Water-Level Difference Controller for Main Canals

Abstract

The operation of main irrigation canals is complicated in situations in which the operator does not have full control over the canal inflow, or in which 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 downstream, thus creating problems further down the system. In previous work, the writer 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, errors will occur at the upstream-most canal pool. An alternative scheme is offered here in which the canal check gates are controlled on the basis of 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 is 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 inflow and outflow do not match, it effectively treats the canal as a storage reservoir. When in equilibrium, operators will be able to judge the actual flow rate mismatch by the rate of change of water 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 (CAIDD), Eloy, AZ.