Enhancing Terrestrial Net Primary Productivity Estimation with EXP-CASA: A Novel Light Use Efficiency Model Approach

TL;DR

This paper introduces the EXP-CASA model, a novel approach that enhances the accuracy of terrestrial Net Primary Productivity estimation by addressing limitations of the classic CASA model through innovative functions and data integration.

Contribution

The study develops the EXP-CASA model, improving NPP estimation accuracy and model stability, and demonstrates its superior performance over existing models using long-term observational data.

Findings

EXP-CASA outperforms other NPP products with R^2=0.68

The model shows strong adaptability to various vegetation indices

Parameters remain stable over time series

Abstract

The Light Use Efficiency model, epitomized by the CASA model, is extensively applied in the quantitative estimation of vegetation Net Primary Productivity. However, the classic CASA model is marked by significant complexity: the estimation of environmental stress parameters, in particular, necessitates multi-source observation data, adding to the complexity and uncertainty of the model's operation. Additionally, the saturation effect of the Normalized Difference Vegetation Index (NDVI), a key variable in the CASA model, weakened the accuracy of CASA's NPP predictions in densely vegetated areas. To address these limitations, this study introduces the Exponential-CASA (EXP-CASA) model. The EXP-CASA model effectively improves the CASA model by using novel functions for estimating the fraction of absorbed photosynthetically active radiation (FPAR) and environmental stress, by utilizing long-term observational data from FLUXNET and MODIS surface reflectance data. In a comparative analysis of NPP estimation accuracy among four different NPP products, EXP-CASA ($R^2 = 0.68, RMSE= 1.1gC\cdot m^{-2} \cdot d^{-1}$) outperforms others, followed by GLASS-NPP, and lastly MODIS-NPP and classic CASA. Additionally, this research assesses the EXP-CASA model's adaptability to various vegetation indices, evaluates the sensitivity and stability of its parameters over time, and compares its accuracy against other leading NPP estimation products. The findings reveal that the EXP-CASA model exhibits strong adaptability to diverse vegetation indices and stability of model parameters over time series. By introducing a novel estimation approach that optimizes model construction, the EXP-CASA model remarkably improves the accuracy of NPP estimations and paves the way for global-scale, consistent, and continuous assessment of vegetation NPP.