets as contributed by individual FIFE scientists: (1) A flag (IDATA) was assigned to unreliable flux data as defined by the presence of anomalies (“spikes”) in the sensible and latent heat flux time series, discontinuities generated when the temperature and humidity profiles crossed over at dawn and dusk, and data points isolated by the erasure of neighboring unreliable data. In addition, Bowen ratio data were also considered to be unreliable whenever the surface energy budget was reported as nonzero (Rn + λE + H + G ≠ 0). (2) A simple procedure was used to adjust the net radiation values reported by two of the Bowen ratio stations to appear as if they were using the same type of net radiometer as most of the other Bowen ratio stations deployed in FIFE 1987. Corresponding changes were made to the associated sensible and latent heat fluxes for these two stations; the ground heat fluxes were left unchanged. At one eddy correlation site (site 26), the sensible and latent heat fluxes were suspected to be systematically underestimated; these were proportionately increased to match the site net radiation flux minus a FIFE‐area‐average ground heat flux. (3) An additional flag (IDF) was assigned to data based on modeling results obtained from the simple biosphere (SiB) model (see Sellers et al., 1992b). Nonzero values of IDF denote periods when the flux data time series diverge significantly from a set of calibrated model results, suggesting a bias in the data or indicating that SiB could not reproduce the observed latent and sensible heat flux time series using a reasonable set of surface parameters. An IDF flag was assigned to each flux station for each intensive field campaign (IFC). (4) Using the IDATA and IDF flags to identify and exclude questionable data points, a site‐averaged surface heat flux data set was computed for each IFC from the remaining data. This is compared to a similar “hand‐filtered” data set produced by Betts and Ball as published by Strebel et al. (1994).

Development and evaluation of continuously weighing mini-lysimeters

Micrometeorological methods for measuring evapotranspiration are based on the assumption that the water vapor flux is constant with respect to height within the first few meters above the ground, and consequently are not applicable where an area is not extensively homogeneous. An increasing emphasis on energy and water flux studies in heterogeneous terrain (e.g. urban and hilly environments) has created a need to develop alternative approaches to quantify the latent heat flux. Lysimetry is an established alternative that permits the direct measurement of evapotranspiration from the ground surface and thus can provide accurate point estimates of the latent heat flux in heterogeneous environments. To date, the continuously weighing lysimeters that have been developed and extensively tested are large (surface area generally greater than 2 m 2), stationary and expensive to construct and maintain. These instruments are also inappropriate for most geographic studies of energy and water budgets in heterogeneous environments because knowledge of the spatial variability of evapotranspiration and quantification of the horizontal transport of moisture and heat require multiple, simultaneous flux determinations. This research tests two continuously weighing, portable mini-lysimeters (<0.2 m 2) designed for practical application in heterogeneous environments. The performance of the lysimeters are compared with evaporative flux measurements obtained using eddy correlation instrumentation from an extensive homogeneous surface. The soil moisture status of the monolith and the stomatal resistance of the plants within the lysimeter are compared with simultaneous measurements from the surrounding grass field. The results show that the evaporative flux, soil moisture and stomatal resistance measurements from both lysimeters compare favorably with measurements that are representative of their surroundings.

Estimates of sensible heat flux from observations of temperature fluctuations

Comparisons between sensible heat flux measured using eddy correlation instrumentation and estimated using the temperature fluctuation method are presented for four types of surface in West Africa. Agreement between measured and estimated values is good. Regression of estimated on measured sensible heat flux gave a mean slope of 0.98 with a mean r2 of 0.94 for bare soil, mature millet, fallow savannah and tiger bush. Estimates of heat flux from temperature fluctuations measured by an instrument mounted beneath a tethered balloon are also shown to be in close agreement with eddy correlation measurements made at the surface (regression slope = 0.98, r2 = 0.84). The results provide evidence that the ratio Σθ/θ×is indeed a universal function of z/L for all the surface types considered.

Bowen ratio, eddy correlation, and portable chamber measurements of sensible and latent heat flux over irrigated spring wheat

Measurements of the latent ( LE) and sensible ( H) heat flux density in the atmospheric boundary layer of irrigated crops have applications for understanding processes in agriculture and meteorology and for water management. The objective of this research was to compare measured Bowen ratios and calculated LE and H from four Bowen ratio systems (BR1–BR4) of different design with each other and with fluxes measured by three sets of eddy correlation instrumentation ( H and LE) and a portable chamber ( LE). Measurements were made on 9 and 10 April 1989 in an irrigated wheat field at the Maricopa Agricultural Center near Maricopa, Arizona. The Bowen ratio system designs varied in terms of temperature and humidity sensors and measurement arm movement. Bowen ratios were lower (more negative) on 9 April for all of the systems. The range of the four Bowen ratios was greatest in the early morning and late afternoon ( ±0.1) and least around noon ( +0.02). Measured net radiation and soil heat flux density were constant in the Bowen ratio LE calculations. The range of daytime LE from the four systems on 9 April and from three on 10 April was 11% and 1% of the mean LE, respectively. The three eddy correlation H measurements were essentially equal to each other. The average eddy correlation H was 82% and 69% of Bowen ratio H on 9 and 10 April, respectively whilst the eddy correlation LE was 77% and 67% of Bowen ratio LE on the two days. On 9 and 10 April, portable chamber LE was greater than Bowen ratio LE during periods of southerly winds owing to the effect of advected energy to the southern field edge where chamber measurements were made. On 10 April, portable chamber LE was 125% of Bowen ratio LE. This study has shown that: (1) Bowen ratios from instrumentation of different designs were similar; (2) eddy correlation H from three systems were similar to each other and were slightly less than Bowen ratio H; (3) eddy correlation LE was consistently and significantly less than Bowen ratio LE; (4) measurements of portable chamber LE on the edge of a field were affected by surrounding conditions.

