Eddy Correlation System Research Articles

Measuring system for the long‐term monitoring of biosphere/atmosphere exchange of carbon dioxide

We describe an ongoing program being carried out in Hungary to investigate the role of the temperate continental region in the global carbon cycle. Carbon dioxide mixing ratios are continuously monitored at 10, 48, 82, and 115 m above the ground on a television transmitter tower, and the atmosphere/surface exchange of CO2 is measured by eddy covariance at 82 m. The region surrounding the tower is typical of rural areas of central Europe, with agricultural fields, forest patches, and small villages. We first describe the layout and the operation of the measuring system designed for the continuous, unattended monitoring of the vertical distribution of CO2 mixing ratio in the lowest 115 m of the atmosphere based on a Li‐Cor model 6251 infrared gas analyzer (IRGA). It provides vertical profile data with a temporal resolution of 8 min. Next, we discuss the measuring system for long‐term, continuous monitoring of the biosphere/atmosphere exchange of CO2. The eddy correlation system is based on a Li‐Cor model 6262 fast‐response IRGA and a Gill ultrasonic anemometer running at 4 Hz sampling frequency. Results are to illustrate the performance of the systems. Among others, they show the occasional accumulation of CO2 in the boundary layer in the Carpathian Basin during winter and the diurnal variation of the vertical distribution of CO2 mixing ratio in summer. A simple method based on similarity theory to calculate vertical fluxes from vertical gradients is presented, which can be used to fill the data gaps that inevitably occur during long‐term eddy correlation measurements. The present study confirms the feasibility of the long‐term tall‐tower CO2 flux and mixing ratio measurements.

Estimation of heat and momentum fluxes over complex terrain using a large aperture scintillometer

A comprehensive experimental plan has been designed to further investigate the potential and the limitations associated with the use of a large aperture scintillometer (LAS) to infer path average sensible and momentum fluxes over complex surfaces as part of the Semi-Arid Land–Surface–Atmosphere (SALSA) Program. The complexity of the terrain is associated with the type and the cover of the vegetation canopy as well as with changes in topography. Scintillometer based estimates of sensible heat flux and friction velocity are compared to those measured by eddy correlation systems over a grassland patch, a mesquite patch, and over a transect spanning both patches. The results show that considering the complexity of the surface, the overall performance of the scintillometer is relatively good.

CALIBRATION OF THE PENMAN-MONTEITH METHOD TO ESTIMATE LATENT HEAT FLUX OVER A GRASS CANOPY

A study was performed at the Panguilemo Evapotranspiration Investigation Plot (ETIP) located 11 km north of Talca, Chile (35°23' South and 71° West; 110 m above the sea level) to evaluate latent heat flux by using the Penman-Monteith method. Data collected on a 20 minute time intervals included latent heat flux (LE), net radiation (Rn), soil heat flux (G), air temperature (Ta), relative humidity (RH), and wind speed (V). The performance of the Penman-Monteith equation (LEPM) was tested with latent heat flux measurements from the eddy correlation system (LEED) over a grass canopy under different soil moisture conditions. Results indicate that there was a good agreement between LEPM and LEED with an overall standard error of estimate (SEE) of 45 W m · -2 and an absolute relative error of 6.2 %.

Surface heat flux estimation with wind-profiler/RASS and radiosonde observations

A bulk ABL similarity approach was used to make regional estimates of the sensible heat flux by combining surface temperature measurements with mixed layer temperature and wind speed profiles. The mixed layer profiles were measured by a 915 MHz Profiler/Radio Acoustic Sounding System and by radiosondes in north-central Oklahoma at the ARM Southern Great Plains CART Central Facility. A comparison of calculated sensible heat flux values with regional mean values measured at two ground stations showed good agreement with r=0.88 and r=0.76 for the 915 MHz profiler and the radiosonde data sets, respectively. Estimates of friction velocity u * by means of radiosonde wind profiles gave good results when compared to values measured by an surface eddy correlation system with a correlation coefficient r=0.77. However, u * values obtained from wind profiles measured by the 915 MHz profiler were underestimated, because these velocity measurements were systematically too small. The results also show that the 915 MHz Profiler/Radio Acoustic Sounding System is capable of providing the needed temperature measurements to make reasonable regional estimates of sensible heat flux.

