Evaluating the Effect of Transpiration in Hydrologic Model Simulation through Parameter Calibration

Abstract

The effect of evapotranspiration (ET) on water redistribution in hydrological models has already been explored in many studies, but whether correcting plant transpiration (EP) in a hydrologic model can play a comparable role remains unclear. This study aims to evaluate the effect of EP from the perspective of model calibration. Reference EP data were derived from the Biome Biogeochemical Cycles (Biome-BGC) model, with parameters based on long-term field investigation of the Qinghai-Tibetan Plateau. Soil and Water Assessment Tool (SWAT) model calibration and EP-sensitive parameter analysis were conducted in the plateau’s Huangshui basin. Four modeling experiments were explored: (1) SWAT base model; (2) calibration against EP for each subbasin; (3) calibration against runoff at the gauge station; and (4) calibration against both EP and runoff. The effects of EP and runoff were analyzed. The results indicate that calibration against either EP or the runoff process alone is not adequate to produce spatially and temporally accurate water partitioning. Specifically, calibration against EP improves the runoff simulation during wet and warm seasons and the EP spatial pattern across the basin. Parameters involved in vertical water redistribution are sufficient but not necessary conditions to affect the EP process in the SWAT model. Calibration against runoff gives priority to regulating the EP in the watershed as a whole, retaining a negative spatial correlation with the reference EP. To produce a better simulation of EP and runoff, both factors are recommended in the calibration based on the SWAT model parameterization.