Chun-Ta Lai

The dynamic role of root-water uptake in coupling potential to actual transpiration

The relationship between actual (Eact) and potential (Ep) transpiration above a grass-covered forest clearing was investigated numerically and experimentally from simultaneous measurements of soil moisture content profiles, mean meteorological conditions, turbulent heat and water vapor fluxes in the atmospheric surface layer, and soil hydraulic properties for two drying periods. The relationship between Eact/Ep was found to be approximately constant and insensitive to variability in near-surface soil moisture content. To explore this near-constant Eact/Ep, a model that relates potential and actual transpiration and accounts for root-uptake eAciency, potential transpiration rate, and root-density distribution was proposed and field-tested. The total amount of water consumed by the root system was integrated and compared with eddy-correlation latent heat flux measurements (field scale) and total water storage changes (local scale). Model calculations suggested that the deeper and more eAcient roots are primarily responsible for the total water loss within the root zone when the near-surface soil layer approaches their wilting point. ” 1999 Elsevier Science Ltd. All rights reserved.

Multiscale analysis of vegetation surface fluxes: from seconds to years

Abstract The variability in land surface heat (H), water vapor (LE), and CO2 (or net ecosystem exchange, NEE) fluxes was investigated at scales ranging from fractions of seconds to years using eddy-covariance flux measurements above a pine forest. Because these fluxes significantly vary at all these time scales and because large gaps in the record are unavoidable in such experiments, standard Fourier expansion methods for computing the spectral and cospectral statistical properties were not possible. Instead, orthonormal wavelet transformations ( OWT ) are proposed and used. The OWT are ideal at resolving process variability with respect to both scale and time and are able to isolate and remove the effects of missing data (or gaps) from spectral and cospectral calculations. Using the OWT spectra, we demonstrated unique aspects in three appropriate ranges of time scales: turbulent time scales (fractions of seconds to minutes), meteorological time scales (hour to weeks), and seasonal to interannual time scales corresponding to climate and vegetation dynamics. We have shown that: (1) existing turbulence theories describe the short time scales well, (2) coupled physiological and transport models (e.g. CANVEG) reproduce the wavelet spectral characteristics of all three land surface fluxes for meteorological time scales, and (3) seasonal dynamics in vegetation physiology and structure inject strong correlations between land surface fluxes and forcing variables at monthly to seasonal time scales. The broad implications of this study center on the possibility of developing low-dimensional models of land surface water, energy, and carbon exchange. If the bulk of the flux variability is dominated by a narrow band or bands of modes, and these modes “resonate” with key state and forcing variables, then low-dimensional models may relate these forcing and state variables to NEE and LE.

Modelling Vegetation-Atmosphere Co2 Exchange By A Coupled Eulerian-Langrangian Approach

A Eulerian-Lagrangian canopy microclimate model wasdeveloped with the aim of discerning physical frombiophysical controls of CO2 and H2O fluxes. The model couples radiation attenuation with mass,energy, and momentum exchange at different canopylevels. A unique feature of the model is its abilityto combine higher order Eulerian closure approachesthat compute velocity statistics with Lagrangianscalar dispersion approaches within the canopy volume. Explicit accounting for within-canopy CO2,H2O, and heat storage is resolved by consideringnon-steadiness in mean scalar concentration andtemperature. A seven-day experiment was conducted inAugust 1998 to investigate whether the proposedmodel can reproduce temporal evolution of scalar(CO2, H2O and heat) fluxes, sources andsinks, and concentration profiles within and above auniform 15-year old pine forest. The modelreproduced well the measured depth-averaged canopy surfacetemperature, CO2 and H2O concentrationprofiles within the canopy volume, CO2 storageflux, net radiation above the canopy, and heat andmass fluxes above the canopy, as well as the velocitystatistics near the canopy-atmosphere interface. Implications for scaling measured leaf-levelbiophysical functions to ecosystem scale are alsodiscussed.

