Validation of collection of 6 MODIS/Terra and MODIS/Aqua gross primary production in an alpine meadow of the Northern Tibetan Plateau

Moderate Resolution Imaging Spectroradiometer (MODIS) continuously monitors gross primary production (GPP), which is an extremely important component of carbon cycling, at the global scale. Uncertainties about MODIS GPP limit our ability to accurately quantify GPP at the regional scales. The Collection 6 MODIS/Terra and MODIS/Aqua GPP products (i.e. MOD17A2H and MYD17A2H) were compared with the estimated GPP (GPPEC) by eddy covariance measurements in an alpine meadow in the Northern Tibetan Plateau during three consecutive growing seasons of 2005-2007. The Collection 6 MODIS/Terra and MODIS/Aqua fractional photosynthetically active radiation (FPAR) products (i.e. MOD15A2H and MYD15A2H) were also validated. The MOD17A2H and MYD17A2H products tended to overestimate GPPEC by 2.17% and 7.35% in 2005-2007, respectively, although these differences were not significant. The MOD15A2H and MYD15A2H products also tended to overestimate ground-based FPAR (FPAR(G)) by 20.31% and 24.73% in 2005-2007, respectively. The overestimation of FPAR resulted in about 17.51-23.97% overestimation of GPPEC. The default maximum light-use efficiency (emax) of 0.86 g C MJ(-1) only underestimated the ground-based emax (0.88 g C MJ(-1)) by 2.27%, which in turn resulted in about 2.13-2.72% underestimation of GPPEC. The meteorology data errors only caused about 0.48-1.06% underestimation of GPPEC. Therefore, although MODIS Collection 6 GPP had a very high accuracy, the input parameters had relative greater errors in the alpine meadow of the Northern Tibetan Plateau. The differences between MODIS GPP and GPPEC mainly resulted from FPAR, followed by emax and meteorological data.