Editorial Type:
Article
Article Type:
Research Article

Biological and Environmental Controls on Evaporative Fractions at AmeriFlux Sites

Chunlüe Zhou State Key Laboratory of Earth Surface Processes and Resource Ecology, College of Global Change and Earth System Science, Beijing Normal University, and Joint Center for Global Change Studies, Beijing, China

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Kaicun Wang State Key Laboratory of Earth Surface Processes and Resource Ecology, College of Global Change and Earth System Science, Beijing Normal University, and Joint Center for Global Change Studies, Beijing, China

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Print Publication:
01 Jan 2016
DOI:
https://doi.org/10.1175/JAMC-D-15-0126.1
Page(s):
145–161
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Abstract

Knowledge of the evaporative fraction (EF: the ratio of latent heat flux to the sum of sensible and latent heat fluxes) and its controls is particularly important for accurate estimates of water flux, heat exchange, and ecosystem response to climatic changes. In this study, the biological and environmental controls on monthly EF were evaluated across 81 AmeriFlux sites, mainly in North America, for 2000–12. The land-cover types of these sites include forest, shrubland, grassland, and cropland, and the local climates vary from humid to arid. The results show that vegetation coverage, indicated by the normalized difference vegetation index (NDVI), has the best agreement with EF (site-averaged partial correlation coefficient ρ = 0.53; significance level p < 0.05) because of vegetation transpiration demand. The minimum air temperature is closely related to EF (site-averaged ρ = 0.51; p < 0.05) because of the inhibition of respiratory enzyme activity. Relative humidity, an indicator of surface aridity, shows a significant positive correlation with EF (site-averaged ρ = 0.46; p < 0.05). The impacts of wind speed and diurnal air temperature range on EF depend on land-cover types and are strong over grasslands and cropland. From these findings, empirical methods were established to predict monthly EF using meteorological data and NDVI. Correlation coefficients between EF estimates and observations range from 0.80 to 0.93, with root-mean-square errors varying from 0.09 to 0.12. This study demonstrates the varying controls on EF across different landscapes and enhances understanding of EF and its dynamics under changing climates.

Denotes Open Access content.

Corresponding author address: Dr. Kaicun Wang, State Key Laboratory of Earth Surface Processes and Resource Ecology, College of Global Change and Earth System Science, Beijing Normal University, Beijing, 100875, China. E-mail: kcwang@bnu.edu.cn

Abstract

Knowledge of the evaporative fraction (EF: the ratio of latent heat flux to the sum of sensible and latent heat fluxes) and its controls is particularly important for accurate estimates of water flux, heat exchange, and ecosystem response to climatic changes. In this study, the biological and environmental controls on monthly EF were evaluated across 81 AmeriFlux sites, mainly in North America, for 2000–12. The land-cover types of these sites include forest, shrubland, grassland, and cropland, and the local climates vary from humid to arid. The results show that vegetation coverage, indicated by the normalized difference vegetation index (NDVI), has the best agreement with EF (site-averaged partial correlation coefficient ρ = 0.53; significance level p < 0.05) because of vegetation transpiration demand. The minimum air temperature is closely related to EF (site-averaged ρ = 0.51; p < 0.05) because of the inhibition of respiratory enzyme activity. Relative humidity, an indicator of surface aridity, shows a significant positive correlation with EF (site-averaged ρ = 0.46; p < 0.05). The impacts of wind speed and diurnal air temperature range on EF depend on land-cover types and are strong over grasslands and cropland. From these findings, empirical methods were established to predict monthly EF using meteorological data and NDVI. Correlation coefficients between EF estimates and observations range from 0.80 to 0.93, with root-mean-square errors varying from 0.09 to 0.12. This study demonstrates the varying controls on EF across different landscapes and enhances understanding of EF and its dynamics under changing climates.

Denotes Open Access content.

Corresponding author address: Dr. Kaicun Wang, State Key Laboratory of Earth Surface Processes and Resource Ecology, College of Global Change and Earth System Science, Beijing Normal University, Beijing, 100875, China. E-mail: kcwang@bnu.edu.cn
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