Modified soil hydrological schemes for process-based ecosystem model Biome-BGC

Modified soil hydrological schemes were implemented in process-based ecosystem model Biome-BGC to improve water cycle simulation. The new schemes employed a 2-layer soil model and introduced new outflow components. Surface flow is input water exceeding soil infiltration capacity, and a scheme to predict it in the shorter duration than 1 day was proposed. Drainage from bottom soil layer and soil water redistribution between the layers driven by matric potential gradient and gravity were also modeled. The modified model was tested using the meteorological data and site parameters distributed with the original version. In comparison with the original, the modified model calculated 14% larger mean annual outflow. It also calculated a rapid outflow under dry antecedent soil condition, and reduced outflow peaks with slow drainage recession after soil wetted up, which were not simulated by the original model. The ecosystem carbon cycle responded to the change in soil water budget and net primary production (NPP) decreased by the modified model. Newly introduced parameters were determined automatically, so that users can use the same input data and parameter set for the original version. These simple schemes can be applied in other process-based ecosystem models.