## 5 Conclusions  
In this paper, a laboratory study was conducted on the depth average velocity in a rectangular open channel. Based on the flow division theory and Log-Law, a correlation for calculating the location of depth average velocity on a vertical line from the bottom normal direction wall of different regions in an open rectangular channel is proposed. The main findings of this study are as follows.  
Figure 10: Velocity distribution and the location of depth average velocity in the center line under different experimental conditions ((**a–f**) show results for different aspect ratios).  
1. The division lines cross above and on the water surface (Partition 1), whereas the location of depth average velocity can be calculated using Equation (20), Equation (21), and Equation (26) and the locations of the points which stand for the depth average velocity in different regions can be calculated using Equation (23), Equation (34), Equation (35) and Equation (36). According to the comparison between calculation and measurement, the current approach gives an average value of \(E\), which is from 11.9% to 13.8%, thus proving the correctness of the flow division theory and the universality of the fitting correlation.
2. The division lines cross below the water surface (Partition 2), while the location of depth average velocity can be calculated using Equation (20), Equation (21), Equation (26), and Equation (29). It is found that the obtained results are consistent with the experimental measurements.
3. According to the comparison, it is found that the proposed correlation achieves an accurate matching both in Regions I and II, especially in the center line of the channel. It may be due to the fact that it is far from the side wall and is less affected by the shear stress of the side wall.  
Therefore, the proposed correlation will simplify the procedure of vertical measuring profile measurements by only measuring the characteristic points representing the mean velocity of each region, and then, calculating the flow in different regions using the velocity-area method. It will be useful for flow measurement and the management of irrigation practices.  
Conceptualization, Y.H., J.C., and Z.M.; Methodology, Z.W., J.C., and Z.M.; Software, T.L., J.C., and Z.M.; Validation, Y.H., J.C., and Z.M.; Writing--Original Draft Preparation, Z.M., Z.W., and Y.H.; Project Administration, Y.H.; Funding Acquisition, Y.H., J.C., and L.Z.  
The authors express gratitude for the financial support from the National Key R&D Program of China (Grant Nos. 2017 YFC0403203, 2016 YFC0400207, 2017 YFD0701000 and 2016 YFD200700). National Natural Science Foundation of China (Grant No. 51590248). Jilin Province Key R&D Plan Project (20180201036 SF). Chinese Universities Scientific Fund (Grant Nos. 2019 TJC108, 10710301, 1071-31051012 and 1071-3105136I). Open Fund of Synergistic Innovation Center of Jiangsu Modern Agricultural Equipment and Technology, Jiangsu University (Grant No. 409160002). Open Fund of State Laboratory of Information Engineering in Surveying, Mapping and Remote Sensing, Wuhan University (Grant No. 19R06).  
The authors are very grateful for the support of the fund.  
The authors declare no conflict of interest.