Vapour flux density and transpiration rate comparisons in a stand of Maritime pine ( Pinus pinaster Ait.) in Les Landes forest

Measurements of sap flow made on eight trees in a stand of Maritime pine ( Pinus pinaster Ait.) are presented, and compared with potential evaporation and concurrent measurements of stomatal conductance, water potential, and the total vapour flux as measured by eddy correlation instrumentation. When the probable transpiration from the understorey is taken into account, there is good agreement between the two methods.

Carbon dioxide, water vapor and sensible heat exchanges of a grain sorghum canopy

With the aid of eddy correlation instrumentation, the components of the energy budget and CO2 flux were measured over grain sorghum grown at Mead, Nebraska. Diurnal patterns of sensible heat, latent heat, CO2 and momentum flux are examined for typical days. On a mostly clear day when the crop leaf area index was 3.7, net radiation reached a mid-day peak of 560 W m-2, while sensible and latent heat fluxes peaked at 50 and 460 W m-2, respectively. The peak CO2 flux occurring just prior to solar noon was 1.5 mg m-2(ground area) s-1. CO2 flux (respiration from plants, soil and roots) in the early evening was about -0.28 mg m-2 (ground area) s-1.

Turbulent transport of carbon dioxide over a paddy field

Turbulent fluctuations in CO2 concentrations over a paddy field are measured by a fastresponse device with an open sensing path. This IR device coupled with a sonic anemometer constitutes an eddy correlation instrument to measure CO2 fluxes. Three experiments were conducted in the surface layer over paddy 90 cm high. The stability (z − d)/L ranged from -0.14 to 0.20, where L denotes the Monin-Obukhov length.

A Study of Eddy Fluxes Over a Forest

Eddy correlation instruments mounted above a plantation of Pinus radiato near Mt. Gambier, South Australia, have been operated during two periods of intensive effort, in May and October, 1972. Measurements of the Reynolds stress and of wind speed gradients show that the zero plane for momentum is located at about d = 0.8h (where h is the height of the trees), and that the roughness length of the surface is about 30% of the difference (h—d).Sensible heat fluxes and temperature gradients give a displacement length not significantly different from that applicable in the momentum case, but the roughness length for sensible heat transfer is smaller than that for momentum, by about a factor of 3.Advective effects are found to be important, particularly when the fetch across the canopy is less than about 0.8 km (corresponding in our case to an effective fetch/height ratio of between 100 and 200). Long-fetch cases allow an evaluation of the heat storage (S) in the canopy and in the air below the height of eddy flux measurement. The rate of heat storage is found to be about 60 ± 20 W m−2 per °C h−1 of canopy temperature change (for a densely packed forest with trees about 13 m high), which is compatible with measurements of the biomass and assumed specific heats. The residual heat energy at about 6 m above the effective zero plane, unaccounted for by the various measured fluxes, is found to be related to the difference in net radiation over grassland and forest.During daytime, the forest is found to lose heat by turbulence in much the same manner as pasture, with fluxes of similar magnitude (although possibly differing to the extent of differences in ground flux, albedo and emissivity, for example) and giving similar Bowen ratios. At night, however, the evaporation from the forest tends to continue as heat is supplied by the cooling canopy. This is in direct contrast to the usual situation over pasture, where the heat storage is not of sufficient magnitude to result in this behavior.

Measurements of eddy‐fluxes over the sea from an off‐shore oil rig

AbstractTwo eddy correlation instruments (Fluxatrons) and associated anemometers were installed on the Marlin natural gas platform, which stands in Bass Strait about 60 m deep and 40 km off the Victorian coast (lat. 38° 14′S, long. 148° 13′E). The sensing heads were mounted 5 m above the sea surface on a boom extending from the rig. At this height the rig did not appear to affect the wind flow.The measured Reynolds stresses give an average value of 0.0011 ± 0.0001 for the drag coefficient referred to the 10 m wind speed (31 half hour observations over the wind speed range 2–10 m s−1). Below about 5 m s−1 the drag coefficient is not significantly different from that of an aerodynamically smooth surface. Above this speed, it increases with velocity, reaching about 0.0015 at 10 m s−1.The measured sensible heat‐fluxes are small, showing that evaporation dominates sensible heat in the sea‐air energy budget. There is no reason to dispute the bulk aerodynamic approach, in which the heat‐flux is written as H = p cp d ua Δθa. The data do not allow an accurate estimate of the transfer coefficient, d, which is found here to be 0.0014 (±25 per cent), insignificantly different from the value found for the case of evaporation from inland lakes.Heat budget considerations imply that about 60 per cent of the day‐time net radiation was used to heat the water body.The success of the experiment suggests the feasibility of using oil rigs as platforms for further air‐sea interaction studies. The stability of such platforms, even at very high wind speeds, is particularly attractive.

The evaporation from a swamp

The evaporation from an Australian swamp of area 3,000 ha has been measured over several daylight 1 2 - h periods during three days in summer, at the same time as measurements of the evaporation from a lake 16 km away. The swamp evaporation was determined using an eddy-correlation instrument. This was used also in preliminary measurements at the lake, to verify the appropriate constant in a “bulk property”-formula for lake evaporation, enabling estimates to be made of the latter from measurements of wind travel, dry- and wet-bulb temperatures, and water surface temperature. The results suggest that, except immediately after rain, the swamp lost water at a rate significantly lower than the lake. This is attributed to the lower albedo of the clear water surface of the lake, the shelter given by the reeds in the swamp to the water surface and the internal resistance to water movement of the reeds themselves.