The simulation of water vapor and carbon dioxide fluxes over a rice paddy field by modified soil-plant-atmosphere model (mSPA)

The exchanges of CO₂ and water vapor over a rice paddy field have been examined with a two-layer canopy model (mSPA model). Four cases were selected for detailed examination during growing season (June to September in 2003). Case 1 is characterized by low LAI, small root biomass and high vapor pressure deficit while Case 2 is characterized by near maximum. LAI, large root biomass and moderate vapor pressure deficit. Cases 3 and 4 are characterized by senescence period. The simulated diurnal variations of turbulent fluxes agree well with observations in Cases 1 and 2. However, the model fails to simulate both latent heat and CO₂ fluxes during late growing season (Cases 3 and 4). These errors were investigated in terms of the assimilation and stomatal conductance (A-g_s) relationship and the result indicated the need to consider seasonality of A-g_s relationship and biophysical parameter for accurate simulation of water vapor and CO₂ fluxes during late growing season. Sensitivity test was performed to investigate the influence of uncertainties (±30%) of prescribed model parameters on simulated cumulative GPP and evaporation during growing season. The cumulative evaporation showed little sensitivity to both LAI and V_cmax due to compensating effect of soil evaporation while the cumulative GPP showed a larger sensitivity to LAI (9-16%) than V_cmax (6-9%). For hydraulic parameters such as stem hydraulic conductivity and root resistivity, model showed little sensitivity. It is due to low water stress during the growing season in Korea.