Comparison of four EVI-based models for estimating gross primary production of maize and soybean croplands and tallgrass prairie under severe drought

Accurate estimation of gross primary production (GPP) is critical for understanding ecosystem response to climate
variability and change. Satellite-based diagnostic models, which use satellite images and/or climate data
as input, are widely used to estimate GPP. Many models used the Normalized Difference Vegetation Index
(NDVI) to estimate the fraction of absorbed photosynthetically active radiation (PAR) by vegetation canopy
(FPARcanopy) and GPP. Recently, the Enhanced Vegetation Index (EVI) has been increasingly used to estimate
the fraction of PAR absorbed by chlorophyll (FPARchl) or green leaves (FPARgreen) and to provide more accurate
estimates of GPP in such models as the Vegetation Photosynthesis Model (VPM), Temperature and Greenness
(TG) model, Greenness and Radiation (GR) model, and Vegetation Index (VI) model. Although these EVI-based
models perform well under non-drought conditions, their performances under severe droughts are unclear. In
this study, we run the four EVI-based models at three AmeriFlux sites (rainfed soybean, irrigated maize, and
grassland) during drought and non-drought years to examine their sensitivities to drought. As all the four models
use EVI for FPAR estimate, our hypothesis is that their different sensitivities to drought are mainly attributed to
the ways they handle light use efficiency (LUE), especially water stress. The predicted GPP from these four
models had a good agreement with the GPP estimated from eddy flux tower in non-drought years with root
mean squared errors (RMSEs) in the order of 2.17 (VPM), 2.47 (VI), 2.85 (GR) and 3.10 g C m−2 day−1 (TG).
But their performances differed in drought years, the VPM model performed best, followed by the VI, GR and
TG, with the RMSEs of 1.61, 2.32, 3.16 and 3.90 g C m−2 day−1 respectively. TG and GR models overestimated
seasonal sum of GPP by 20% to 61% in rainfed sites in drought years and also overestimated or underestimated
GPP in the irrigated site. This difference in model performance under severe drought is attributed to the fact
that the VPMuses satellite-based Land Surface Water Index (LSWI) to address the effect of water stress (deficit)
on LUE and GPP,while the other three models do not have such a mechanism. This study suggests that it is essential
for these models to consider the effect of water stress on GPP, in addition to using EVI to estimate FPAR, if
these models are applied to estimate GPP under drought conditions.