Generalization method of karst underground river in SWAT：An example of the Daotian river watershed in Bijie, Guizhou

The existence of karst underground rivers seriously affect the general applicability of Soil and Water Assessment Tools (SWAT) in karst areas. To solve this problem, this paper proposes a method based on Digital Elevation Model (DEM) preprocessing combined with SWAT watershed automatic recognition function. It exposes the karst underground river to the surface, and simplifies the complex surface underground binary structure into a surface unitary structure. The application of this method is demonstrated by an example of the Daotian River watershed in Bijie, Guizhou Province. The results show that， (1) compared with the model established without generalization, the area of the basin identified by the model is increased by 30.73%, the number of subbasins increased by 29.27%, and the number of hydrological response units increased by 43.82%, respectively； (2)When the parameters are taken as the maximum range of physical significance, the p-factor of the model established without generalization is 0.64, showing a good SWAT model can not be established without using this method to generalize the underground river； (3) The SWAT model with the generalized underground river is used to perform simulation with a month step length. It yields calibration period R2 = 0.96, NS = 0.96, validation period R2 = 0.94, and NS= 0.93, indicating a good result. The generalization method of karst underground river makes SWAT both in watershed division and simulation more reasonable.