Global estimates of evapotranspiration and gross primary production based on MODIS and global meteorology data

Abstract

The simulation of gross primary production (GPP) at various spatial and temporal scales remains a major challenge for quantifying the global carbon cycle. We developed a light use efficiency model, called EC-LUE, driven by only four variables: normalized difference vegetation index (NDVI), photosynthetically active radiation (PAR), air temperature, and the Bowen ratio of sensible to latent heat flux. The EC-LUE model may have the most potential to adequately address the spatial and temporal dynamics of GPP because its parameters (i.e., the potential light use efficiency and optimal plant growth temperature) are invariant across the various land cover types. However, the application of the previous EC-LUE model was hampered by poor prediction of Bowen ratio at the large spatial scale. In this study, we substituted the Bowen ratio with the ratio of evapotranspiration (ET) to net radiation, and revised the RS-PM (Remote Sensing-Penman Monteith) model for quantifying ET. Fifty-four eddy covariance towers, including various ecosystem types, were selected to calibrate and validate the revised RS-PM and EC-LUE models. The revised RS-PM model explained 82% and 68% of the observed variations of ET for all the calibration and validation sites, respectively. Using estimated ET as input, the EC-LUE model performed well in calibration and validation sites, explaining 75% and 61% of the observed GPP variation for calibration and validation sites respectively. Global patterns of ET and GPP at a spatial resolution of 0.5°latitude by 0.6°longitude during the years 2000-2003 were determined using the global MERRA dataset (Modern Era Retrospective-Analysis for Research and Applications) and MODIS (Moderate Resolution Imaging Spectroradiometer). The global estimates of ET and GPP agreed well with the other global models from the literature, with the highest ET and GPP over tropical forests and the lowest values in dry and high latitude areas. However, comparisons with observed GPP at eddy flux towers showed significant underestimation of ET and GPP due to lower net radiation of MERRA dataset. Applying a procedure to correct the systematic errors of global meteorological data would improve global estimates of GPP and ET. The revised RS-PM and EC-LUE models will provide the alternative approaches making it possible to map ET and GPP over large areas because (1) the model parameters are invariant across various land cover types and (2) all driving forces of the models may be derived from remote sensing data or existing climate observation networks.

Groundwater-dependent ecosystems: recent insights from satellite and field-based studies

“…phenology, LAI) and function (e.g. ET, gross primary productivity) has become increasingly sophisticated (Glenn et al, 2010;Yuan et al, 2010;Jung et al, 2011;Rossini et al, 2012;Kanniah et al, 2013;Ma et al, 2013;Nagler et al, 2013) and increasingly applied to realworld applications of water resources management (Scott et al, 2008;Glenn et al, 2010;Barron et al, 2014;Doody et al, 2014). Remote sensing (RS) provides a robust and spatially explicit means to assess not only vegetation structure and function but also relationships amongst these and climate variables.…”

“…Direct evidence that vegetation is using groundwater can be obtained by comparing the stable isotope composition of groundwater, soil water, surface water (if relevant) and xylem water (Thorburn et al, 1993;Zencich et al, 2002;Lamontagne et al, 2005;O'Grady et al, 2006a, b;Kray et al, 2012;Busch et al, 1992;Ehleringer and Dawson, 1992;. This method is very effective in semi-arid regions where groundwater is derived from snowmelt or winter precipitation (which is isotopically lighter than summer precipitation) (Ehleringer and Dawson, 1992;Jobbagy et al, 2011).…”

“…This method is very effective in semi-arid regions where groundwater is derived from snowmelt or winter precipitation (which is isotopically lighter than summer precipitation) (Ehleringer and Dawson, 1992;Jobbagy et al, 2011). When sufficient differences in isotopic composition exist among sources of water, the dominant source used by different species at different times of year can be identified (Zencich et al, 2002). …”

Spatial variation in annual actual evapotranspiration of terrestrial ecosystems in China: Results from eddy covariance measurements

“…In recent years, several studies have quantified the spatial patterns of AET at regional (e.g., Li et al, 2014;Liu et al, 2013a;Wang et al, 2013) and global scales (e.g., Mu et al, 2011;Yan et al, 2012;Yuan et al, 2010) using process-based or remote sensing models. Although the spatial patterns described by different models generally agreed well with each other for a given region (Mueller et al, 2011;Liu et al, 2013a), large obvious uncertainties in AET values still remain.…”

Construction and progress of Chinese terrestrial ecosystem carbon, nitrogen and water fluxes coordinated observation

“…At the same time, a series of important ecological phenomena and their controling mechanism were illustrated. For example, the 'light depress' at ecosystem scale in temperate and alpine grasslands (Fu et al, 2006(Fu et al, , 2009, the nonlinear response of ecosystem carbon flux to temperature variation , heterogeneous response of soil respiration to temperature Jia et al, 2013;Song et al, 2013), the spatio-temporal variation of ecosystem light use efficiency (Yuan et al, 2010;Zhang et al, 2006c;Wu et al, 2008), and the 'carbon pool' in mountain area (Yao et al, 2012). Such studies enhance the understanding of the biotic and abiotic controlling mechanism of ecosystem CO 2 flux across different temporal scales, and the response and adaptation of ecosystem flux to global change.…”