Assessment of a relaxed eddy accumulation for measurements of fluxes of biogenic volatile organic compounds: study over arable crops and a mature beech forest

A relaxed eddy accumulation (REA) system, based on the design by Beverland et al. (Journal of Geophysics Research 101 (D17) 22, 807-22, 815), for the measurement of biogenic VOC species was evaluated by intercomparison with an eddy correlation CO 2 flux system over a mature deciduous beech canopy ( Fagus Sylvatica) during the FOREXNOX program. Measurements from a site where winter wheat and barley ( Hordeum Vulgare and Triticum Aestivum) were being harvested are also presented. The system was inter-compared with two different eddy correlation systems for measuring CO 2 fluxes. Good results were obtained with correlation coefficients for the REA system ranging from 0.71 to 0.82, lending further confidence in the use of this technique. Daily averaged biogenic emissions from the wheat and barley canopies were significantly larger than expected, likely a result of harvesting. Fluxes measured over the beech canopy were also larger than might be expected from northern latitude deciduous forests.

Seasonal variation of boreal forest surface conductance and evaporation

Long term measurements (June 1994 to December 1996) of evaporation were made in a boreal forest in central Sweden. Fluxes were measured continuously with eddy-correlation systems from a 100m tower. Surface conductance and potential evaporation were estimated using the Penman–Monteith equation. The overall average evaporation during the 947-days observation period was 1.07mmd−1. The average evaporation from June to December 1994 was 1.25mmd−1, 1.07mmd−1 during 1995, and 0.97mmd−1 during 1996. Maximum daily rates were typically 4mmd−1 around mid-summer in 1994 and 1995 and slightly less in 1996. During the winter period from November to March, the evaporation sometimes reached 0.5mmd−1. Generally, the actual evaporation followed the dynamics of the potential evaporation fairly well. As a total for the entire period, the actual evaporation accounted for 918mm, or 69% of the potential evaporation (1332mm). There were three major time periods with a considerable evaporation deficit: the late summer (July–August) 1994, the late summer (August) 1995, and the spring to early summer (March–June) 1996 (Table 1Table 1Sums of actual and potential evaporation in NorundaPeriodE (mm)Epot (mm)E/Epot (%)July 1994 to October 1996916133169July–August 199413218073August 19956510463March–May 199611624348December 1994 to January 19954.670.15December 1995 to January 19962.690.80). The first two periods were correlated with low soil moisture while the last probably was caused by low winter temperatures. A surface conductance model showed little dependence on soil moisture. Radiation and vapour pressure deficit were the most important factors.When soil water was not limiting, the diurnal courses of surface conductance showed a steep increase in the morning followed by an almost linear decrease, the coniferous trend caused by stomatal control. During periods of water stress a midday depression occurred after the morning maximum. During wintertime, the surface conductance hardly ever approached to zero.A simple model for surface conductance based on global radiation and vapour pressure deficit fitted the estimated data fairly well except for calm nights with poor turbulent mixing. This was apparently due to water vapour storage within and above the canopy. Including the friction velocity in the model parameterisation improved the fitting considerably.

A new model for the structure function of integrated water vapor in turbulence

Turbulent fluctuation of integrated water vapor in the troposphere is one of the major noise sources in radio interferometry. Processed integrated water vapor estimates from microwave radiometers colocated with interferometers have been used to set bounds on this uncertainty. The bound has been in the form of a calculated structure function, which is a measure of temporal or spatial decorrelation of fluctuations. In this paper a new model is presented for the estimation of the structure function in the absence of radiometer measurements. Using this model, the structure function can be estimated using measurements or estimates of a limited number of meteorological parameters. These parameters include boundary layer depth, surface heat and humidity fluxes, entrainment humidity flux, average virtual potential temperature in the boundary layer, and geostrophic wind speed. These parameters can be found or estimated from radiosonde and surface eddy correlation system data. The model is based on a framework of turbulence meteorology and provides excellent agreement when compared with state‐of‐the‐art atmospheric turbulence simulations. Results of preliminary comparisons with ground truth show some excellent agreement, as well as some problems. The performance of the new model exceeds that of one current model.

