Abstract
Water use efficiency (WUE) is a useful indicator to illustrate the interaction of carbon and water cycles in terrestrial ecosystems. MODIS gross primary production (GPP) and evapotranspiration (ET) products have been used to analyze the spatial and temporal patterns of WUE and their relationships with environmental factors at regional and global scales. Although MODIS GPP and ET products have been evaluated using eddy covariance flux measurements, the accuracy of WUE estimated from MODIS products has not been well quantified. In this paper, we evaluated WUE estimated from MODIS GPP and ET products against eddy covariance measurements of GPP and ET during 2003–2008 at eight sites of the Chinese flux observation and research network (ChinaFLUX) and conducted sensitivity analysis to investigate the possible key contributors to the bias of MODIS products. Results show that MODIS products underestimate eight-day water use efficiency in four forest ecosystems and one cropland ecosystem with the bias from −0.36–−2.28 g·C·kg−1 H2O, while overestimating it in three grassland ecosystems with the bias from 0.26–1.11 g·C·kg−1 H2O. Mean annual WUE was underestimated by 14%–54% at four forest sites, 45% at one cropland site and 7% at an alpine grassland site, but overestimated by 66% and 9% at a temperate grassland site and an alpine meadow site, respectively. The underestimation of WUE by MODIS data results from underestimated GPP and overestimated ET at four forest sites, while MODIS WUE values are significantly overvalued mainly due to underestimated ET in the three grassland ecosystems. The maximum light use efficiency and fraction of photosynthetically-active radiation (FPAR) were the two most sensitive factors to the estimation of WUE derived from the MODIS GPP and ET algorithms. The error in meteorological data partly caused the overestimation of ET and accordingly underestimation in WUE in subtropical and tropical forests. The bias of MODIS-produced WUE was also derived from the uncertainties in eddy flux data due to gap-filling processes and unbalanced surface energy issue. Their contributions to the uncertainty in estimated WUE at both eight-day and annual scales still need to be further quantified.
Highlights
Water use efficiency (WUE), the ratio of carbon assimilation or productivity to water loss, is a valuable and commonly-used index to investigate the relationship between terrestrial carbon and water cycles [1,2,3]
The objectives of this paper are to evaluate WUE estimated from MODIS gross primary productivity (GPP) and ET products against eddy covariance measurements of GPP and ET at eight sites of the Chinese flux observation and research network (ChinaFLUX) and to the identify key factors that influence the accuracy of WUE derived from MODIS data
The results show that the decrease in leaf area index (LAI) by 50% for Dinghushan evergreen mixed forest (DHS) and Xishuangbanna evergreen broadleaf forest (XSBN) led to a reduction of 0.2% in ET and an increase of 0.2% in WUE, as well as a 200% increase in LAI for Yucheng cropland (YC) only caused by a 0.4% increase in ET and a 0.4% decrease in WUE
Summary
Water use efficiency (WUE), the ratio of carbon assimilation or productivity to water loss, is a valuable and commonly-used index to investigate the relationship between terrestrial carbon and water cycles [1,2,3]. Improving WUE is an effective way of saving plant water consumption and optimizing water management. It has been extensively studied at different scales (leaf scale, canopy scale and regional scale) [4,5,6]. WUE can be calculated by the ratio of gross primary productivity (GPP) to evapotranspiration (ET). Eddy covariance techniques provide an effective way to quantify the magnitudes, patterns and changes in WUE at the ecosystem level over varied vegetation types [7,8,9,10,11,12]. It is still difficult to obtain dynamic WUE maps over a large region or even at the global level based on eddy flux data measured at a limited number of tower sites
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