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A thermodynamically based model is presented to estimate daily actual evapotranspiration (ET) of a grass site closely resembling reference grass as defined by the Food and Agriculture Organization of the United Nations (FAO) under nonadvective conditions, from Meteosat Second Generation (MSG) imagery. The model presented here is derived from the thermodynamic theory by Schmidt combined with an atmospheric boundary layer model. Daily net radiation over the (reference) grass surface is parameterized as a function of global radiation, which can be estimated from MSG observations. It is then shown that ET over the grass area can be estimated using remotely sensed daily global radiation and air temperature as input only. The validation relied on observations gathered in Cabauw, a site closely resembling the reference grass, as defined by the FAO. The comparison with in situ data indicated a bias of 2.8 W m−2 and an RMSE of 7.7 W m−2. The possibility of using the approach developed here to provide reference crop evapotranspiration ETo is discussed. Because of the ambiguousness of ETo definition regarding local advection effects, it should be noted that explicitly advection-free conditions are dealt with. It is pointed out that in semiarid regions local advection cannot be ignored.

Routine information on regional evapotranspiration (ET) and dryness index is essential for agricultural water management, drought monitoring, and studies of water cycle and climate. However, this information is not currently available for the East Africa highlands. The main purpose of this study is to develop (1) a new methodology that produces spatially gridded daily ET estimates on a (near) real-time basis exclusively from satellite data, and (2) a new dryness index that depends only on satellite data and weather forecast data. The methodology that calculates daily actual ET involves combining data from two sensors (MODIS and SEVIRI) onboard two kinds of platforms (Terra – polar orbit satellite and MSG – geostationary orbit satellite). The methodology is applied to the East African highlands, and results are compared to eddy covariance measurements at one site. Results show that the methodology produces ET estimates that accurately reproduce the daily fluctuation in ET but tends to underestimate ET on the average. It is concluded that the synergistic use of the polar-orbiting MODIS data and the geostationary-orbiting SEVIRI data has potential to produce reliable daily ET, but further research is needed to improve the accuracy of the results. This study also proposes an operational new dryness index that can be calculated from the satellite-based daily actual ET estimates and daily reference ET estimates based on SEVIRI data and weather forecast air temperature. Comparison of this index against ground measurements of daily actual ET at one site indicates that the new dryness index can be used for drought monitoring.

This study introduces four methods for determining turbulent water vapour and carbon dioxide flux densities, the evapotranspiration and CO2 flux respectively. These methods combine scintillometer measurements with point-sampling measurements of scalar quantities and consequently have a faster statistical convergence than the eddy-covariance method. The scintillometer measures the friction velocity and stability averaged over space, allowing the time averaging to be a minute or less in homogenous conditions. This paper aims to thoroughly test the methods by analysing their sensitivity to the variables that go into the method and validate the methods with 30-min eddy-covariance data. Introduced are: the Bowen-variance method, the flux-variance method, the structure-parameter method, and the energy-balance method. Sensitivity analysis shows that each method is sensitive to the turbulence measurements of the scalar quantities that are specific to the method, as well as to the friction velocity. This demonstrates that the accuracy of the flux results from a correct representation of the turbulence variables used by the methods. Furthermore, a 30-min flux validation shows that the methods compare well to the independent eddy-covariance fluxes. We found that the structure-parameter method performs best – a low scatter (the correlation coefficient, r = 0.99) and a 5% underestimation were observed. Also the other methods perform well, although the energy-balance did not close, because storage terms and CO2 flux were neglected. Furthermore, during the night the variance methods were influenced by non-stationarity in the measurement signal. Finally, we suggest using the correlation coefficients between temperature and scalar quantities to acquire the sign of the fluxes. Data for this study were gathered in May–June 2009 over a wheat field near Merken, Germany, in the framework of the TransRegio32 program.

Simultaneous scintillometer measurements at multiple wavelengths (pairing visible or infrared with millimetre or radio waves) have the potential to provide estimates of path-averaged surface fluxes of sensible and latent heat. Traditionally, the equations to deduce fluxes from measurements of the refractive index structure parameter at the two wavelengths have been formulated in terms of absolute humidity. Here, it is shown that formulation in terms of specific humidity has several advantages. Specific humidity satisfies the requirement for a conserved variable in similarity theory and inherently accounts for density effects misapportioned through the use of absolute humidity. The validity and interpretation of both formulations are assessed and the analogy with open-path infrared gas analyser density corrections is discussed. Original derivations using absolute humidity to represent the influence of water vapour are shown to misrepresent the latent heat flux. The errors in the flux, which depend on the Bowen ratio (larger for drier conditions), may be of the order of 10%. The sensible heat flux is shown to remain unchanged. It is also verified that use of a single scintillometer at optical wavelengths is essentially unaffected by these new formulations. Where it may not be possible to reprocess two-wavelength results, a density correction to the latent heat flux is proposed for scintillometry, which can be applied retrospectively to reduce the error.

This paper evaluates four methods to obtain accurate averaged flux estimates under conditions of non-stationary turbulence. In Part I (Van Kesteren et al., 2012), we introduced and evaluated these four combined methods for 30-min averaging intervals, notably the flux-variance method, the Bowen-variance method, the structure-parameter method, and the energy-balance method. The aim of this paper, Part II, is to validate the accuracy of the 1-min flux estimates of the CO2 flux, FCO2, and the evapotranspiration/latent-heat flux, LvE. Furthermore, we use the 1-min fluxes to investigate flux and vegetation responses under conditions of non-stationary turbulence. Using several validation methods, we show that both the eddy-covariance method and the energy-balance method are unsuitable for estimating fluxes over 1-min averaging intervals. The three other combined methods are more successful in determining 1-min fluxes. The random error is approximately half that of the eddy-covariance method, but still some issues limit the success. The Bowen-variance method has a +0.09 systematic error and moreover, 30% of the data had to be omitted, because the method requires more stringent conditions. Furthermore, the flux-variance method has a -0.15 systematic error. The structure-parameter method performs best of all methods and accurately resolves 1-min fluxes. With this method, we do a final validation with a different data set and show that also under dry conditions the method accurately resolves FCO2, although LE was more difficult to resolve. In the last part, the structure-parameter method is successfully applied under conditions of non-stationary turbulence. We show that LvE and FCO2 have a different step response upon abrupt changes in solar radiation, because different processes drive these fluxes. Also, we observe a 2-min time lag between solar radiation and 1-min fluxes and show the relevance of taking this into account for determining light-response curves of the plants for both 1-min and 30-min averaging intervals. Furthermore, we show the relevance of 1-min fluxes for studying the light-response curves of plants for conditions with different temperature and humidity. Finally, we show that accurate estimates of 1-min averaged canopy resistances can be determined via the resistance expressions for sensible heat and LvE. As such, we show that vegetation can indeed modify its canopy resistance significantly within several minutes. (c) 2013 Elsevier B.V. All rights reserved.

