To solve the problem of estimating and verifying stream flows without direct observation data; we extend existing techniques for estimating stream flows in ungauged zones, coupling a hydrological model with a hydrodynamic model, using the Poyang Lake basin as a test case. We simulated stream flows in the land covered area of the ungauged zone by building a SWAT model for the entire catchment area covering gauged stations and the land covered area; then estimated stream flows in the water covered area of the ungauged zone using the simplified water balance equation. To verify the results, we built two scenarios (original and adjusted scenarios) using the Delft3D model. In this study, the original scenario did not take stream flows in the ungauged zone into consideration, unlike the adjusted scenario that accounts for the ungauged zones. Experimental results show there was a narrower discrepancy between the stream flows observed at the outlet of the lake and the simulated stream flows in adjusted scenario. Using our technique, we estimated that the ungauged zone of Poyang Lake produces stream flows of approximately 180 billion m<sup>3</sup>; representing about 11.4 % of the total inflow from the entire watershed. We also analysed the impact of the stream flows in ungauged zone on the water balance between inflow and outflow of the lake. These results, incorporating the estimated stream flow in ungauged zone, significantly improved the water balance as indicated by R<sup>2</sup> with higher value and percent bias with lower value, as compared to the results when the stream flows in the ungauged zone were not taken into account, R<sup>2</sup> with lower value and percent bias with higher value. The method can be extended to other lake, river, or ocean basins where observation data is unavailable.