A coupled hourly water-carbon flux model at plot and field scales for water-saving irrigated rice paddy

Based on spot-scale measurements of lysimeter-based water fluxes and chamber-based carbon fluxes (ETWML or FSTC), and field- scale measurements of eddy covariance-based water and carbon fluxes (ETEC or FEC), a coupled water-carbon flux model was developed to predicted simultaneously hourly water and carbon fluxes at either spot or field scale in the water-saving irrigated paddy in Tai Lake Region of China. The coupled model integrated Penman-Monteith and Farquhar-von Caemmerer-Berry (FvCB) sub-models with the Jarvis sub-model, and was calibrated based on measured fluxes in 2015 and validated using data in 2016–2017. The performance of the coupled model was assessed by comparing simulated values with measured hourly ETWML, FSTC, ETEC and FEC under various conditions, including day/night/all day periods, different canopy coverage levels, and seasonal variations during rice seasons. The results revealed notable differences in the parameters of the Jarvis sub-model between plot and field scales. The ratio of leaf dark respiration to leaf maximum carboxylation rate were respectively calibrated as 0.041 and 0.047 to reflect the respiration of soil-vegetation system at plot and field scales. The maximum carboxylation rate (Vm) reached its peak during the jointing-booting stage and then decreased until the end of the rice season, and the Vm values at the plot scale were considerably higher than those at the field scale. The coupled model respectively exhibited poor and acceptable performance in estimating water and carbon fluxes during nighttime, and performed well during daytime or throughout the day. Additionally, the coupled model performed much better in estimating diurnal and seasonal variations in fluxes under high leaf area index conditions compared to low leaf area index conditions.