A scaling factor, S, is derived to account for the difference in path-weighted measurement heights of a combined system consisting of a large-aperture scintillometer (LAS) and a millimetre-wave scintillometer (MWS), operating at wavelengths of 0.88 μm and about 3 mm respectively, and designed to determine the area-averaged latent heat flux. This work extends an earlier derivation of Zef, the effective height for the LAS yielding the area-averaged sensible heat flux. The LAS and MWS have different path-weighting functions, therefore, in general, it is expected that the path-weighted beam heights are different, for paths other than with the beam parallel to the land surface. Their difference will depend on the detail of the experimental set-up. The objective is to introduce a scaling factor that transforms the MWS refractive-index structure parameter measurement to the same path-weighted height of the LAS measurement. By applying S as a prelude to the calculation of the temperature and humidity structure parameters, and the sensible and latent heat fluxes, these quantities are made representative of the same measurement height, thus simplifying the application of two-wavelength scintillometry. The equations presented here enable the analysis of LAS-MWS paths such that they can be selected to optimize S towards its ideal value of unity. For this purpose we provide a new analytical approximation of the LAS path-weighting function. The importance of accounting for S is demonstrated by example applications over varying topography as well as for slanted beams.

Eddy covariance (EC) and large aperture scintillometer (LAS) measurements were collected over an irrigated olive orchard near Marrakech, Morocco. The tall, sparse vegetation in the experimental site was relatively homogeneous, but during irrigation events spatial variability in soil humidity was large. This heterogeneity caused large differences between the source area characteristics of the EC system and the LAS, resulting in a large scatter when comparing sensible heat fluxes obtained from LAS and EC. Radiative surface temperatures were retrieved from thermal infrared satellite images from the Landsat Enhanced Thematical Mapper+ and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellites. Using these images in combination with an analytical footprint model, footprint-weighted radiative surface temperatures for the footprints of the LAS and the EC system were calculated. Comparisons between the difference in measured sensible heat fluxes and the difference in footprint-weighted radiative surface temperature showed that for differences between the footprint-weighted radiative surface temperatures larger than ±0.5 K, correlations with the difference in measured sensible heat flux were good. It was found that radiative surface temperatures, obtained from thermal infrared satellite imagery, can provide a good indication of the spatial variability of soil humidity, and can be used to identify differences between LAS and EC measurements of sensible heat fluxes resulting from this variability.

The success of weather forecasting and climate prediction essentially depends on our ability to understand and describe the energy and water cycles at various spatial and temporalscales.ThishasbeenrecognisedbytheWMOininitiatingtheGlobalEnergy and Water Cycle Project (GEWEX) as one of the major research activities within the framework of the World Climate Research Programme (WCRP). GEWEX covers measurements and modelling activities from the local up to the global scale and also provides an umbrella for a number of so-called continental-scale experiments (CSEs) that focus on the investigation of various aspects of the energy and water cycles in different geographic and climate regions of the earth. Recently, the efforts from the different CSEs have been brought together during a two-year Coordinated Enhanced Observation Period (CEOP) by combining long-term observations at a number of globally distributed reference sites, the analysis of global satellite data products and global climate model simulations in a number of the leading climate research centres of the world (Lawford et al. 2004). TheinternationalBalticSeaExperiment(BALTEX)astheEuropeancontribution to GEWEX is intended to describe and quantify the energy and water budget over the Baltic Sea Drainage Basin, an area of about 2 × 10 6 km 2 covering large parts of

The Monin–Obukhov similarity theory (MOST) functions fe and f T , of the dissipation rate of turbulent kinetic energy (TKE). e, and the structure parameter of temperature, C 2 , were determined for the stable atmospheric surface layer using data gathered in the context of CASES-99. These data cover a relatively wide stability range, i.e. ζ=z/L of up to 10, where z is the height and L the Obukhov length. The best fits were given by fe = 0.8 + 2.5ζ and f T = 4.7[ 1+1.6(ζ)2/3], which differ somewhat from previously published functions. e was obtained from spectra of the longitudinal wind velocity using a time series model (ARMA) method instead of the traditional Fourier transform. The neutral limit fe =0.8 implies that there is an imbalance between TKE production and dissipation in the simplified TKE budget equation. Similarly, we found a production-dissipation imbalance for the temperature fluctuation budget equation. Correcting for the production-dissipation imbalance, the ‘standard’ MOST functions for dimensionless wind speed and temperature gradients (φ m and φ m ) were determined from fe and f T and compared with the φ m and φ h formulations of Businger and others. We found good agreement with the Beljaars and Holtslag [J. Appl. Meteorol. 30, 327–341 (1991)] relations. Lastly, the flux and gradient Richardson numbers are discussed also in terms of fe and f T .

The large-aperture scintillometer (LAS) is by now a generally accepted device for routinely obtaining the area-averaged sensible heat flux, H, on a scale of up to 10 km. It is an optical instrument that consists of a transmitter and receiver. In practice, the LAS beam height often varies along the path due to a variety of reasons. This study will explain what effective height to use in such situations, when analyzing scintillometer data to derive H. Several aspects are covered: a slanted path over flat terrain, structured terrain, and varying path height due to the curvature of the earth's surface. To test the derived effective height formulation the authors present LAS data taken in September and October 1996 at a rangeland site in Sonora, Mexico. In experiment 1, the LAS was set up over a slant path, ranging roughly between 10 and 45 m above the surface over a 3200-m path. In experiment 2, a horizontal LAS path was used at approximately 30 m over a pathlength of 1100 m. The resulting sensible heat fluxes were compared with eddy-covariance data and show satisfactory results for both the full and one of the approximate formulations of the effective height.

An incoherent scintillometer with 0.31-m aperture was tested along a 9.8-km path over grassland. Scintillometer derived heat fluxes were compared with in situ eddy covariance measurements. Albeit with considerable scatter, the fluxes compared well during daytime. During nighttime credible fluxes are also obtained. The scintillometer functioned satisfactory for 96␘f the 7-week period

To test the applicability of the scintillation method over a heterogeneous area an experiment was carried out in the summer of 1998 in Flevoland (The Netherlands). In the patchy area only four crops were grown namely sugar beet, potatoes, wheat and onions. From eddy covariance measurements it was found that the heterogeneity was mainly caused by differences in thermal properties. No variations in the aerodynamics roughness length were observed. Two large aperture scintillometers were installed at a height of 11.6 and 20.4 m. A good resemblance was found between the sensible heat fluxes derived from both LAS instruments and the area-averaged fluxes obtained from the in-situ eddy covariance measurements. The slight underestimation of the lower LAS could be assessed using a blending height model and an analytical footprint model. The results also indicated that when scintillometer measurements are made below the blending height the violation to Monin–Obukhov Similarity Theory is small and that reasonable fluxes can be obtained from path-averaged structure parameters.

