Application of dropping head ponding test on calculation of water use efficiency in concrete lined canal

Abstract: Canal water use efficiency calculation by using ponding test shall be assisted with canal discharge. Ponding test is one of the tests to observe seepage quantity. Furthermore, to calculate canal water use efficiency, it is efficient to test low seepage canals such as concrete lined canal. The observation object in ponding test is a length of canal water statically. The test results could reflect seepage quantity influence factors, such as canal material, section form and depth of canal water. Canal water use efficiency calculation with inflow-outflow test is based on flow difference between upstream section and downstream section, which could reflect seepage process dynamically. According to canal discharge design formulas, four factors were concluded in canal discharge calculation: discharge section area, Chezy coefficient, hydraulic radius (or wetted width and longitudinal slope. Ponding test has two measurements according to the range of water level, one is constant head and the other is dropping head. It was assumed that as canal cross-section is designed, its discharge section area and R(X) could be calculated by depth of canal water, and different depth of canal water will lead to different discharge section area, R(X) and canal water use efficiency undoubtedly. If the canal discharge varies during the period of irrigation, the dropping head ponding test was fit for analyzing the relations between h and canal water use efficiency. The water balance theory was used to describe the process of canal water use efficiency calculation, and the relative parameters regulation was analyzed when depth of canal water was increased. Based on those, we set the canal discharge of unit time as the water volume, which could reflect canal discharge characteristic. It was assumed that W flowed along canal by v, if the length of canal was 1 km, canal water use efficiency calculation was described as six basic steps: 1) Applying ponding test to representative canal, and establishing the relationship between depth of canal water and seepage quantity; 2) Observing canal discharge to establish relationship between depth of canal water and canal discharge; 3) Letting time equal 1 second, and calculating the length of water volume; 4) Calculation of water seepage time; 5) Calculation of water seepage volume; and 6) Calculation of canal water use efficiency. Ponding test and canal discharge observation were both applied on the south 4th branch canal of main canal in Shijin Irrigation District, and dropping head method was adopted in ponding test, depth of canal water ranged from 0.294 to 0.940 m. Establishment process of relationship between depth of canal water and seepage quantity can be described as seven steps: 1) Making a scatter plot with X-axis as time and Y-axis as depth of canal water, and R2 of the regressing the function was 0.99; 2) Discretizing depth of canal water interval 0.294-0.940 into intervals of length 0.001 m, calculate ti for hi. 3) Calculating water volume for each interval; 4) Calculating seepage time corresponding to each water volume; 5) Calculating seepage quantity in each interval; 6) Calculating the percentage of evaporation in total lost water, and correct the seepage quantity in each interval; and 7) Making a scatter plot with X-axis as depth of canal water of each interval and Y-axis as corrected seepage quantity of each interval, R2 of the regression function was 0.99998. The sample size of canal discharge was 92, and corresponding depth of canal water fell in between 0.56 and 0.99 m. Through canal water use efficiency calculation, the results showed that canal water use efficiency was increased with depth of canal water and ranged from 0.988 to 0.991, and seepage quantity met the criterion of “Standard for engineering technique of seepage prevention on canal”. While depth of canal water >0.7 m, the trend of canal water use efficiency was increasing smoothly and lightly. Hence, it was necessary to regard canal water use efficiency calculation by dropping head ponding test as a routine work for those canals which were working on different canal discharge.