Below-canopy and soil CO 2 fluxes in a ponderosa pine forest

Below-canopy eddy covariance measurements of CO 2 flux ( F cb) and soil surface CO 2 flux measurements ( F s) were made seasonally in a ponderosa pine forest in central Oregon in 1996 and 1997. The forest ecosystem has a very open canopy, and it is subject to drought and high vapor pressure deficits in summer. Below-canopy flux measurements in March, May, and August 1997 showed increasing effluxes from the forest floor as soils warmed. In July 1996, daytime F cb measurements appeared to have been influenced by photosynthetic uptake of CO 2 by ground vegetation. We did not see a similar diurnal trend in F cb data in August 1997, probably because photosynthesis may have decreased with senescence of ∼1/3 of the pine canopy and the herbaceous species. On 4 days in August 1997, the mean nocturnal F s (2.6 ± 0.08 μmol m −2 s −1) was lower than nocturnal F cb (3.5 ± 0.28 μmol m −2 s −1) by 26%, and daytime F s was lower than nocturnal F cb by 18%, possibly because F cb includes respiration by understory and the lower portions of trees. The mean nocturnal NEE calculated from above-canopy flux and storage in the canopy airspace ( F ca + F stor) at this time was 2.8 ± 0.40 μmol m −2 s −1, 23% lower than ecosystem respiration calculated from chamber measurements on soils, wood, and foliage. The largest difference was observed on a more a turbulent night ( u * = 0.30 m s −1) when F ca + F stor was even significantly less than F cb and F s. Our hypothesis is that under calm conditions (e.g. u * < 0.15 m s −1 as observed on three of the nights), F ca is negligible and has no impact on the CO 2 budget. Under weak wind conditions (e.g. u * = 0.30 m s −1), F ca begins to become significant and fluxes missed by the above-canopy eddy correlation system degrade the CO 2 budget. Under windy conditions, the above-canopy eddy correlation measurement is a good approximation and the CO 2 budget improves again. Below-canopy flux measurements provided useful temporal information for understanding seasonal differences in diel patterns, while the chambers allowed us to characterize spatial variation in CO 2 fluxes. It is important to measure below-canopy fluxes along with above-canopy fluxes throughout the year to understand CO 2 exchange components and annual contributions to the carbon budget of open canopy forest systems.

Long‐term measurements of boreal forest carbon balance reveal large temperature sensitivity

AbstractWe present results from two years’ net ecosystem flux measurements above a boreal forest in central Sweden. Fluxes were measured with an eddy correlation system based on a sonic anemometer and a closed path CO2 and H2O gas analyser. The measurements show that the forest acted as a source during this period, and that the annual balance is highly sensitive to changes in temperature. The accumulated flux of carbon dioxide during the full two‐year period was in the range 480–1600 g CO2 m–2. The broad range is caused by uncertainty regarding assessment of the night‐time fluxes. Although annual mean temperature remained close to normal, the results are partly explained by higher than normal respiration, due to abnormal temperature distribution and reduced soil moisture during one growing season. The finding that a closed forest can be a source of carbon over such a long period as two years contrasts sharply with the common belief that forests are always carbon sinks.

細線熱電対乾湿計を用いた渦相関法による降雨時の顕熱・潜熱フラックス測定

Sensible and latent heat flux above a deciduous forest was measured directly by eddy correlation method using a sonic anemometer and a fine thermocouple psychrometer during rainfall. Analizing the power spectum of the wind speed components, the sonic anemometer (Kaijo Denki, WA-200) was considered to have operated well even in heavy rainfall. The fine thermocouple psychrometer had two problems; one was the heating by solar radiation and another was the slow response time. Nevertheless, it was regarded to be accurate enough when used in rainy days with low solar radiation and low fluctuation of air temperature and humidity. The dry-bulb of the fine thermocouple psychrometer was small enough, so it seldome caught rain drops; only one measurement out of 130 was affected by a rain drop. Error in calculated heat flux caused by catching a rain drop on the dry-bulb of the fine thermocouple psychrometer was estimated. Standard error was expected to be 10W m-2 both for sensible and latent heat flux. As a conclusion, the eddy correlation system with a sonic anemometer and a fine thermocouple psychrometer was available for measurement of sensible and latent heat flux during rainfall.

Eddy correlation and relaxed eddy accumulation measurements of CO 2 fluxes over grassland

Eddy correlation measurements of CO 2 fluxes over grassland on peat soil were performed from April to October 1994. In this article the measurement system is described and some results are discussed. The closed path system was compared with an open path eddy correlation system developed at the Royal Netherlands Meteorological Institute (KNMI) in December 1994. Fluxes deduced from the results of the two systems agree within 10%. The eddy correlation system was adapted to perform eddy correlation and relaxed eddy accumulation at the same time. Measurements over a pasture in the Netherlands showed an agreement between eddy correlation and relaxed eddy accumulation CO 2 fluxes within 10%.

