This paper proposes a new strategy for control of open irrigation canals based on fractional order controllers. A methodology is developed to design PI fractional controllers combined with Smith predictors which are robust to changes in the time delay. This method is applied to solve the effective control problem of an open irrigation canal. Simulated results of a standard PI controller, PI plus Smith predictor controller, and the controller developed in this paper are compared when applied to the dynamical model of a real irrigation canal pool.