Sensible heat flux estimation by flux variance and half‐order time derivative methods

This study is the first to contrast two similarity theory methods, the flux variance and the half-order time derivative, over a wide range of atmospheric stability and surface roughness conditions. These two methods were selected because they require only single-level temperature measurement to estimate sensible heat flux. The data used were collected over bare soil, a grass-covered forest clearing, and an even-aged pine forest. For all three sites the flux variance method estimated the sensible heat flux relatively well for unstable atmospheric conditions. The half-order time derivative method was found to be sensitive to the parameterization of the eddy diffusivity, especially for the grass and bare soil field sites. Overall, the flux variance method was able to reproduce the measured sensible heat flux with greater accuracy than the half-order time derivative methods for the three experiment sites. range of atmospheric stability and roughness conditions, using the same data sets. Therefore it is envisaged that this study will provide a quantitative assessment of the general applicability of these two indirect methods for such a wide range of condi- tions as well as contrast their relative performance on the same data set.

Quantifying net ecosystem exchange by multilevel ecophysiological and turbulent transport models

To quantify the interplay between scalar sources and sinks (Sc) and net ecosystem exchange (NEE), ‘‘forward’’ and ‘‘inverse’’ approaches have been proposed. The canonical form of forward approaches is a one-dimensional ecophysiological-radiative transfer scheme coupled to turbulent transport theory. In contrast, inverse approaches strictly rely on turbulent transport theory and mean scalar concentration as their primary input to infer Sc and NEE. While the formulation of both approaches have evolved over the past decade, no systematic comparison between them was undertaken for the same data set, and over a wide range of atmospheric conditions. Our objective is to compare the predicted Sc and NEE from these two approaches with eddy-covariance measurements. The results show that the forward method outperformed all three inverse methods for unstable and neutral conditions on short time scales (� 30 min) but yielded comparable results at longer time scales. Poor agreement was obtained under stable conditions for all models. Hence, for modeling event-based flux variations, forward models are preferred. Since the forward method requires detailed knowledge of ecophysiological, drag, radiative transfer and other canopy attributes, all of which are difficult to obtain on a routine basis, a symbiotic use of forward and inverse approaches is most advantageous. � 2002 Elsevier Science Ltd. All rights reserved.

Isoscapes to address large-scale Earth science challenges

Sugar cane cropping for biofuel production reduces water discharge from a northern Indian basin and threatens downstream communities. Regulators want to partition blame between climate change—induced declines in mountain snowpack and excessive evaporation from poorly managed fields. In the same basin, a tiger is found shot. Is it the nuisance animal that has been tormenting local communities, or is it a different animal poached from the upland forests?

An automated sampler for collection of atmospheric trace gas samples for stable isotope analyses

Research focused on the isotopic composition of CO2 exchange between terrestrial ecosystems and the atmosphere has been historically constrained by the need for personnel to be present at remote field sites for sample collection. In practice, this has limited sampling frequency and duration, and potentially even biases sampling events to fair weather periods. We have developed an automated sampling system that can be installed and used for unattended collection of 100-ml air samples in remote areas. The sampler was designed with the primary goal of collecting samples for analysis of CO2 concentration and its isotopic composition in ecosystem-atmosphere flux research, but several other potential applications are also discussed. Laboratory tests examined potential artifacts associated with sampler components. These tests included evaluation of potential isotopic exchange between atmospheric CO2 and sampler component materials and the effects of sample exposure to these materials for up to 5 days and under a wide range of temperatures (10–50 ◦ C). Some of the rejected component materials influenced either CO 2 mole fraction or CO2 isotopic content. Exposure of air at subambient CO2 concentrations to all sampler components in an intact system for 5 days resulted in a [CO2] value that was 0.9mol mol −1 higher than for an equivalent sample collected by the sampler but not stored. Associated exposure-induced errors in δ 13 Co f CO 2 were generally small, ranging between 0.03 and 0.17‰ for 0 day versus 5 days exposure, respectively. These error values were within the sampling precision associated with a PreCon continuous flow mass spectrometer analysis. A more substantial exposure-induced error was observed for δ 18 Oi n CO 2 (0.29 and 0.88‰, respectively). The potential for isotopic exchange between CO2 and sampler components increased under a combination of elevated temperature and multiple-day storage treatments. These errors were small and of similar magnitude between 10 and 40 ◦ C, but unacceptably large at 50 ◦ C. Finally, we compared Keeling plots created with samples collected by the sampler with those collected simultaneously by a manual method and found no detectable differences between the two approaches. Based on these results, we conclude that sampler induced isotopic exchange for air samples held up to 5 days between 10 and 40 ◦ C is largely within the overall precision limits of a PreCon continuous flow mass spectrometer measurement.

