Anne Verhoef

A system to measure surface fluxes of momentum, sensible heat, water vapour and carbon dioxide

An eddy covariance system is described which has been developed jointly at a number of European laboratories and which was used widely in HAPEX-Sahel. The system uses commercially available instrumentation: a three-axis sonic anemometer and an IR gas analyser which is used in a closed-path mode, i.e. air is brought to the optical bench by being ducted down a sampling tube from a point near the sonic anemometer. The system is controlled by specially written software which calculates the surface fluxes of momentum, sensible and latent heat and carbon dioxide, and displays them in real time. The raw turbulent records can be stored for post-processing. Up to five additional analogue instruments can be sampled at up to 10 Hz and digitised by the sonic anemometer. The instruments are described and details of their operation and connection are presented. The system has relatively low power consumption and can operate from appropriate solar cells or rechargeable batteries. Calibration of the gas analyser needs to be performed typically every 2 or 3 days, and, given that the system requires minimal maintenance and is weather insensitive, it can be operated for the routine collection of surface flux data for extended periods. There are a number of corrections which have to be applied in any eddy covariance system and we describe the system of transfer functions which define our system. Some representative results showing the potential of the system are presented.

Some Practical Notes on the Parameter kB−1 for Sparse Vegetation

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.

Fluxes of carbon dioxide and water vapour from a Sahelian savanna

Abstract Simultaneous measurements of atmospheric CO 2 flux, F c , and latent heat flux, E , from a shrub savanna in Niger, West Africa, were made by eddy correlation. The vegetation at the study site consisted of scattered shrubs with an understorey of grasses and herbs. The measurements made available some of the first data on CO 2 and H 2 O exchange for an semi-natural, mixed plant community, growing in the semi-arid tropics. Such data are necessary for the development of improved soil-vegetation-atmosphere models, able to describe the complex interplay between atmospheric CO 2 , vegetation conductance and the surface energy balance of the Sahel in global climate models. In this framework, the effect of the extreme and highly variable environment, represented by the saturation deficit, D , and integrated water content of the upper soil layers, Φ, was discussed. F c and E were measured throughout the transition from the wet to the dry season (September–October) during the HAPEX-Sahel Intensive Observation period, in 1992. At the same time, leaf stomatal conductances, g 1 , of the shrubs and two understorey herb species was measured. Daily totals of F c decreased by 50% (peak values declined from −10 to −5 μmol m −2 s −1 ) over 3 weeks following the last rainfall of the wet season. During the same period, g 1 decreased roughly four-fold for all sampled species. D appeared to be the main controlling parameter in the exchange of CO 2 and H 2 O. g 1 and ‘water use efficiency’, | F c E |, were well correlated with D , which also considerably influenced the response of F c to photosynthetically active radiation, Q p . Simultaneously, a decreasing Φ caused lower values of g 1 and F c . However, soil moisture had little effect on the empirical relationships found between atmospheric variables ( D or Q p ) and g 1 or F c .

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

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

A SVAT scheme describing energy and CO2 fluxes for multi-component vegetation: calibration and test for a Sahelian savannah.

Abstract A soil vegetation atmosphere transfer scheme (SVAT), describing the fluxes of heat, water vapour and CO 2 between a multi-component vegetated land surface and the atmosphere, has been developed. The SVAT has been calibrated and tested using 30 days of micrometeorological and physiological measurements collected over a sparse savannah in the Sahel. The model explicitly represents the four major functional components of the savannah: shrubs (C 3 photosynthesis), grasses (C 4 ), herbs (C 3 ) and bare soil. The leaf conductance model for the shrub, grass and herb components was calibrated against porometry measurements made in the field. The performance of the SVAT was tested against independent surface flux and temperature measurements. Agreement between the model predictions and measurements of the total net radiation, sensible and latent heat fluxes, net CO 2 exchange and surface temperatures was good: in all cases at least 80% of the variance in the measurements was explained by the model. Separate flux and surface temperature predictions for the four surface components were satisfactory, although complete verification was difficult as data were lacking for some variables. Predicted water use efficiency (WUE) of the vegetation showed a strong, non-linear dependence on vapour pressure deficit, especially for the bushes and the grasses. WUE for the grasses was about three times as large as the values found for the bushes and herbs. A SVAT model like this can be employed to address the possible effects of CO 2 enrichment and climate change on the competitive balance between different species in a plant community or in research concerning productivity and water use efficiency in mixed crops, such as agroforestry systems.

