A gravitation vortex type water turbine is a water turbine that uses gravitational vortices generated upon water draining from a tank bottom and can generate power at low head and low flow rate. In our case, given its operation in a free surface flow field, it is critical to quantitatively understand the performance and hydraulic loss at various flow rates for its design and operation. Accordingly, we investigated the influence of flow rates on the performance of the gravitation vortex type water turbine by conducting experiments and free surface flow analysis. Using the analysis results, we proposed a loss analysis method and quantitatively evaluated the hydraulic loss. We found that the effective head and the turbine efficiency increased as the flow rate increased; hence, the turbine output increased at a rate greater than the increase rate of the flow rate. Our study revealed that among the losses that occurred in the water turbine, the tank loss and tank outlet loss were the most dominant, followed by the friction loss inside the tank, whereas the runner loss and friction loss in the runner were small. (C) 2020 Elsevier Ltd. All rights reserved.