The performance of the Scintec displaced-beam small aperture scintillometer (DBSAS) in the stable boundary layer (SBL) is investigated using data gathered during the CASES-99 experiment in Kansas, U.S.A. The DBSAS is superior to the eddy-covariance method in determining vertical fluxes of sensible heat and momentum close to the ground and/or over short (< 1 min) averaging intervals. Both aspects are of importance in the shallow and non-stationary SBL. The friction velocity, u*, the temperature scale, θ*, and from these the sensible heat flux, H, were calculated from the indirectly determined dissipation rate, ε, and the structure parameter of temperature, CT2, by the DBSAS, which was operated over a path length of 112 m. All these variables are compared with eddy-covariance data for 10-minute time averages. Previously reported systematic errors in the DBSAS, overestimation of u* for low u* values and underestimation of u* for high u* values, have in part been dealt with by adjusting the beam displacement distance from 2.7 mm to 2.6 mm in the calculations. The latter adjustment is presented as a working hypothesis, not a general solution.

In the stable boundary layer it is often observed that turbulence is not continuous in space and time. This discontinuous, intermittent turbulence causes alterations from the mean evolution of the stratified atmospheric boundary layer, which may result in an oscillatory type of behavior of the near-surface wind speed and temperature. This paper focuses on an intermittency generating mechanism that arises from a direct interaction of the lower atmosphere (first tens of meters) with the vegetation surface, without interaction with the air aloft. This atmosphere–surface intermittency (ASI) is associated with the essential elements of the stable boundary layer (SBL): strong surface cooling, the supply of mechanical energy by the synoptic pressure gradient, and the limiting effect of stratification on mixing efficiency. In Part I it is shown that the essence of ASI can be captured by a system of three coupled nonlinear differential equations. This simplified system shows both intermittent and nonintermittent flow regimes for different circumstances. In the present paper, this system is studied analytically, following a system dynamics approach. The transition between the different flow regimes is identified as a Hopf bifurcation. This property is used to derive a dimensionless parameter, which is a function of external parameters, such as radiative forcing and pressure gradient. With this dimensionless parameter the equilibrium behavior of the system (i.e., intermittent or nonintermittent) can be predicted exactly. As such this parameter is used to classify SBL regimes. It is shown that the proposed classification parameter provides different information about the state of the SBL than other typical SBL classification parameters such as z/L and Ri.

Measurements made with a large aperture scintillometer were compared to those made using an eddy correlation system over a uniform barley field. In spite of wind profile measurements indicating that conditions were not ideal for the application of Monin–Obukhov theory, good agreement was found between the instruments for measuring sensible heat fluxes as long as an effective value for the displacement height was used in the equations. This effective value was calculated from measurements made using sonic anemometers, although these data indicated a value well outside the normally accepted range. It is suggested that this is a consequence of the changes in atmospheric turbulence produced by a hill upwind of the experimental site.

Measurements carried out in Northern Finland on radiation and turbulent fluxes over a sparse, sub-arctic boreal forest with snow covered ground were analysed. The measurements represent late winter conditions characterised by low solar elevation angles. During the experiment (12-24 March 1997) day and night were about equally long. At low solar elevation angles the forest shades most of the snow surface. Therefore an important part of the radiation never reaches the snow surface but is absorbed by the forest. The sensible heat flux above the forest was fairly large, reaching more than 100 W m-2. The measurements of sensible heat flux within and above the forest revealed that the sensible heat flux from the snow surface is negligible and the sensible heat flux above the forest stems from warming of the trees. A simple model for the surface energy balance of a sparse forest is presented. The model treats the diffuse and direct shortwave (solar) radiation separately. It introduces a factor that accounts for the shading of the ground at low solar elevation angles, and a parameter that deals with the partial transparency of the forest. Input to the model are the direct and diffuse incoming shortwave radiation. Measurements of the global radiation (direct plus diffuse incoming shortwave radiation) above the forest revealed a considerable attenuation of the global radiation at low solar elevation. A relation for the atmospheric turbidity as function of the solar elevation angle is suggested. The global radiation was simulated for a three month period. For conditions with a cloud cover of less than 7 oktas good agreement between model predictions and measurements were found. For cloud cover 7 and 8 oktas a considerable spread can be observed. To apply the proposed energy balance model, the global radiation must be separated into its diffuse and direct components. We propose a simple empirical relationship between diffuse shortwave and global radiation as function of cloud cover.

As part of an international experiment in which several methods of estimating the actual evapotranspiration were compared, two large aperture scintillometers (LAS) and a small micrometeorological station were installed in the Gediz Basin near Menemen, Turkey during the summer of 1998 by the Meteorology and Air Quality Group of the Wageningen Agricultural University. The instruments will be used to provide “ground-truth” sensible heat fluxes. At the first site, a LAS was set up over a transect of the valley from Belen to Suluklu with a path length of 2700 m. The second LAS (a path length of 670 m) together with a small micrometeorological station was installed in an irrigated cotton field west of the first site. However, due to experimental problems the scintillation method for the second site could not be applied. Instead the variance method, which just as the scintilliation method is based on the Monin–Obukhov similarity theory, was used. The 24 h average sensible heat fluxes of the valley for the entire growing season are presented and special attention is given to the LANDSAT overpass dates June 26 and August 29. The results presented in this paper show that the scintillometer is a robust and reliable “stand-alone” system that is able to provide operational estimates of areal averaged sensible heat fluxes.

Simple analytic approximate solutions arepresented for the set of equations that follows fromthe Monin–Obukhov flux-profile relationships using thestability functions of Dyer (unstable case) andBeljaars–Holtslag (stable case). Several publicationsare devoted to the same subject, however the currentapproach contains some new features, namely: (a) itappears to be more accurate for unstable situationsand (b) it applies also to the general case where windspeed (u) and potential temperature(¿) are given at different levels. In order toillustrate the accuracy of the approach a comparisonwith the actual solutions is presented for someselected combinations of ¿ and u levelstypical for various practical applications.

