Ray Leuning

Maximum conductances for evaporation from global vegetation types

We compare independent data sets of the maximum stomata1 conductance (gsmax, for single leaves) and bulk surface conductance (G,,,,, for a vegetated surface including the plant canopy and soil) for evaporation. Data were obtained from field measurements, restricted to periods with plentiful soil water, adequate light, high relative humidity and moderate temperature. The data encompass most major vegetation types and a wide range of leaf area index (A). Observed G smax is not systematically dependent on A; and takes average values of 20 and 33 mm s-’ for natural vegetation and agricultural crops. A similar pattern exists in the g,,, data, which yield remarkably consistent average values of 6 and 12 mm s-l, respectively, for natural vegetation and crops. Overall, the ratio G,,,,/g,,,, is consistently close to 3, for seven major vegetation types of diverse structure. A simple model accounts for the close relationship between g,,,, and G smax, and in particular how G,,,, is conservative against A because of the compensating decrease in plant canopy evaporation and increase in soil evaporation as A diminishes. The results are important for development of parameters for biosphere-atmosphere interactions in models.

A two-leaf model for canopy conductance, photosynthesis and partitioning of available energy I:: Model description and comparison with a multi-layered model

An one-layered, two-leaf canopy model which calculates the fluxes of sensible heat, latent heat and CO2 separately for sunlit and shaded leaves is presented. The two-leaf model includes: (1) a simple but robust radiation model, (2) an improved leaf model accounting for the interaction of conductance and photosynthesis and the response of stomata to water vapour pressure deficit and available soil water and (3) a new parameterisation of radiative conductance which simplifies solution of the leaf energy balance equation. Comparisons with a multi-layered model show that predicted fluxes of CO2, latent and sensible heat fluxes usually agree within 5% over a range leaf area index typical of a wheat crop grown in a temperate climate. The two-leaf model is computationally 10 times more efficient than the multi-layered model and is suitable for the incorporation into regional and global climate models. For a hypothetical canopy with a leaf area index of 5 under very dry (vapour pressure deficit of air of 2 kPa) and sunny conditions, the net canopy photosynthesis and latent heat fluxes calculated by the two-leaf model agree with those by the multi-layered model within 10% for the whole range of soil water conditions (from very dry to wet) and the sensible heat fluxes of the canopy calculated by the two-leaf model agree with those by the multi-layered model within 25 W m ˇ2 (or usually within 15%). For a canopy with leaf area index less than 2, the differences in the modelled fluxes of canopy CO2, latent or sensible heat are less than 5% between the multi-layered model and two-leaf model. Our results show that the two-leaf model can predict net photosynthesis, latent and sensible heat fluxes of a canopy quite accurately under a wide range of soil water availability and meteorological conditions, as compared with the multi-layered model. # 1998 Elsevier Science B.V. All rights reserved.

A re-evaluation of long-term flux measurement techniques - Part I: Averaging and coordinate rotation

Experience of long term flux measurements over tall canopiesduring the last two decades has revealed that the eddy flux of sensible plus latentheat is typically 30% smaller than the available radiant energy flux. This failureto close the energy balance is less common close to the surface over short roughnessbut is still sometimes seen, especially in complex topography. These observationscast doubt on the results obtained from long term flux studies where daily and annualnet ecosystem exchange is usually the small difference between large positive andnegative fluxes over 24 h. In this paper we investigate this problem by examiningsome fundamental assumptions entailed in analysis of surface exchange by the eddyflux method.In particular, we clarify the form and use of the scalar conservation equation thatunderlies this analysis and we examine the links between averaging period androtation of coordinates in the situation where coordinates are aligned with thewind vector. We show that rotating coordinates so that the x axis is alignedwith the mean wind vector has the effect of high pass filtering the scalar covariance,¯wc, such that contributions to the aerodynamic flux from atmosphericmotions with periods longer than the averaging period are lost while those of shorterperiod are distorted.We compare the effect of computing surface exchange by averaging many shortperiods, in each of which the coordinates are rotated so that the mean verticalvelocity is zero (the method currently adopted in most long-term flux studies),with analysis in long-term coordinates and show a systematic underestimationof surface exchange in the former case. This is illustrated with data from threelong-term forest field sites where underestimations of sensible and latent heatfluxes of 10–15% averaged over many days are seen.Crucial factors determining the loss of flux are the averaging period T, themeasurement height and the content of the scalar cospectrum at periods longerthan T. The properties of this cospectrum over tall canopies in both homogeneousand complex terrain are illustrated by measurements at our three sites and we see thatover tall canopies on flat ground in convectiveconditions, or on hilly sites in near neutralflow, the scalar cospectra have much more low frequency contentthan classical surface-layerspectral forms would predict. We believe that the filtering of this low frequencycovariance by the averaging-rotation operations in common use is a large contributoryfactor to the failure to close the energy balance over tall canopies.