Spatial and temporal variations in net carbon flux during HAPEX-Sahel

Micrometeorological measurements of the surface flux of carbon dioxide were made at a number of spatially separate sites within the HAPEX-Sahel experimental area. Differences in the timing of plant development caused by differences in rainfall (both quantity and frequency) over the experimental area exert a major influence on the absolute magnitude of the surface flux of carbon made simultaneously over the experimental area. Surface fluxes are presented for the three major types of vegetation in the Sahelian landscape, namely, millet, savannah and tiger bush. The atmospheric and surface controls on the surface flux of CO2 are described for these three land surfaces and the fluxes are compared between sites. The comparison shows that modellers need to take into account the heterogeneity not only of the type of surface vegetation but also the variability in the life cycle stage of the vegetation, when scaling up to larger areas.

Seasonal variation of surface energy balance over two Sahelian surfaces

The seasonal variation of the surface energy balance (SEB) (net radiation, soil heat flux, and sensible and latent heat fluxes) for a savanna and an open natural forest (tiger-bush) located in the Sahel is presented. Daily averages of these four fluxes were obtained using standard micrometeorological instruments:methods (radiometry, soil heat flux plates and the eddy correlation technique). Because sensible and latent heat flux data were not available during the dry season (and occasionally missing during the wet season), estimates from a simple linear equation, which relates sensible heat flux to the difference between surface and air temperature, were used instead. This produced a practically continuous time series of the surface energy balance for a period of over 1 year. Net radiation over both surfaces appeared to be very similar, varying roughly between 75 W m 2 (dry winter months) and 175 W m 2 (wet summer months) on a daily basis. During the winter months, soil heat flux had maximum values of about 8W m 2 , while maximum values during the summer were approximately 8Wm 2 . Sensible heat flux varied between approximately 20 W m 2 (wet season) and 100 W m 2 (end of dry season) with generally slightly higher values for the savanna than for the tiger-bush, especially during the dry autumn and winter months. During the wet season, evaporation reached values of 100‐150 W m 2 . Both surfaces were predicted to exhibit some evaporation during the dry season; approximately 5‐10 W m 2 for the savanna and approximately 20 W m 2 for the tiger-bush. The course of these fluxes is supported by secondary observations such as rooting depth and greenness of the vegetation, indicating that this method may be a useful tool if gaps in long-term time series exist. The information presented will be useful as verification data for climate modelling and can be used as ground truth data for remote sensing. Copyright

Adsorption of Water Vapor by Bare Soil in an Olive Grove in Southern Spain

Abstract Data for water vapor adsorption and evaporation are presented for a bare soil (sandy loam, clay content 15%) in a southern Spanish olive grove. Water losses and gains were measured using eight high-precision minilysimeters, placed around an olive tree, which had been irrigated until the soil reached field capacity (∼0.22 m3 m−3). They were subsequently left to dry for 10 days. A pair of lysimeters was situated at each of the main points of the compass (N, E, S, W), at a distance of 1 m (the inner set of lysimeters; ILS) and 2 m (the outer set of lysimeters; OLS), respectively, from the tree trunk. Distinct periods of moisture loss (evaporation) and moisture gain (vapor adsorption) could be distinguished for each day. Vapor adsorption often started just after noon and generally lasted until the (early) evening. Values of up to 0.7 mm of adsorbed water per day were measured. Adsorption was generally largest for the OLS (up to 100% more on a daily basis), and increased during the dry down. This was ...

Soil evaporation from sparse natural vegetation estimated from Sherwood numbers.

For various purposes and applications it is convenient to have a simple technique available that produces reliable estimates about the contribution of the soil sensible and latent heat of a crop canopy or natural vegetation to the total fluxes. This is especially of importance in the case of a sparse vegetation where the bare soil is the major component. Under low wind conditions a free convective state often occurs which offers an opportunity to make a simple assessment of the soil sensible heat contribution to the total sensible heat flux. In this case there exists a unique relation between the surface Rayleigh number and the surface Nusselt number. The same technique can be applied to the vapour flux by using a unique relation between the surface Rayleigh number and the surface Sherwood number, if the soil surface is wet. The last condition occurs after a rainy period. Mostly, however, the upper soil layer is dry and the soil evaporation will be limited by the surface resistance to evaporation. If the relation between soil moisture and the so-called ‘soil Bowen ratio coefficient’, cw, as proposed by Massman (1992) is known, a simple correction to the potential soil evaporation can be applied. During the HAPEX-Sahel experiment the above-mentioned technique has been applied to a natural vegetation under semi-arid conditions. Moreover, the modelled soil evaporation has been verified by micro-lysimeter data. It appeared that the proposed technique is promising and is in agreement with the measurement results.

A NEW METHOD TO DETERMINE THE ZERO-PLANE DISPLACEMENT

A new method is presented to determine the zero-plane displacement of tall vegetation from single-level measurementsof the standard deviation of the vertical wind speed and the kinematicsensible heat flux. A reasonable result is obtained for tiger bush,a vegetation type in the Sahel.