Since most land surfaces are non-uniform on meteorological model scales, usually applied flux-profile relationships to obtain surface flux densities cannot be used. Therefore, some modellers assume that these relations are scale-invariant if the local bulk transfer parameters are replaced by effective parameters. This approach leads to wrong results for grid cells that contain subgrid areas that differ largely with respect to stability. Hence, in such cases, the surface energy balance needs to be solved for each subgrid area separately. Therefore, to obtain the correct flux densities in all situations, for each grid cell in the horizontal domain of meteorological models, subgrid areas that differ with respect to stability need to be identified.

The use of a large-aperture scintillometer to estimate sensible heat flux has been successfully tested by several investigators. Most of these investigations, however, have been confined to homogeneous or to sparse with single vegetation-type surfaces. The use of the scintillometer over surfaces made up of contrasting vegetation types is problematic because it requires estimates of effective roughness length and effective displacement height in order to derive area-average sensible heat from measurements of the refractive index. In this study an approach based on a combination of scintillometer measurements and an aggregation scheme has been used to derive area-average sensible heat flux over a transect spanning two adjacent and contrasting vegetation patches: grass and mesquite. The performance of this approach has been assessed using data collected during the 1997 Semi-Arid Land-Surface-Atmosphere field campaign. The results show that the combined approach performed remarkably well, and the correlation coefficient between measured and simulated area-average sensible heat flux was ∼0.95. This is of interest because this approach offers a reliable means for validating remotely sensed estimates of surface fluxes at comparable spatial scales.

The crop factor method is applied to unstressed maize, in which grass is the reference crop, and the evapotranspiration of the latter is determined with the simplified equation of Makkink as proposed by De Bruin (1987). Using the crop factor published by Feddes (1987), a good agreement is found with measurements collected during the growing season of 1985 at the Sinderhoeve (51° 59′ N, 5° 45′ E) near Renkum in The Netherlands. It appears that Makkink's method yields slightly better results than the Penman–Monteith equation using surface resistances fitted to the data set itself. © 1998 John Wiley & Sons, Ltd.

Surface fluxes and soil moisture were measured in situ during the Echival Field Experiment in Desertification-threatened Areas (EFEDA), executed in Castilla la Mancha, Central Spain. Although the observation network had a high density (20 flux towers and 46 soil moisture plots), the area-average values at a scale of 100 km for a Mediterranean agricultural landscape were difficult to deduce. An attempt with remote sensing data has been made to study the possibility to extrapolate in situ measurements to a coarser scale. A remote sensing flux algorithm was applied to estimate (1) actual evaporation, (2) evaporative fraction, (3) bulk surface resistance and (4) top soil moisture from spectral radiances at different spatial scales. The spatial variability and area-average values were computed for two different golden days (June 12 and 29). The evaporative fraction (latent heat flux/net available energy) estimated from remote sensing was within the error bounds of the values derived from instrumented flux towers in 85% of the cases compared. The bulk surface resistance derived from the remote sensing flux algorithm could be related successfully to in situ near-surface soil moisture measurements conducted with Time Domain Reflectometers (TDR). Unfortunately, the relationship between resistance and soil moisture is shown to be space and time dependent. Nevertheless, after calibration with field scale measurements, instantaneous relationships between resistance and top soil moisture could be applied to estimate soil moisture at unsampled locations. Thereafter, the area-average top soil moisture could be assessed. It was concluded that an arithmetic means of the distributed field measurements of evaporation ( n = 13) and soil moisture ( n = 46) gives a wrong indication of the area-average values at a scale of 100 km, and that weighting factors for the areal integration can be derived from remote sensing data. During a 17-day drying period in the Special Observation Period, top soil moisture in Castilla la Mancha at a scale of 100 km decreased from 0.16 to 0.10 cm 3 cm −3 resulting in an increase of the areal bulk surface resistance from 661 to 1166 s m −1 . The associated daily evaporation at this scale decreased from 2.0 to 1.3 mm d −1 .

This paper deals with the parameter kB21, the logarithm of the ratio between momentum and heat roughness length, of sparsely vegetated surfaces and bare soil. The bare soil surface is included as a reference, since it is fairly homogenous and smooth, having no distinguishable roughness elements. The mean value of kB21 is about 8 for the vineyard and 12 for the savannah. These values are significantly greater than kB 21 5 2, which is usually assumed to hold for vegetation. The mean value of kB21 for bare soil is small and negative, which agrees with the literature. A large variation of kB 21 during the day is measured for all three surfaces. This behavior has been observed for sparse vegetation in previous studies. Some authors explained the phenomenon with a vertical movement of the source of heat through the day as solar angle varies, or with the use of an inappropriate value of effective surface temperature to calculate kB21. For the first time, this diurnal variation is measured for a smooth surface, the bare soil, for which neither explanation is valid. A sensitivity study reveals that the calculated kB21 is very sensitive to measuring errors in the micrometeorological variables and errors in the roughness length for momentum. This explains the large range in observed kB21 values for one particular surface type. In addition, several semiempirical expressions for kB21 from the literature are tested. Two well-established formulas, both based on a simple combination of Reynolds and Prandtl numbers, appear to produce the best estimates of daily averaged kB21 values. None of the formulas are able to describe the diurnal variation. The authors conclude that the concept of kB21 is questionable as it is based upon extrapolating a theoretical profile through a region where this profile does not hold, toward a ‘‘surface temperature’’ that is difficult to define and to measure. It should therefore be avoided in meteorological models, for example, by applying canopy boundary layer resistances. Unfortunately, in remote sensing, the bulk transfer equations are up to now the only option, which therefore requires the use of kB21.

This paper explores the possibility to use a model for assimilation rate, An, to calculate stomatal conductance, gs. The relation between stomatal conductance, gs, and net photosynthetic rate, An, is studied for grapevine (Vitis Vinifera L. cv Airen). Data were collected in a dry Mediterranean vineyard in Central Spain, under a wide range of temperature and humidity conditions, but with sufficient water supply. The data confirm the existence of a strong correlation between An and gs. However, the relation is strongly influenced by leaf-to-air specific humidity deficit, Ds. This influence can be expressed by means of a linear relation between Ds and the ratio f, which is the ratio of internal to external CO2 concentration, CiCs, corrected for CO2 concentration. The data were used to tune a coupled An−gs model using as much standard coefficients as possible. Using the linear relation between Ds and f the model also takes stomatal humidity responses into account. A model test reveals that the current approach is able to simulate the photosynthetic rate and stomatal conductance of grapevines reasonably well.