Long-term comparison of energy flux calculation methods over an agricultural field

Since March 1990 micrometeorological measurements were carried out over an agricultural field with varying land use (wheat, barley, sunflowers, mustard) using a profile mast and an energy balance mast with an eddy correlation system for the sensible heat flux. Soil temperature, soil heat flux, soil moisture and precipitation were measured as well. Long-term measurements allow statistical analysis of the energy fluxes and comparisons of different methods for their calculation (eddy correlation, flux profile, Bowen ratio and the residual method). For the sensible heat flux a good agreement was found using these different methods after applying all necessary corrections. The latent heat flux shows greater deviations in the daily cycle between the flux profile method and the residual method due to the shape of the humidity profiles which often and especially at night show a maximum at heights between 1 m and 4 m, even if the soil is free of vegetation. This could be a consequence of the patchiness of the agricultural area, the position of the station on top of a hillock or high water absorption of the soil, respectively. The residual method seems to give more reliable results for the actual evapotranspiration than the flux profile method or the Bowen ratio method if an eddy correlation system is used to determine the sensible heat flux. Differences in the soil heat flux measured with heat flux plates and determined using the profiles of soil temperature and soil moisture can be explained by the heat flux plates being a disturbance to the soil matrix.

The mechanisms involved in the natural ventilation of greenhouses

Although natural ventilation is one of the major mechanisms that controls the greenhouse climate, our understanding of the underlying processes remains insufficient to allow accurate prediction of the rates of such exchanges. This paper deals with the physical mechanisms involved in natural ventilation of a greenhouse equipped with continuous lateral windows, and uses the following experimental procedures: • • air exchange rate measurements, using tracer gas or heat and water balance techniques; • • direct determination of the air and heat flows through an opening, using an eddy correlation system, comprising a sonic anemometer and a fine wire thermocouple; • • measurements of mean and turbulent pressure differences at ground level between inside and outside. The methods employed allow the prediction of greenhouse air exchange rates as well as the characterization of its components: a steady effect resulting from the combination of both mean wind-related and stack effects and a turbulent effect linked to wind speed fluctuations. Local estimations of total, mean and turbulent flows are provided: a wind parallel to the greenhouse axis produces an inflow at the leeward half and an outflow at the windward half. The mean flow of sensible heat is estimated between 55% and 80% of the total flux so that the turbulent flow does not exceed 45% of the total. Local estimations of total, mean and turbulent flows are compared with air exchange rate measurements using the decay rate method and a good agreement between both approaches is demonstrated.

Flux agreement above a Scots pine plantation

The surface energy exchange of 12m high Scots pine plantation at Hartheim, Germany, was measured with a variety of methods during a 11-day period of fine weather in mid-May 1992. Net radiation and rate of thermal storage were measured with conventional net radiometers, soil heat flux discs and temperature-based storage models. The turbulent fluxes discussed in this report were obtained with an interchanging Bowen ratio energy budget system (BREB, at 14 m), two one-propeller eddy correlation systems (OPEC systems 1 and 2 at 17m), a 1-dimensional sonic eddy correlation system (SEC system 3) at 15 m, all on one “low” tower, and a 3-dimensional sonic eddy correlation system (SEC system 22) at 22 m on the “high” tower that was about 46 m distant. All systems measured sensible and latent heat (H and LE) directly, except for OPEC systems 1 and 2 which estimated LE as a residual term in the surface energy balance. Closure of turbulent fluxes from the two SEC systems was around 80% for daytime and 30% for night, with closure of 1-dimensional SEC system 3 exceeding that of 3-dimensional SEC system 22. The night measurements of turbulent fluxes contained considerable uncertainty, especially with the BREB system where measured gradients often yielded erroneous fluxes due to problems inherent in the method (i.e., computational instability as Bowen's ratio approaches −1). Also, both eddy correlation system designs (OPEC and SEC) appeared to underestimate |H| during stable conditions at night. In addition, both sonic systems (1- and 3-dimensional) underestimated |LE| during stable conditions. The underestimate of |H| at night generated residual estimates of OPEC LE containing a “phantom dew” error that erroneously decreased daily LE totals by about 10 percent. These special night problems are circumvented here by comparing results for daytime periods only, rather than for full days. To summarize, turbulent fluxes on the low tower from OPEC system 2 and the adjacent SEC system 3 were in reasonable agreement, while the BREB system appeared to overestimate H and underestimate LE; H and LE measured by SEC system 22 on the high tower were lower than from OPEC and SEC3 on the low tower. The turbulent flux measurements tended to converge, but the data exhibit unexplained differences between days, between systems, and between locations.

