A new method is proposed to design a class of robust fractional order PI controller (FPI) based on frequency specifications for water distribution in a main irrigation canal pool. The robustness features of the obtained FPI controller are compared with the ones of an equivalent standard PI controller with the same design frequency specifications. A justification of its enhanced properties it is also provided. The interest of such fractional order controllers is justified by the fact that dynamical parameters of main irrigation canal pools may change drastically in function of its operation regimes. The designed FPI controller was implemented in a PLC of the Siemens company (Simatic 300) and was installed in a real main irrigation canal pool. The real time experimental results carried out comparing both FPI and standard PI controllers for different discharge regimes showed the superiority of the obtained FPI controller over the standard PI controller in terms of time domain performance and robustness. These results proved that the proposed design method leads to an efficient realistic FPI controller for main irrigation canal pools. Copyright © 2007 International Federation of Automatic Control All Rights Reserved.