Eddy-covariance measurements of CO2 exchange above a Sahelian savanna consisting of small shrubs over a continuous herb layer were made during the HAPEX-Sahel experiment in Niger, West Africa. The measurements were made near-continuously during an 8 week period, covering the main part of the rainy season and three weeks at the beginning of the dry season. In this paper the measurements are corrected for in-canopy storage of CO2 and the nightime measurements used to derive respiration functions for the soil, roots and aerial plant material. Photosynthetic CO2 uptake was calculated and the measurements compared to simulations using a biochemical photosynthesis model in a simple (“big-leaf”) implementation, with average stomatal conductance given as an independent input. This model was able to reproduce the measurements (RMS error 1.95 μmol m−2 s−1) with only minor parameter adjustment.

Turbulent fluctuations of atmospheric properties near the soil-vegetation-atmosphere interface have been investigated in a situation of strong local advection. To this end, an analysis has been made of data from the 1987 field experiment in ‘la Crau’ (France), where an extensive dry terrain with sparse vegetation was situated upwind from an irrigated field completely covered with grass. The change in surface roughness was negligible, the main effect being a change in surface heat flux. In the experiment, turbulence measurements were made of horizontal and vertical wind speed, temperature and humidity at various locations over both terrains. In this study, 12 runs of 1 h are analysed, roughly half of which were made when an advective inversion was present over the downstream terrain. Both vertical and horizontal velocity fluctuations are almost unaffected by the change in terrain. Temperature fluctuations are influenced by the fact that over the downstream terrain production of variance almost ceases and advection of variance becomes important. This results in a deviation of the σ θ /θ ∗ = ƒ(z/L) relation found for homogeneous situations. Also, the temperature spectra of the unstable runs are affected, showing a different inertial subrange roll-off rate. Humidity fluctuations adjust rapidly to the new surface and, for all but the lowest frequencies, behave as if measured over homogeneous terrain.

Measurements of sensible heat flux, radiometric surface temperature, air temperature, and wind speed made at eight semiarid rangeland sites were used to investigate the sensible heat flux-aerodynamic resistance relationship. The individual sites covered a wide range of vegetation (0.1-4 m tall) and cover (3%-95% bare soil) conditions. Mean values of k/B, a quantity related to the resistance of heat versus momentum transfer at the surface, for the individual sites varied between 3.5 and 12.5. A preliminary test of the utility of an excess resistance based on the mean value of k/B showed that the difference between the mean estimated and measured sensible heat fluxes varied +/- 60 W/sq m for the eight semiarid sites. For the eight sites the values of k/B were plotted against the roughness Reynolds number. The plot showed considerable scatter with values ranging between and beyond the theoretical curves for bluff rough and permeable rough surfaces.

The standard deviation of temperatureσ T is proposed as a temperature scale and $$v_ * \equiv \overline {w\prime T\prime } /\sigma _T $$ as a velocity scale to describe the behaviour of turbulent flows in the Atmospheric Surface Layer (ASL), instead ofθ * andu * of the Monin—Obukhov similarity theory, and ofT f andU f used for free convection stability conditions. On the basis of experimental evidence reported in the literature, it is shown thatσ T αT f andv * αU f in the free convection region, andσ T αθ * andv * αU * in nearneutral and stable conditions. This implies that the proposed scales can be applied for all stabilities. Furthermore, a new length scale is proposed and its relation with Obukhov length is given. Also, a simple semi-empirical expression is presented with whichσ T andv * can be evaluated in a rather simple way. Some examples of practical applications are given, e.g., a stability classification for unstable conditions.

With the temperature fluctuation method, the sensible heat flux and friction velocity can be evaluated from standard deviation of the temperature and horizontal wind speed at one level. In principle, for unstable conditions the governing equations have to be solved by iteration. In this note, an approximate analytic solution is presented. Also, the solution for stable situations is discussed.

First results are shown of a project aiming to estimate daily values of reference crop evapotranspiration ET0 from geo-stationary satellite imagery. In particular, for Woreta, a site in the Ethiopian highland at an elevation of about 1800 m, we tested a radiation-temperature based approximate formula proposed by Makkink (MAK), adopting ET0 evaluated with the version of the Penman-Monteith equation described in the FAO Irrigation and Drainage paper 56 as the most accurate estimate. More precisely we used the latter with measured daily solar radiation as input (denoted by PMFAO-Rs). Our data set for Woreta concerns a period where the surface was fully covered with short green non-stressed vegetation. Our project was carried out in the context of the Satellite Application Facility on Land Surface Analysis (LANDSAF) facility. Among others, the scope of LANDSAF is to increase benefit from the EUMETSAT Satellite Meteosat Second Generation (MSG). In this study we applied daily values of downward solar radiation at the surface obtained from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) radiometer. In addition, air temperature at 2 m was obtained from 3-hourly forecasts provided by the European Centre for Medium-Range Weather Forecasts (ECMWF). Both MAK and PMFAO-Rs contain the psychrometric "constant", which is proportional to air pressure, which, in turn, decreases with elevation. In order to test elevation effects we tested MAK and its LANDSAF input data for 2 sites in the Jordan Valley located about 250 m b.s.l. Except for a small underestimation of air temperature at the Ethiopian site at 1800 m, the first results of our LANDSAF-ET0 project are promising. If our approach to derive ET0 proves successfully, then the LANDSAF will be able to initiate nearly real time free distribution of ET0 for the full MSG disk.

Several authors have determined the sensitivity of transpiration to different environmental parameters using the Penman-Monteith equation. In their studies the interaction between transpiration and, for example, the humidity of the air is ignored: the feedback with the planetary boundary layer (PBL) is not accounted for. Furthermore, surface-layer (SL) feedback (e.g., stability effects in the surface layer) is often neglected.In our study, both PBL feedback and SL feedback are accounted for by coupling the big-leaf model to a detailed model for the PBL. This study provides an analysis of the sensitivity of transpiration to net radiation calculated after an albedo change, aerodynamic resistance calculated after a change in the aerodynamic roughness, and surface resistance. It is shown that PBL feedback affects the sensitivity of transpiration to the tested variables significantly. The sensitivity of transpiration to surface resistance and to aerodynamic resistance, or aerodynamic roughness, is decreased by the PBL feedback. In contrast, PBL feedback enlarges the sensitivity of transpiration to the net radiation, or albedo, and appears to be highly dependent on the specific conditions, especially on the aerodynamic roughness of the vegetation. It is recommended that future sensitivity studies for prognostic use account for PBL feedback.

