The objective of the study was to investigate the measurement error while determining water discharge in open canals of irrigation system by “slope-area” method. The essence of the given method involves the use of two sensors of water level equipped with means of remote transmission of readings and located in measuring wells of the upper and lower gauging target. According sensors, water levels in targets, level difference between the upper and lower target are determined and water discharge is calculated. By the methods of statistical processing of experimental data, measurement accuracy of hydraulic gradient was determined at the site of lined canal of irrigation net with prismatic channel while using level gauge with an absolute measurement error from ∆h = 0.2 mm to ∆h = 2.0 mm. Using the relationships of G. V. Zheleznyakov and B. B. Danilevich, the values of absolute measurement error of slope and water discharge depending on the absolute measurement error of water level at different spaces between the upper and lower gauging target were calculated. Comparative analysis of measurement errors for the slope of water surface and water discharge was done by two methods which requires leveling zeroes of level gauges and which doesn’t. According to the results of the analysis it was determined that the main source of errors, while calculating hydraulic gradient, is the errors at level difference measuring. For reliable determination of water discharge by the proposed way using “slope-area” method (with measurement error less than 5 %) for measuring the level difference of water free surface with an accuracy 0.2 mm, it should be used the reach of a canal with the distance between slope water gauge station 60–80 m for the method which requires leveling of zeroes of level gauges, and 40–60 m for the method which doesn’t require leveling of zeroes.
Keywords: irrigation system, water accounting, “slope-area” method, level difference, sensor of water level, gauging target, level-measuring well.
