Evaluation of MODIS-based estimates of water-use efficiency in Amazonia

ABSTRACT

This study aimed to assess the spatial-temporal patterns of water-use efficiency (WUE) obtained through MODIS gross primary productivity (GPP) and evapotranspiration (ET) products (MOD17 for GPP and MOD16 for ET) in the Upper Tapajos and Curua-Una River basins, located in the oriental flank of the Amazon region, and to validate the results with flux tower measurements within the Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA) project. The spatial variation of WUE was primarily related to the larger presence of forested areas in the Upper Tapajos River basin (western part) compared with the Curua-Una River basin (eastern part), which is situated within the so-called arc of deforestation. Temporally, WUE showed a pronounced seasonal pattern, varying with the dry and wet seasons in the region. A decrease of ~3% in WUE was observed during the dry season, which was related to the low water availability and increased vapour pressure deficit during the dry period, which induces stomatal closure, leading to a decline in the photosynthetic rate. Comparison between the WUE estimates obtained by MODIS data and observations from the LBA towers showed an average error of 17%, varying between ~12% and ~28% for the different sites. MODIS WUE depends on the accuracy of both GPP and ET estimation. In this sense, we highlight that improvements in both MODIS GPP and ET products are necessary and should focus on reducing the uncertainties related to the biophysical vegetation parameters and meteorological data that serve as input information in the algorithms.

Acknowledgements

Gabriel de Oliveira acknowledges CNPq (Grant No. 52521/2012-7) and CAPES (PDSE program) (Grant No. 8210/2014-4) agencies for providing research fellowships. Luiz E. O. C. Aragao acknowledges the financial support of FAPESP (Grant No. 50533-5/2013) and CNPq (Grant No. 304425/2013-3) agencies.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the National Council for Scientific and Technological Development (CNPq) [304425/2013-3,52521/2012-7]; Coordination for the Improvement of Higher Education Personnel (CAPES) [8210/2014-4]; São Paulo Research Foundation (FAPESP) [50533-5/2013].