In this study different parameterizations for land surface models currently employed in meteorological models at ECMWF [Tiled ECMWF Surface Scheme for Exchange Processes over Land (TESSEL)] and NCEP (Noah) are evaluated for a semiarid region in Ghana, West Africa. Both schemes utilize the Jarvis–Stewart approach to calculate canopy conductance as the critical variable for partitioning the available energy into sensible and latent heat flux. Additionally, an approach within Noah is tested to calculate canopy conductance based on plant physiology (A-gs method), where the photosynthetic assimilation is coupled to the leaf stomatal conductance.All parameterizations were run offline for a seasonal cycle in 2002/03 using observations as forcings at two test sites. The two locations are in the humid tropical southern region and in the drier northern region. For the purpose of forcing and evaluation, a new set of data has been utilized to include surface fluxes obtained by scintillometry. The measurements include the rapid wet-to-dry transition after the wet season at both sites.As a general trend, it has been found that during the wet period of a season net radiation is described well by all parameterizations. During the drying process the errors in modeled net radiation increased at both sites. The models perform poorly in simulating soil heat fluxes with larger errors for TESSEL for both sites. The evolution in time for sensible heat flux and latent heat flux was tackled in different ways by the utilized parameterizations and sites with enhanced model performance for the more southern site. Soil moisture in the upper soil layers is modeled with small errors for the different parameterizations.Key adjustments for reducing net radiation during the dry period of a season are discussed. In particular, the ratio of roughness length of momentum and heat was found to be an important parameter, but will require seasonal adjustments.

To monitor seasonal water consumption of agricultural fields at large scale, spatially averaged surface fluxes of sensible heat (H) and latent heat (LvE) are required. The scintillation method is shown to be a promising device for obtaining the area-averaged sensible heat fluxes, on a scale of up to 10 km. These fluxes, when combined with a simple available energy model, can be used to derive area-averaged latent heat fluxes. For this purpose, a Large Aperture Scintillometer (LAS) was operated continuously for more than one year over a tall and sparse irrigated oliveyard located in south-central Marrakesh (Morocco). Due to the flood irrigation method used in the site, which induces irregular pattern of soil moisture both in space and time, the comparison between scintillometer-based estimates of daily sensible heat flux (H-LAS) and those measured by the classical eddy covariance (EC) method (H-EC) showed a large scatter during the irrigation events, while a good correspondence was found during homogenous conditions (dry conditions and days following the rain events). We found, that combining a simple available energy model and the LAS measurements, the latent heat can be reliably predicted at large scale in spite of the large scatter (R-2 = 0.72 and RMSE = 18.25 W m(-2)) that is obtained when comparing the LAS against the EC. This scatter is explained by different factors: the difference in terms of the source areas of the LAS and EC, the closure failure of the energy balance of the EC, and the error in available energy estimates. Additionally, the irrigation efficiency was investigated by comparing measured seasonal evapotranspiration values to those recommended by the FAO. It was found that the visual observation of the physical conditions of the plant is not sufficient to efficiently manage the irrigation, a large quantity of water is lost (approximate to 37% of total irrigation). Consequently, the LAS can be considered as a potentially useful tool to monitor the water consumption in complex conditions.

A simple satellite-based algorithm for estimating actual evaporation based on Makkink’s equation is applied to a seasonal cycle in 2002 at three test sites in Ghana, West Africa: at a location in the humid tropical southern region and two in the drier northern region. The required input for the algorithm is incoming solar radiation, air temperature at standard level, and the green-vegetation fraction. These data are obtained from Meteorological Satellite (Meteosat) and Moderate-Resolution Imaging Spectroradiometer (MODIS) images. The observation period includes the rapid wet-to-dry transition after the wet season. Incoming solar radiation and air temperature are validated against local measurements at the three sites. It is found that the incoming solar radiation obtained from Meteosat corresponds well with the measurements. For air temperature from Meteosat data, the diurnal cycle is realistically reproduced but is in need of a bias correction. The algorithm output is compared with the evapotranspiration data obtained from hourly large-aperture scintillometer observations and simultaneous “in situ” measurements of net radiation and soil heat flux. It is found that the actual evapotranspiration can be monitored using the modified Makkink method, with daily mean errors of between 5% and 35% of measured evapotranspiration and a seasonal error smaller than 5%. Furthermore, it appears that the algorithm realistically describes the daily cycle of evapotranspiration.

This study examines the seasonal cycle of the components of the surface energy balance in the Volta basin in West Africa as part of the GLOWA-Volta project. The regional climate is characterized by a strong north-south gradient of mean annual rainfall and the occurrence of pronounced dry and wet seasons within one annual cycle, causing a strong seasonal variation in the natural vegetation cover. The observations are conducted with a combined system, consisting of a Large Aperture Scintillometer (LAS) for areally aver- aged sensible heat flux, radiometers and sensors for soil heat flux. For comparisons the eddy-covariance (EC) method providing the fluxes of momentum, sensible and latent heat is utilized as well. The measurements of a seasonal cycle in 2002/2003 were gathered including the rapid wet-to-dry transition after the wet season at two locations in Ghana, one in the humid tropical southern region and one in the northern region. A direct comparison and the energy balance closure of the two methods are investigated for daytime and nighttime separately. An attempt is made to understand and explain the differences between the two methods and the closure of energy budget found for these. It is found that the two systems correspond well during daytime. During nighttime the LAS seems to perform more realis- tically than the EC system. Considering the fact that a LAS system is much easier to use in the climate conditions of the Volta basin, it is concluded that the LAS approach is very suitable in this type of climate conditions. Surface conductances are estimated by rearranging the Penman-Monteith equation and compared to a Jarvis-type model optimised for savan- nah conditions. It is found that temperature dependence should be included in the conduc- tance formulation in contrast to earlier findings. Based on the findings the gathered dataset can be used for further model studies of the climate and environment of West Africa.

The performance of a combined large aperture scintillometer (LAS) and a millimetre wave scintillometer (MWS) for estimating surface fluxes of sensible and latent heat over natural landscape is investigated, using data gathered during LITFASS-2003. For this purpose the LAS–MWS system was installed in a moderate heterogeneous landscape over a path length of 4.7 km with an effective beam height of 43 m. The derived surface fluxes have been compared with aggregated eddy-covariance (EC) measurements. The fluxes of sensible and latent heat from the LAS–MWS combination, as well as sensible heat fluxes of the single LAS, agreed fairly well with the EC-based fluxes, considering the uncertainties of the similarity stability functions and observed energy imbalance.

