Changes in Paddy Rice Net Primary Production (NPP) in Thailand (2007-2018): Nakhon Sawan Case Study

Net primary production (NPP) is the amount of carbon and energy that enters ecosystems via plants; that is, it determines the amount of energy available for transfer through food web dynamics. Moreover, NPP estimates plant growth and provides quantitative information for sustainable resource management. The study aims to assess the variation of NPP in paddy rice cultivation in the Nakhon Sawan province during 2007, 2012, and 2018. The Carnegie-Ames-Stanford Approach (CASA) model, widely conducted for terrestrial ecosystem NPP estimations, was used in this study. The CASA model is a light-use efficiency process model that fully considers environmental conditions and vegetation characteristics. The satellite image has been based on the 250-m MODIS Vegetation Indices coded as TERRA product MOD13Q1 and 500-m MODIS Net Evapotranspiration coded as EVAPOTRANSPIRATION product MOD16A2. NPP was calculated in terms of monthly values aggregated to a year as gC m^-2 y^-1. The GIS maps presented the spatial distribution of paddy rice areas and NPP values. The results showed that the annual paddy rice NPP increased gradually between 2007, 2012, and 2018, reaching 832, 809, and 825 gC m^-2 y^-1, respectively. The variation of NPP is influenced by climatic factors. Solar radiation has the strongest correlation with NPP (r = 0.90) at p-value < 0.05, followed by temperature () (r = 0.66), water () stress factor (r = 0.40), and the fraction of wavelengths absorbed by vegetation (FPAR) (r = 0.35).