Runoff simulation by SWAT model based on parameters transfer method in ungauged catchments of middle reaches of Jialing River

The runoff simulation and forecast in ungauged catchment plays an important role. There are lots of catchments belonging to the medium and small size in China. Many of them have no or lack hydrologic station due to the limited environment. It limits the effective management of water resources and the flood forecast without hydrological data. Hence, hydrological research related to solve the data missing is very important, and regionalization is the most useful method which is using the calibration parameters from the gauged catchments in the ungauged catchments of the same region through a certain way. There are several regionalization methods, such as parameters transfer, regression-based parameters and spatial interpolation, of which parameters transfer method is a commonly used approach at present. In this study, 9 catchments in the middle reaches of Jialing River were selected to assess the applicability of parameters transfer method in ungauged catchment based on distributed model SWAT (soil and water assessment tool). How to search the donor catchment and how to transfer the parameters were the most important, so the study explored 3 parameters transfer methods (spatial proximity, physical similarity and comprehensive similarity) and the model performance was determined on 3 test catchments(Zhaojiaci, Changtanqiao and Qingxi). The spatial proximity selected the geographically closest gauged catchments to be the donor catchment; some attributes were selected to calculate the similar degree between test catchment and donor catchment in the physical similarity; the comprehensive similarity regarded the distance as an attribute based on the physical similarity; Zhaojiaci was taken as an example to explore the process of parameters transfer methods. The results showed that: 1) The runoff simulation accuracy was uncertain when randomly selecting donor catchment, and some donor catchments had near distance with test catchment but would have a low NS and R² value; 2) In this study, the number of donor catchments had a great influence on the results of spatial proximity and physical similarity method, and the parameters calculated by multiple donor catchments had a better simulation precision than single donor catchment; 3) The comprehensive similarity result (0.662 value was larger than 0.80 by using parameters transfer method, which satisfied the requirements of runoff simulation precision. Therefore, the SWAT model is suitable for the ungauged catchments in middle reaches of Jialing River based on the parameters transfer method. This study has strengthened the application of distributed model SWAT in ungauged catchment in Jialing River, and further strengthened the application in water resources utilization and agricultural non-point source pollution control. The next step research is to increase the number of donor catchments, and to seek the effective watershed attributes.