Evidence is presented that in the stable atmospheric surface layer turbulent fluxes of heat and momentum can be determined from the standard deviations of longitudinal wind velocity and temperature, ru and rT respectively, measured at a single level. An attractive aspect of this method is that it yields fluxes from measurements that can be obtained with two- dimensional sonic anemometers. These instruments are increasingly being used at official weather stations, where they replace the standard cup anemometerwind vane system. With methods such as the one described in this note, a widespread, good quality, flux network can be established, which would greatly benefit the modelling community. It is shown that a 'variance' dimensionless height (fr) defined from ru and rT is highly related to the 'conven- tional' dimensionless stability parameter f ¼ z=L, where z is height and L is the Obukhov length. Empirical functions for fr are proposed that allow direct calculation of heat and momentum fluxes from ru and rT. The method performs fairly well also during a night of intermittent turbulence.

The RAPID field experiment took place in August - September 1999 at a site 25km south of Twin Falls, Idaho, USA. The experiment concerned micrometeorological observations over extensive, well-irrigated fields covered with the fast-growing crop alfalfa. During daytime, on a number of days the sensible heat flux was negative and the latent heat flux exceeded net radiation. The energy required for the latent heat flux to be larger than net radiation has to be advected from elsewhere. As the fields were large, we refer to this process as "regional advection". "Local advection", on the other hand, refers to advection effects, where the wet to dry transition is on a field scale. Evidence is presented that the RAPID data are subject to regional advection conditions. A simple model, based on Penman-Monteith, is derived that describes the regional-advection case rather well. The influence of wind speed under those conditions is illustrated using data and the model. The correlation coefficients between temperature and horizontal wind component appear to be good indicators for advection

Understanding and modeling water exchange in and and semiarid ecosystems is complicated by the very heterogeneous distribution of vegetation and moisture inputs, and the difficulty of measuring and validating component fluxes at a common scale. We combined eddy covariance (EC), sap flow, and stable isotope techniques to investigate the responses of transpiration and soil evaporation to an irrigation event in an olive (Olea europaea L.) orchard in Marrakech, Morocco. The primary goal was to evaluate the usefulness of stable isotope measurements of water vapor in the turbulent boundary layer for partitioning evapotranspiration under Such dynamic conditions. The concentration and deuterium isotope composition (delta(2)H) of water vapor was collected from different heights within the ecosystem boundary layer of the olive canopy before and over several days following a 100mm surface irrigation. 'Keeling plots' (isotope turbulent mixing relationships) were generated from these data to estimate the fractions of evaporation and transpiration contributing to the total evapotranspiration (ET) flux. Transpiration accounted for 100% of total ET prior to irrigation, but only 69-36% of ET during peak midday fluxes over the 5-day period following irrigation. The rate of soil evaporation and plant transpiration at the stand level was calculated from eddy covariance measurements and the evaporation and transpiration fractions from isotope measurements. Soil evaporation rate was positively correlated with daily atmospheric vapor pressure deficit (D), but transpiration was not. Component fluxes estimated from the isotope technique were then compared to those obtained from scaled sap flow measurements. Sap flow in multiple-stemmed trees increased following the irrigation, but large single-stemmed trees did not. We matched the source area for eddy covariance estimates of total ET fluxes with scaled sap flow estimates developed for the different tree types. Soil evaporation was determined from the difference between total ET and the scaled sap flow. Ecosystem-level transpiration and soil evaporation estimated by the isotope approach were within 4 and 15% of those estimated by scaled sap flow, respectively, for periods of peak fluxes at midday. Our data illustrate the utility of the isotope 'Keeling plot' approach for partitioning ET at the ecosystem scale on short time steps and the importance of accurate spatial representation of scaled sap flow for comparison with eddy covariance measurements of ET. (C) 2004 Elsevier B.V. All rights reserved.

In this paper a classification of stable boundary layer regimes is presented based on observations of near-surface turbulence during the Cooperative Atmosphere¿Surface Exchange Study-1999 (CASES-99). It is found that the different nights can be divided into three subclasses: a turbulent regime, an intermittent regime, and a radiative regime, which confirms the findings of two companion papers that use a simplified theoretical model (it is noted that its simpliflied structure limits the model generality to near-surface flows). The papers predict the occurrence of stable boundary layer regimes in terms of external forcing parameters such as the (effective) pressure gradient and radiative forcing. The classification in the present work supports these predictions and shows that the predictions are robust in a qualitative sense. As such, it is, for example, shown that intermittent turbulence is most likely to occur in clear-sky conditions with a moderately weak effective pressure gradient. The quantitative features of the theoretical classification are, however, rather sensitive to (often uncertain) local parameter estimations, such as the bulk heat conductance of the vegetation layer. This sensitivity limits the current applicability of the theoretical classification in a strict quantitative sense, apart from its conceptual value

¶During the LITFASS-98 experiment, local flux measurements were performed over five different types of underlying surface (grass, barley, triticale, pine forest, water) in a heterogeneous landscape using eddy covariance and profile techniques over a three week time period in June, 1998. Estimates of the area-integrated sensible heat flux during daytime were obtained from continuous measurements with a large aperture scintillometer (LAS) along a 4.7 km path. The calculation of a mean diurnal cycle of the fluxes during the experiment revealed significant differences between the main land use classes. A land-use weighted average of the sensible heat flux was found to be in good agreement with the LAS based estimate, which in turn was supported by other regionally integrated flux estimates from budget considerations and aircraft measurements for a few case studies. The profiles of turbulent quantities measured along a 99 m-tower significantly deviate from “idealised” profiles measured over homogeneous terrain. Peculiarities in the profile structure could be attributed to the heterogeneity of the terrain, namely to the differences in the surface characteristics of the footprint areas for the different tower levels.

A large-aperture scintillometer (LAS) was operated continuously during a period of more than one year over a heterogeneous land surface in Central Europe at the transition between marine and continental climates. The LAS measurements of the refractive index structure parameter, C N2, were used to estimate the sensible heat flux. This was possible for about 60 to 80␘f the time under daytime conditions during the summer, with lower values obtained for the cold season (October to March). Using data from a three-week long field experiment, the LAS-based heat flux was compared with a weighed average of local heat flux measurements over the main land use classes (forest, agriculture, water) in the area, resulting in reasonable agreement. LAS-based heat fluxes were then used for comparison with the heat flux values of a numerical weather prediction model. An over-prediction of the model heat flux was found in summer but the modelled values were lower than the LAS derived data during the cold season

The friction velocity (u*) and the sensible heat flux density (H) determined with a displaced-beam small aperture scintillometer (DBSAS) and a hot-film eddy correlation system are compared. Random errors in the DBSAS are relatively small, compared to scatter found with two eddy-correlation systems. Assuming that the hot-film system yields the `true' fluxes, the DBSAS appears to overestimate u* when u* is less than ≈0.2 m s-1 and to underestimate u* at high wind speeds. This implies that the DBSAS measurements of the inner scale length of turbulence, l0, a direct measure for the dissipation rate of kinetic turbulent energy, are biased. Possible causes for these results are discussed in detail. A correction procedure is presented to account for effects of random noise and of so-called inactive turbulence or sensor vibrations. The errors in u* cause errors in the DBSAS measurements of the structure parameter of temperature CT2. The derived H appears to be less sensitive to errors in l0 and CT2, because errors in these quantities tend to cancel out.

