Submitted to: International Conference on Water Resources Engineering Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 6/22/1996
Publication Date: N/A
Interpretive Summary: Water is becoming a scarce resource and agricultural water users are under pressure to use water more judiciously. For many large irrigation projects, the operation of the canals that convey and distribute water to the farms influences the farmers' ability to use efficiently the resource. Infrastructural improvements (e.g., conversion to pressurized pipelines) can facilitate the operation of the delivery system but are typically very expensive relative to changes in operations. Operations can be improved by developing analytical tools for determining control actions. One such tool is a mathematical model that computes the water delivery schedule needed at the upstream end of a canal to provide a specified downstream water demand. Calculation of the needed inflow is complex because the system serves various users simultaneously and changes at one location in the canal affect deliveries at other locations. Several schemes have been developed to solve this problem. In this paper, we compare three schemes with examples. Further, we propose an improvement to one scheme which provides reasonable results with less computational difficulties than a theoretically more precise scheme. Beneficiaries of this research are other scientists working in various aspects of canal control, as well as, designers and operators of open-channel water delivery systems.
Technical Abstract: Open-channel water distribution systems are difficult to operate because changes in withdrawal rates at one point in the canal affect water levels and delivery rates at other points in the system. Inadequate canal operation in response to changing demands causes unreliable and unequitable deliveries to users and may endanger the physical integrity of the delivery ystructures. Thus, improving canal operations is an important strategy for improving agricultural water management and efficiency of use. A method for computing canal operations to satisfy predetermined demand schedules by solving the unsteady flow equations backwardly in space, has been available for several years. Practical application of the method, which uses a method-of-characteristics formulation, has been limited partly because of user unfriendliness of the software and computational limitations. Two finite-difference schemes have been recently proposed for solving the unsteady flow problem backward in space. Therefore, a study was conducted to compare these methods with the more theoretically correct method-of- characteristics. The objective of this work was to identify a strategy that can be used under a wider range of canal design conditions and can produce relatively accurate results. One of the newly proposed finite- difference methods ignored the effects of the backward characteristic and was found to be conceptually incorrect. The second scheme, which locally linearizes the governing equations, produced inaccurate results under certain design conditions. When we made this scheme fully nonlinear, results were as accurate as those obtained by the method-of-characteristics and the procedure was found to be more robust.