Stream flow simulation and verification in ungauged zones by coupling hydrological and hydrodynamic models: a case study of the Poyang Lake ungauged zone

To solve the problem of estimating and verifying stream flow without direct observation data, we estimated stream flow in ungauged zones by coupling a hydrological model with a hydrodynamic model, using the Poyang Lake basin as a test case. To simulate the stream flow of the ungauged zone, we built a soil and water assessment tool (SWAT) model for the entire catchment area covering the upstream gauged area and ungauged zone, and then calibrated the SWAT model using the data in the gauged area. To verify the results, we built two hydrodynamic scenarios (the original and adjusted scenarios) for Poyang Lake using the Delft3D model. In the original scenario, the upstream boundary condition is the observed stream flow from the upstream gauged area, while, in the adjusted scenario, it is the sum of the observed stream flow from the gauged area and the simulated stream flow from the ungauged zone. The experimental results showed that there is a stronger correlation and lower bias (R-2 = 0.81, PBIAS = 10.00 %) between the observed and simulated stream flow in the adjusted scenario compared to that (R-2 = 0.77, PBIAS = 20.10 %) in the original scenario, suggesting the simulated stream flow of the ungauged zone is reasonable. Using this method, we estimated the stream flow of the Poyang Lake ungauged zone as 16.4 +/- 6.2 billion m(3) a(-1), representing similar to 11.24% of the annual total water yield of the entire watershed. Of the annual water yield, 70% (11.48 billion m(3) a(-1)) is concentrated in the wet season, while 30% (4.92 billion m(3) a(-1)) comes from the dry season. The ungauged stream flow significantly improves the water balance with the closing error decreased by 13.48 billion m(3) a(-1) (10.10% of the total annual water resource) from 30.20 +/- 9.1 billion m(3) a(-1) (20.10% of the total annual water resource) to 16.72 +/- 8.53 billion m(3) a(-1) (10.00% of the total annual water resource). The method can be extended to other lake, river, or ocean basins where observation data is unavailable.