Evaporation and canopy characteristics of coniferous forests and grasslands

Canopy-scale evaporation rate (E) and derived surface and aerodynamic conductances for the transfer of water vapour (gs and ga, respectively) are reviewed for coniferous forests and grasslands. Despite the extremes of canopy structure, the two vegetation types have similar maximum hourly evaporation rates (Emax) and maximum surface conductances (gsmax) (medians = 0.46 mm h-1 and 22 mm s-1). However, on a daily basis, median Emax of coniferous forest (4.0 mm d-1) is significantly lower than that of grassland (4.6 mm d-1). Additionally, a representative value of ga for coniferous forest (200 mm s-1) is an order of magnitude more than the corresponding value for grassland (25 mm s-1). The proportional sensitivity of E, calculated by the Penman-Monteith equation, to changes in gs is >0.7 for coniferous forest, but as low as 0.3 for grassland. The proportional sensitivity of E to changes in ga is generally ±0.15 or less.Boundary-line relationships between gs and light and air saturation deficit (D) vary considerably. Attainment of gsmax occurs at a much lower irradiance for coniferous forest than for grassland (15 versus about 45% of full sunlight). Relationships between gs and D measured above the canopy appear to be fairly uniform for coniferous forest, but are variable for grassland. More uniform relationships may be found for surfaces with relatively small ga, like grassland, by using D at the evaporating surface (D0) as the independent variable rather than D at a reference point above the surface. An analytical expression is given for determining D0 from measurable quantities. Evaporation rate also depends on the availability of water in the root zone.Below a critical value of soil water storage, the ratio of evaporation rate to the available energy tends to decrease sharply and linearly with decreasing soil water content. At the lowest value of soil water content, this ratio declines by up to a factor of 4 from the non-soil-water-limiting plateau. Knowledge about functional rooting depth of different plant species remains rather limited. Ignorance of this important variable makes it generally difficult to obtain accurate estimates of seasonal evaporation from terrestrial ecosystems.

Eddy-covariance CO2 flux measurements using open- and closed-path CO2 analysers : corrections for analyser water vapour sensitivity and damping of fluctuations in air sampling tubes

Methods of calibrating infrared CO2 analysers for sensitivity to CO2 and water vapour are described. Equations to correct eddy covariance CO2 flux measurements are presented for: (i) analyser cross-sensitivity to water vapour and the effects of density fluctuations arising from atmospheric fluxes of water vapour and sensible heat, (ii) flux losses caused by signal processing and limited instrument frequency response for open- and closed-path CO2 analysers, and (iii) flux losses resulting from damping of concentration fluctuations in a tube used to sample air for closed-path CO2 analysers. Examples of flux corrections required for typical instruments are presented.

A sampler for measuring atmospheric ammonia flux

Abstract This paper describes the design, construction and testing of a simple sampling device for determining ammonia fluxes in the atmosphere. Sampler performance was predicted theoretically and tested in a wind tunnel, the laboratory and in the field. The results showed that air flowed through the sampler at a rate linearly proportional to the external wind speed and that the ammonia it contained was absorbed quantitatively by the sampler. The mass of ammonia, M , collected by the instrument during a sampling period, t , is thus proportional to the mean convective flux density of ammonia, ( uϱ N ) since M = ( uϱ N )t , where u is the wind velocity, ϱ N the ammonia density, A is the effective cross-sectional area of the sampler, and the overbar represents a time-mean.

