Title: Distributing flow mismatches in supply-constrained irrigation canals through feedback control Authors
Submitted to: USCID International Conference
Publication Type: Proceedings
Publication Acceptance Date: May 4, 2009
Publication Date: March 26, 2010
Citation: Clemmens, A.J., Strand, R.J. Distributing flow mismatches in supply-constrained irrigation canals through feedback control. In: Proceedings of USCID International Conference, March 23-26, 2010, Sacramento, California. p. 41-52. Interpretive Summary: Canals that deliver water for irrigation of crops operate very differently from pressure pipelines, commonly used for domestic water supplies. They operate more like rivers, where it takes time for a change in flow upstream (say to deliver water to a farmer) to arrive downstream (at the farm). In addition, a sudden change upstream arrives gradually downstream. It takes considerable experience for an operator to provide accurate and constant flows to farmers. Inaccurate or changing flows can cause field irrigation system to perform poorly. Sometimes, the water supply is not immediately available to meet water demands. Operators must order the water and then deal with the mismatch between the supply available and the farmer demand. In this paper, a new control system is proposed for large canals which attempts to maintain constant flows to farmers by using the main canal to store water, whether due to shortage or excess. The control method adjusts canal gates based on the difference in water level deviations in two adjacent pools. This method is implemented in computer software: Software for Automated Canal Management (SacMan). Examples are shown where the procedure is used to automatically adjust canal gates on the Central Main Canal at the Central Arizona Irrigation and Drainage District. These results will be of use to irrigation and large water districts, the Bureau of Reclamation, and consultants.
Technical Abstract: The operation of main irrigation canals is complicated in situations where the operator does not have full control over the canal inflow, or where there are very long transmission distances from the point of supply, or both. Experienced operators are able to control the canal, but often supply errors are simply passed to downstream, thus creating problems further down the system. In previous work, the senior author showed that it is important to contain such errors and not let them pass downstream. With automatic upstream level control, all flow errors are passed to the downstream end of the canal. Distant downstream water level control requires full control of canal inflow. Without this, most errors will occur toward the upstream end of the canal. An alternative scheme is offered here where the canal check gates are controlled based on the relative water level error between adjacent pools. The scheme uses a simple linear model for canal pool response. The scheme is implemented as a multiple-input, multiple-output scheme and solved as a Linear Quadratic Regulator (LQR). Thus all gates respond to relative deviations from water-level set point. The scheme works to keep the relative deviations in all pools the same. If the canal has more inflow than outflow, the scheme will adjust gates so the water levels in all pools will rise together with the same deviation from set point. It thus distributes the error over the entire canal. When in equilibrium, operators will be able to judge the actual flow rate mismatch by the rate of change of these levels. The scheme acts like a combination of upstream level and distant downstream level control. It was tested on a simulation model of the Central Main Canal at the Central Arizona Irrigation and Drainage District, Eloy, AZ.