The May?October energy budget of a Scots pine plantation at Hartheim, Germany

This paper describes measurements of the Hartheim forest energy budget for the 157-day period of May 11 – Oct. 14, 1992. Data were collected as 30-min means. Energy available to the forest was measured with net radiometers and soil heat flux discs; sensible heat exchange between the canopy and atmosphere was measured with two “One-Propeller Eddy Correlation” (OPEC) systems, and latent energy (evapotranspiration orET) was determined as a residual in the surface energy balance equation. Net rediation, change in thermal storage, and sensible heat flux were verified by independent measurements during the Hartheim Experiment (HartX, May 11–12), and again during the “HartX2” experiment over 20 days late in the summer (Sep. 10–29). Specifically, sensible heat estimates from the two adjacent OPEC sensor sets were in close agreement throughout the summer, and in excellent agreement with measurements of sonic eddy correlation systems in May and September. The eddy correlation/energy balance technique was observed to overestimate occurrence of dew, leading to an underestimate of dailyET of about 5%. After taking dew into account, estimates of OPECET totaled 358 mm over the 5.1-month period, which is in quite good agreement with an ET estimate of 328 mm from a hydrologic water balance. An observed decrease in forestET in July and August was clearly associated with low rainfall and increased soil water deficit. The OPEC system required only modest technical supervision, and generated a data yield of 99.5% over the period DOY 144–288. The documented verification and precision of this energy budget appears to be unmatched by any other long-term forest study reported to date.

Intra-field variability of scalar flux densities across a transition between a desert and an irrigated potato field

This paper reports on measurements of sensible and latent heat and CO2 fluxes made over an irrigated potato field, growing next to a patch of desert. The study was conducted using two eddy correlation systems. One measurement system was located within the equilibrium boundary layer 800 m downwind from the edge of the potato field. The other measurement system was mobile and was placed at various downwind positions to probe the horizontal transition of vertical scalar fluxes. Latent (LE) and sensible (H) heat fluxes, measured at 4 m above the surface, exhibited marked variations with downwind distance over the field. Only after the fetch to height ratio exceeded 75 to 1 didLE andH become invariant with downwind distance. When latent and sensible heat fluxes were measured upwind of this threshold, significant advection of humidity-deficit occurred, causing a vertical flux divergence ofH andLE.

Determination of evaporation from integrated profiles of humidity and temperature over an inhomogeneous surface

In this study a moderate-sized alfalfa field was downwind of a large dry region. Measured vertical profiles of temperature, humidity and wind at upwind and downwind sides of the field were used to calculate the short-term evaporation rate, as well as the contribution of horizontally transported or advected heat energy to the evaporation. The vertical profiles must be measured at least to the height at which air is modified by the new surface. In this case that height was as large as 18 m over a several hundred meter long traverse. Evaporation rates calculated by such an approach were in very close agreement to surface fluxes measured by an eddy correlation system near the surface. The difference between calculated and measured values averaged 9.5%. The reduction in sensible heat content of the air of the control volume was substantial during passage over the field. If all this energy was assumed to have been used in evaporation, then the advection of heat contributed from 35 to 86% of the total evaporation rate. It appears that for an inhomogeneous surface, knowledge of the distortion of some properties of local boundary layers can yield reliable estimates of local evaporation. The approach has little empiricism as it is based on simple conservation laws.

Interpretation of surface flux measurements in heterogeneous terrain during the Monsoon '90 experiment