Carbon dioxide and methane fluxes from an intermittently flooded paddy field

To assess the role of floodwater in controlling the exchanges of CO 2 and CH4 from soil, floodwater and the canopy in intermittently flooded rice paddies, an intensive field campaign (IREX96) was conducted in Japan during August 1996. Eddy covariance was employed to measure fluxes of heat, water vapor and CO 2. The flux-gradient method was used to determine CH4 fluxes from measured profiles of CH 4 concentrations, with the required eddy diffusivity estimated using a modified aerodynamic approach or CO 2 as a reference scalar. When the paddy was drained, net CO2 uptake from the atmosphere during daytime was 23% less, and nighttime CO2 emissions were almost twice as great, than when the paddy was flooded. The mean daily CO 2 uptake on the drained days was 14.5 g m 2 , <50% of the mean for the flooded days. These differences in the CO2 budget were mainly due to increased CO2 emissions from the soil surface under drained conditions resulting from the removal of the diffusion barrier caused by the floodwater. Small changes in canopy photosynthesis observed between flooded and drained paddies had little influence on the CO 2 budget and could be explained by sensitivity of stomata to humidity saturation deficit. The CH 4 flux for the drained paddy showed distinct diurnal variation with a maximum of 1.3m gC H 4 m 2 s 1 in the afternoon, but after reflooding the peak flux decreased to <0.9m gC H 4 m 2 s 1 . Mean daily CH4 emissions were 28% larger for the drained paddy than when it was flooded. As with the CO 2 flux, the larger CH 4 flux on the drained days can be attributed to reduced resistance of CH4 transfer from the soil to air by removal of the floodwater.

Evaluating the Performance of Land Surface Models

Abstract This paper presents a set of analytical tools to evaluate the performance of three land surface models (LSMs) that are used in global climate models (GCMs). Predictions of the fluxes of sensible heat, latent heat, and net CO2 exchange obtained using process-based LSMs are benchmarked against two statistical models that only use incoming solar radiation, air temperature, and specific humidity as inputs to predict the fluxes. Both are then compared to measured fluxes at several flux stations located on three continents. Parameter sets used for the LSMs include default values used in GCMs for the plant functional type and soil type surrounding each flux station, locally calibrated values, and ensemble sets encompassing combinations of parameters within their respective uncertainty ranges. Performance of the LSMs is found to be generally inferior to that of the statistical models across a wide variety of performance metrics, suggesting that the LSMs underutilize the meteorological information used in...

Comparison of eddy-covariance measurements of CO2 fluxes by open- and closed-path CO2 analysers

Eddy fluxes of CO2 estimated using a sonic anemometer and a closed-path analyser were, on average, 16% lower than those obtained with the same anemometer and an adjacent open-path CO2 analyser. Covariances between vertical windspeed and CO2 density from the closed-path analyser were calculated using data points for CO2 that were delayed relative to anemometer data by the time required for a parcel of air to travel from the tube inlet to the CO2 sensor. Air flow in the intake tube was laminar. Densities of CO2 that had been corrected for spurious fluctuations arising from fluctuations in temperature and humidity were used in the flux calculations. Corrections for the cross-sensitivity of CO2 analysers to water vapour were also incorporated. Spectral analysis of the corrected CO2 signal from the closed-path analyser showed that damping of fluctuations in the sampling tube at frequencies f > 0.1 Hz caused the apparent loss in flux. The measured losses can be predicted accurately using theory that describes the damping of oscillations in a sampling tube. High-frequency response of the closed-path system can be improved substantially by ensuring turbulent flow in the tube, using a combination of high volumetric flow rate and small tube diameter. The analysis of attenuation of turbulent fluctuations in flow through tubes is applicable to the measurement of fluxes of other minor atmospheric constituents using the eddy covariance method.

Nocturnal carbon efflux: reconciliation of eddy covariance and chamber measurements using an alternative to the u*-threshold filtering technique

Micrometeorological measurements made on single towers often underestimate nighttime respiration of terrestrial ecosystems because they cannot account for vertical and horizontal advection, thereby causing systematic errors in estimates of net ecosystem carbon exchange. We show that there is a maximum in the sum of the turbulent flux and change in storage of CO2 in the early evening, Rmax, that is in close agreement with concurrent and independent estimates of net carbon exchange from soil and plant chambers.We hypothesize that the peak occurs because there is a time delay between the onset of radiative cooling and the development of temperature gradients that are strong enough to initiate thermally-driven horizontal and vertical flows that remove the stored CO2. We propose taking advantage of this time delay to develop relationships between Rmax and soil temperature and moisture. The new parameterization leads to realistic values of nighttime respiration, and therefore to improved estimates of net ecosystem exchange.