Estimation of crop gross primary production (GPP): II. Do scaledMODIS vegetation indices improve performance?

Authors

Qingyuan Zhang, Universities Space Research AssociationFollow
Yen-Ben Cheng, Sigma Space Corporation
Alexei I. Lyapustin, Climate and Radiation Laboratory
Yujie Wang, University of Maryland Baltimore County
Xiaoyang Zhang, South Dakota State University
Andrew E. Suyker, University of Nebraska - LincolnFollow
Shashi Verma, University of Nebraska - LincolnFollow
Yanmin Shuai, Earth Resources Technology, Inc.
Elizabeth M. Middleton, Biospheric Sciences LaboratoryFollow

Document Type

Article

Date of this Version

2015

Citation

Agricultural and Forest Meteorology 200 (2015) 1–8, http://dx.doi.org/10.1016/j.agrformet.2014.09.003 0168-1923

Comments

U. S. government work.

Abstract

Satellite remote sensing estimates of gross primary production (GPP) have routinely been made using spectral vegetation indices (VIs) over the past two decades. The Normalized Difference Vegetation Index(NDVI), the Enhanced Vegetation Index (EVI), the green band Wide Dynamic Range Vegetation Index(WDRVI_green), and the green band Chlorophyll Index (CI_green) have been employed to estimate GPP under the assumption that GPP is proportional to the product of VI and photosynthetically active radiation (PAR)(where VI is one of four VIs: NDVI, EVI, WDRVI_green, or CI_green). However, the empirical regressions between VI*PAR and GPP measured locally at flux towers do not pass through the origin (i.e., the zero X–Y value for regressions). Therefore they are somewhat difficult to interpret and apply. This study investigates (1)what are the scaling factors and offsets (i.e., regression slopes and intercepts) between the fraction of PAR absorbed by chlorophyll of a canopy (fAPAR_chl) and the VIs and (2) whether the scaled VIs developed in(1) can eliminate the deficiency and improve the accuracy of GPP estimates. Three AmeriFlux maize and soybean fields were selected for this study, two of which are irrigated and one is rainfed. The four VIs and fAPAR_chl of the fields were computed with the MODerate resolution Imaging Spectroradiometer (MODIS)satellite images. The GPP estimation performance for the scaled VIs was compared to results obtained with the original VIs and evaluated with standard statistics: the coefficient of determination (R²), the root mean square error (RMSE), and the coefficient of variation (CV). Overall, the scaled EVI obtained the best performance. The performance of the scaled NDVI, EVI and WDRVI_green was improved across sites, croptypes and soil/background wetness conditions. The scaled CI_green did not improve results, compared to the original CI_green. The scaled green band indices (WDRVI_green, CI_green) did not exhibit superior performance to either the scaled EVI or NDVI in estimating crop daily GPP at these agricultural fields. The scaled VIsare more physiologically meaningful than original un-scaled VIs, but scaling factors and offsets may vary across crop types and surface conditions.