A network of 9‐m‐tall surface flux measurement stations were deployed at eight sparsely vegetated sites during the Monsoon '90 experiment to measure net radiation, Q, soil heat flux, G, sensible heat flux, H (using eddy correlation), and latent heat flux, λE (using the energy balance equation). At four of these sites, 2‐m‐tall eddy correlation systems were used to measure all four fluxes directly. Also a 2‐m‐tall Bowen ratio system was deployed at one site. Magnitudes of the energy balance closure (Q + G + H + λE) increased as the complexity of terrain increased. The daytime Bowen ratio decreased from about 10 before the monsoon season to about 0.3 during the monsoons. Source areas of the measurements are developed and compared to scales of heterogeneity arising from the sparse vegetation and the topography. There was very good agreement among simultaneous measurements of Q with the same model sensor at different heights (representing different source areas), but poor agreement among different brands of sensors. Comparisons of simultaneous measurements of G suggest that because of the extremely small source area, extreme care in sensor deployment is necessary for accurate measurement in sparse canopies. A recently published model to estimate fetch is used to interpret measurements of H at the 2 m and 9 m heights. Three sites were characterized by undulating topography, with ridgetops separated by about 200–600 m. At these sites, sensors were located on ridgetops, and the 9‐m fetch included the adjacent valley, whereas the 2‐m fetch was limited to the immediate ridgetop and hillside. Before the monsoons began, vegetation was mostly dormant, the watershed was uniformly hot and dry, and the two measurements of H were in close agreement. After the monsoons began and vegetation fully matured, the 2‐m measurements of H were significantly greater than the 9‐m measurements, presumably because the vegetation in the valleys was denser and cooler than on the ridgetops and hillsides. At one lowland site with little topographic relief, the vegetation was more uniform, and the two measurements of H were in close agreement during peak vegetation. Values of λE could only be compared at two sites, but the 9‐m values were greater than the 2‐m values, suggesting λE from the dense vegetation in the valleys was greater than elsewhere.

Carbon dioxide and latent heat flux measurements in a windbreak-sheltered orchard

An eddy correlation system for carbon dioxide and latent heat flux measurement comprising a twin-channel, closed-path infra-red gas analyzer and sonic anemometer is described. Its performance was examined by comparing latent heat fluxes measured concurrently with an open-path sensor over windbreak-sheltered kiwifruit ( Actinidia deliciosia var. deliciosia). Measurements were carried out at the height of surrounding windbreaks (6 m). Excellent average agreement was found between latent heat fluxes measured by either system with no evidence of bandwidth limiting by the closed-path components. This agreement gives confidence that carbon dioxide fluxes were also correctly measured. It also provides validation of the density corrections for eddy fluxes which differ significantly for closed- and open-path sensors. Carbon dioxide fluxes were closely related to solar radiation, reaching 800 μg m −2 s −1 towards the canopy early in the afternoon. Water-use efficiency was approximately 7 mg of CO 2 per g of water evaporated and showed little dependence on either radiation or water use. Night-time fluxes were erratic in low wind conditions showing infrequent large excursions which are attributed to the action of occasional gusts sweeping out high levels of carbon dioxide accumulated from canopy and under-storey respiration. During windier conditions, night-time fluxes were typically 200 μg m −2 s −1 away from the canopy. The role of windbreaks in isolating the canopy from the free airstream was evident in the large diurnal variation in ambient carbon dioxide concentrations (640–800 mg m −3) observed 3–4 m above the height of the crop.

Evaporation from Lake Kinneret: 1. Eddy correlation system measurements and energy budget estimates

Evaporation rates over Lake Kinneret were directly and continuously measured by means of the eddy correlation system (ECS) for the relatively long periods of 20 and 44 days at the beginning and end of the summer (first and second periods), respectively. Results clearly show that wind speed and stability of thermal stratification over the water surface strongly affect evaporation from the lake. Consequently, large differences in measured daily evaporation rates were apparent during both periods. Evaporation rates, estimated by the energy budget method (EBM) were compared with the corresponding values measured directly by the ECS. Large differences were found between the estimated hourly evaporation rates and the measured ones, in both the values and the timing of maximal rates. In addition, the EBM calculated negative nocturnal evaporation rates, while the ECS measured positive values. Differences between estimated and measured evaporation rates monitored on a daily basis were also noted. The differences were larger for the first period, characterized by sharav events denoting climatic instability. High evaporation rates resulting from strong winds were not depicted by EBM estimates. The distributions of hourly evaporation rates over a 24 hour period or of the daily rates over an entire period as estimated by the EBM were very similar to those of net radiation, while corresponding distributions resulting from ECS measurements were very similar to those of wind speed. The daily average evaporation rate for the first period, calculated by the EBM, was smaller than the corresponding value resulting from ECS measurements. When applied on a hourly basis, the EBM gave a much more similar estimate. For the second period, there were no significant differences between the daily average based on the computed values (for both daily and hourly intervals) and the corresponding average based on the measured ones.