Zhongwang Wei

Partitioning of evapotranspiration using high-frequency water vapor isotopic measurement over a rice paddy field

Partitioning ecosystem evapotranspiration (ET) into soil evaporation (E) and transpiration (T) is crucial for understanding hydrological processes. In this study, by using high-frequency isotope measurements and continuous surface water measurements, we investigated the isotope ratios in soil-vegetation-atmosphere transfer and the physical mechanisms involved over a paddy field for a full growing season. The isotopic signals of δET, δT, and δE were determined by the Keeling plot method, surface water isotopic measurements, and the Craig-Gordon model, respectively. The fraction of transpiration in evapotranspiration (FT) ranged from 0.2 to 1, with an almost continuous increase in the early growing season and a relatively constant value close to 1 later in the year. The result was supported by FT derived from simulated T and eddy correlation measured ET. The seasonal change in the transpiration fraction could be described quite well as a function of the LAI (FT = 0.67LAI0.25, R2 = 0.80), implying that transpiration plays a dominant role in the soil-vegetation-atmosphere continuum during the growing season. The two end-member uncertainty analysis suggested that further improvement in the estimation of δT and δET is necessary for partitioning evapotranspiration using the isotopic method. In the estimation of δET, the assumptions underlying Keeling plot method were rarely met and the uncertainty was quite large. A high frequency of precise isotopic measurements in surface water was also necessary for δT estimation. Furthermore, special care must be taken concerning the kinetic fractionation parameter in the Craig and Gordon Equation for δE estimation under low-LAI conditions. The results demonstrated the robustness of using isotope measurements for partitioning evapotranspiration.

Influences of Root Hydraulic Redistribution on N2O Emissions at AmeriFlux Sites

It has long been suspected that root hydraulic redistribution (HR) affects the carbon and nitrogen cycles. Nitrous oxide (N2O) is an important greenhouse gas and is the primary stratospheric ozone-depleting substance. To our knowledge, the influences of HR on N2O emissions have not been investigated. Here we use the HR schemes of Ryel et al. and Amenu and Kumar incorporated into CLM4.5 to examine N2O emissions at five AmeriFlux sites. The results show that HR reduced N2O emissions by 28–92% in the four natural ecosystems experiencing a dry season, whereas it had a very limited effect on the Corn Belt site that has strong emissions but with no distinct dry season. We hypothesize that N2O emissions in ecosystems with a distinct dry season are likely overestimated by CENTURY-based Earth system models. Plain Language Summary The findings of this study suggest that hydraulic redistribution (HR) may play an important role in N2O emissions from agricultural regions that have a clearly defined dry season. For example, the expansive corn-growing regions of China are all located in the monsoon area with a distinct dry season. We hypothesize that the HR mechanism acts to significantly reduce N2O emissions in these regions. HR may also play an important role in limiting N2O emissions in the Amazonian regions with a tropical monsoon climate, where forests have been converted to agricultural use.

A Meta-Analysis of Open-Path Eddy Covariance Observations of Apparent CO2 Flux in Cold Conditions in FLUXNET

AbstractOpen-path eddy covariance systems are widely used for measuring the CO2 flux between land and atmosphere. A common problem is that they often yield negative fluxes or physiologically unreasonable CO2 uptake fluxes in the nongrowing season under cold conditions. In this study, a meta-analysis was performed on the eddy flux data from 64 FLUXNET sites and the relationship between the observed CO2 flux and the sensible heat flux was analyzed. In theory, these two fluxes should be independent of each other in cold conditions (air temperature lower than 0°C) when photosynthesis is suppressed. However, the results show that a significant and negative linear relationship existed between these two fluxes at 37 of the sites. The mean linear slope value is −0.008 ± 0.001 µmol m−2 s−1 per W m−2 among the 64 sites analyzed. The slope value was not significantly different among the three gas analyzer models (LI-7500, LI-7500A, IRGASON/EC150) used at these sites, indicating that self-heating may not be the only ...