Evaluation of New MODIS LAIchl Product in Cropland Using the Community Land Model Version 4 (CLM4)

Leaf area index (LAI) and the fraction of photosynthetically radiation (PAR) absorbed by vegetation (FPAR) have been widely used to estimate vegetation photosynthesis, i.e., gross primary production (GPP). The MOD15A2 LAI/FPAR product is one of the standard products derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) imagery. However, only the PAR absorbed by chlorophyll of the canopy, not the PAR absorbed by the foliage or by the entire canopy, is used for photosynthesis. Therefore, we use a canopy-leaf radiative transfer model PROSAIL2 with MAIAC retrieved surface reflectance to derive the fraction of PAR absorbed by the foliage (fAPAR_foliage), the fraction of PAR absorbed by chlorophyll of the canopy (fAPAR_chl), LAI, and photosynthetic section of LAI: LAI_chl=LAI*fAPAR_chl/fAPAR_foliage. The default LAI values in the Community Land Model Version 4 (CLM4) are mean values for each biome type in each grid derived from MOD15A2 LAI. The objective of this study is to assess the performance of LAI versus LAI_chl in CLM GPP simulation. Three AmeriFlux tower sites (US-Ne1, US-Ne2 and US-Ne3) located in Nebraska are selected. Three LAI data sets of each site have been implemented into CLM to simulate GPP: (1) default LAI with CLM4.0, (2) MOD15A2 LAI of each site, and (3) LAI_chl of each site. Differences among measured GPP and simulated GPP derived from CLM default LAI, MOD15A2 LAI and LAI_chl have been analyzed. The results indicate that the new LAI_chl product shows good potential for increasing accuracy of GPP simulations and improving vegetation phenology dynamics response to climate variability over croplands in CLM4.