Impact of atmospheric convection on south Tibet summer precipitation isotopologue composition using a combination of in situ measurements, satellite data, and atmospheric general circulation modeling

Precipitation isotopologues recorded in natural archives from the southern Tibetan Plateau may document past variations of Indian monsoon intensity. The exact processes controlling the variability of precipitation isotopologue composition must therefore first be deciphered and understood. This study investigates how atmospheric convection affects the summer variability of δ18O in precipitation (δ18Op) and δD in water vapor (δDv) at the daily scale. This is achieved using isotopic data from precipitation samples at Lhasa, isotopic measurements of water vapor retrieved from satellites (Tropospheric Emission Spectrometer (TES), GOSAT) and atmospheric general circulation modeling. We reveal that both δ18Op and δDv at Lhasa are well correlated with upstream convective activity, especially above northern India. First, during days of strong convection, northern India surface air contains large amounts of vapor with relatively low δDv. Second, when this low-δDv moisture is uplifted toward southern Tibet, this initial depletion in HDO is further amplified by Rayleigh distillation as the vapor moves over the Himalayan. The intraseasonal variability of the isotopologue composition of vapor and precipitation over the southern Tibetan Plateau results from these processes occurring during air mass transportation.

Regional CO2 fluxes inferred from mixing ratio measurements: Estimates from flask air samples in central Kansas, USA

We estimated regional fluxes of carbon dioxide (CO2) using mixing ratios measured in a tallgrass prairie in central Kansas, USA over 3 yr (2002–2004). Glass flasks were used to collect whole air samples in the midafternoon for determining CO2 mixing ratios and their carbon isotopic composition. Regional CO2 fluxes were calculated assuming atmospheric boundary layer (ABL) approaches an equilibrium state on a monthly basis. CO2 mixing ratios derived from the marine boundary layer data were used as a proxy to represent those in the free troposphere, which allowed for determining a boundary layer CO2 gradient primarily resulting from surface exchange.We estimated temporal changes in the ABL height for this region on a monthly basis (600–1700 m asl for a 5-yr average between 1997 and 2001) from European Center for Medium-Range Weather Forecasts (ECMWF) model data. Accordingly, we estimated the rate of entrainment (flux density) by interpolating NCAR/NCEP reanalysis data to the estimated ABL height. Our study differentiates from previous studies in several aspects: (1) we used flask-based mixing ratio measurements; (2) only discrete midday CO2 mixing ratio data were used to construct weekly CO2 gradients between free troposphere and the ABL and (3) we propose a new means for estimating monthly values of vertical transport. Modelled regional CO2 fluxes were compared to net ecosystem exchange (NEE) of CO2 fluxes measured by eddy covariance method. Assuming negligible verticalCO2 gradients between mid-ABL and the surface layer and with no correction applied, calculatedNEE showed a general agreement with measured NEE fluxes throughout the growing season. Using CO mixing ratio data, we show that fossil fuel burning contributed negligible CO2 fluxes in summer but partially explained the discrepancy between modelled regional CO2 fluxes and measured NEE in winter. This wintertime fossil fuel input was consistent with carbon isotope measurements of CO2.We demonstrate in this study that CO2 mixing ratios subsampled at midday in the surface layer can be used to gain insights into regional CO2 flux exchange in the U.S. Great Plains area.

On-Site Calibration for High Precision Measurements of Water Vapor Isotope Ratios Using Off-Axis Cavity-Enhanced Absorption Spectroscopy

AbstractStable isotope ratio measurements of atmospheric water vapor (δ18Ov and δ2Hv) are scarce relative to those in precipitation. This limitation is rapidly changing due to advances in absorption spectroscopy technology and the development of automatically calibrated field-deployable instrument systems. These systems allow high throughput, in situ monitoring of the temporal variability in δ18Ov and δ2Hv. This paper presents a robust calibration procedure for reliable, high-precision δ18Ov and δ2Hv measurements at less than hourly intervals in this study. The method described here was developed and tested using a coupled system consisting of a commercial water vapor isotopic source device and a commercial water vapor isotope analyzer [Los Gatos Research (LGR) model WVIA-24] based on the off-axis integrated cavity output spectroscopy (off-axis ICOS) technique. The isotope analyzer shows a time-dependent response that varies with water vapor mixing ratio, suggesting the need of regular (hourly) calibratio...