Abstract
It remains a challenging issue to accurately estimate the fraction of absorbed photosynthetically-active radiation (FPAR) using remote sensing data, as the direct and diffuse radiation reaching the vegetation canopy have different effects for FPAR. In this research, a FPAR inversion model was developed that may distinguish direct and diffuse radiation (the DnD model) based on the energy budget balance principle. Taking different solar zenith angles and diffuse PAR proportions as inputs, the instantaneous FPAR could be calculated. As the leaf area index (LAI) and surface albedo do not vary in a short periods, the FPAR not only on a clear day, but also on a cloudy day may be calculated. This new method was used to produce the FPAR products in the Heihe River Basin with the Moderate-Resolution Imaging Spectroradiometer (MODIS) LAI and surface albedo products as the input data source. The instantaneous FPAR was validated by using field-measured data (RMSE is 0.03, R2 is 0.85). The daily average FPAR was compared with the MODIS FPAR product. The inversion results and the MODIS FPAR product are highly correlated, but the MODIS FPAR product is slightly high in forest areas, which is in agreement with other studies for MODIS FPAR products.
Highlights
fraction of absorbed photosynthetically-active radiation (FPAR) is the fraction of the incoming absorbed photosynthetically-active radiation (PAR) that a plant canopy absorbs for photosynthesis and growth in the 0.4–0.7 nm spectral range [1]
(1 − α) is the proportion of the radiance absorbed by the land surface, which can be divided into two parts, one is that absorbed by vegetation, namely FPAR, the other is that absorbed by soil, noted as Asoil
FPAR was separated into two parts, direct FPAR and diffuse FPAR, as expressed in Equation (2): FPAR = (1 k ) FPARdir k FPARdiff where FPAR is the total PAR, FPARdir is the fraction of absorbed direct PAR reaching the top of the canopy, FPARdiff is the fraction of absorbed diffuse PAR reaching the top of the canopy and k is the proportion of diffuse PAR
Summary
FPAR is the fraction of the incoming absorbed photosynthetically-active radiation (PAR) that a plant canopy absorbs for photosynthesis and growth in the 0.4–0.7 nm spectral range [1]. FPAR is a very important parameter in calculating gross primary productivity (GPP) using the light-use efficiency model. FPAR can be used as an indicator of the state and evolution of vegetation cover; this function can advantageously replace the normalized Difference Vegetation Index (NDVI). The FPAR inversion methods generally use the relationship between FPAR and NDVI [2] or between. FPAR and the Enhanced Vegetation Index (EVI) [3]. There is a linear relationship between FPAR and NDVI [4,5]. In the Carnegie–Ames–Stanford approach (CASA) model, for example, FPAR is calculated using the NDVI linear stretching model [5,6]. Myneni et al used an empirical relationship based on NDVI as a backup algorithm for MODIS FPAR product derivation (MOD15) [1]
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