A large aperture scintillometer (LAS) and radio wave scintillometer (RWS) were installed over a heterogeneous area to test the applicability of the scintillation method. The heterogeneity in the area, which consisted of many plots, was mainly caused by differences in thermal properties of the crops; the variations in the aerodynamic roughness length were small. The water vapour fluxes derived from the combined LAS-RWS system, also known as the two-wavelength method, agreed fairly well with the aggregated water vapour fluxes derived from in-situ eddy covariance measurements. The water vapour fluxes derived from a stand-alone LAS are also presented. It was found that a single LAS and an estimate of the area averaged available energy (using a simple parameterisation scheme) can provide also reasonable area-averaged water vapour fluxes.

In the stable boundary layer (SBL) it is observed often that turbulence is not continuous in space and time. This discontinuous, intermittent turbulence causes alternations from the mean evolution of the stratified atmospheric boundary layer, which may result in an oscillatory type of behavior of the near-surface wind speed and temperature. It is well known that intermittent turbulence in the SBL can be generated by various mechanisms. This paper focuses on an intermittency generating mechanism that arises from a direct interaction of the lower atmosphere (first tens of meters) with the vegetation surface, without interaction with the air aloft. It is shown that the essence of this mechanism can be captured by a 1D bulk model of three coupled nonlinear differential equations. In the present paper, numerical runs with the model show that intermittent turbulence is most likely to occur over land surfaces with low vegetation during clear-sky conditions in the presence of a moderate to low synoptical pressure gradient. The existence of a vegetation layer has a strong influence on intermittency dynamics. Due to its small heat capacity, the vegetation temperature is able to quickly respond to rapidly changing conditions. This in turn affects the stability of the lower atmosphere, causing an important feedback mechanism. In addition, it was found that intermittent behavior of SBL models occurs for various first-order closure schemes with different stability functions. However, stability functions that allow turbulent transport beyond the critical Richardson number effectively suppress intermittent–oscillatory behavior. Currently, the latter type of formulations is often used in numerical weather prediction to prevent excessive SBL cooling in very stable conditions. The advantage of using a simplified SBL model, as proposed in the present paper, is that it allows an analytical study of the system, which, in turn, allows theoretical predictions about the occurrence of intermittent SBL behavior (see the companion paper).

A mesoscale numerical model, incorporating a land-surface scheme based on Deardorffs' approach, is used to study the diurnal variation of the boundary layer structure and surface fluxes during four consecutive days with air temperatures well below zero, snow covered ground and changing synoptic forcing. Model results are evaluated against in-situ measurements performed during the WINTEX field campaign held in Sodankylä, Northern Finland in March 1997. The results show that the land-surface parameterization employed in the mesoscale model is not able to reproduce the magnitude of the daytime sensible heat fluxes and especially the pronounced maximum observed in the afternoon. Additional model simulations indicate that this drawback is to a large extent removed by the implementation of a shading factor in the original Deardorff scheme. The shading factor, as discussed in Gryning et al. (2001), accounts for the fact that in areas with sparse vegetation and low solar angles, both typical for the northern boreal forests in wintertime, absorption of direct solar radiation is due to an apparent vegetation cover which is much greater than the actual one (defined as the portion of the ground covered by vegetation projected vertically). Moreover, the observed asymmetry in the diurnal variation of the sensible heat flux indicates that there might be a significant heat storage in the vegetation. The implementation of an objective heat storage scheme in the mesoscale model explains part of the observed diurnal variation of the sensible heat flux.

In this study, the authors develop and validate an approach to calculate a canopy conductance that can successfully be implemented in an atmospheric model. The approach is based on plant physiology approaches that have developed recently. However, it includes a new analytic formulation to scale the conductance up from leaf to canopy. Furthermore, a new expression that accounts for the effect of soil moisture on the canopy conductance is proposed. The parameters are not optimized locally; rather, values are assigned to them as a function of vegetation type. This approach is validated for three experimental sites: the First International Satellite Land Surface Climatology Project Field Experiment (FIFE)-Kansas area, the Hydrological Atmospheric Pilot Experiment-Modélisation du Bilan Hydrique (HAPEX-MOBILHY) site, and the Cabauw area. The vegetation at these sites is representative for large areas with low vegetation in the world. The results of the plant physiological approach are based on a distinction in C3 and C4 plant types, and these results are found to be better (FIFE-Kansas) and more consistent (HAPEX-MOBILHY) than estimates obtained by a traditional Jarvis-Stewart approach. The parameters of the latter are also obtained from a vegetation classification. For the Cabauw area, both approaches perform comparably. Furthermore, the new soil moisture content response function is found with work well, as compared with previous formulations.

The behaviour of various formulas for evapotranspiration of grass in non-restricted soil water conditions is considered. These are the expressions based on the Penman formula, i.e. "old" Penman, Penman-Monteith, Thom-Oliver and the version recommended more recently by the FAO. Moreover, the Priestley-Taylor and the Makkink formulas are considered, which are radiation-based. Comparisons are made between daily mean values estimated with these formulas and direct measurements. The latter were collected over grass in the period 1979–1982 in the catchment area of the Hupselse Beek (The Netherlands). It was found that if all required input data were measured, the Priestley-Taylor and the "old" Penman formula yielded the best results. The assumption that soil heat flux can be neglected introduces a systematic and a random error of roughly 5ÐThe empirical estimates for net radiation from sunshine duration, temperature and humidity appear to perform rather poorly. These estimates improved significantly if solar radiation was measured directly. The empirical expression proposed by Slob (unpublished) that requires incoming solar radiation only as input, provided better results than the other more complicated expressions. Moreover, this study reveals that evaporation of unstressed grass is primarily determined by the available energy, i.e. good evaporation estimates can be obtained by using simply l E = 0.86(Rn – G). The Makkink method appears to be attractive for practical applications. These findings support the use of Makkink’s formula for routine calculations of crop-reference evapotranspiration as has been done by the Royal Netherlands Meteorological Institute since 1987.