It is well known that, over the past decades water problems has risen to the top of the global development agenda, so reviewing its uses to be under control and reduce its losses is a must. Monitoring of water levels/discharges is considered one of the most important challenges that facing the design/implementation of a new irrigation schemes. A new methodology technique for a downstream control algorithm for on-demand operation of irrigation systems is proposed. The concept within this method is achieved by maintaining the downstream end water level of the channel at a target set point by manipulating the upstream end structure in real-time. The effectiveness of the method is tested using a simulation model that is based on the de St.-Venant equations and the implicit Preissmann scheme. The control model is presented using the MATLAB- Simulink programming to facilitate the use of the designed Proportional Integral Differential (PID) controller for feedback and real-time adjustment of the upstream end structure. The results of the simulations show that the proposed control algorithm is able to provide an effective control of the flow and cope with rapid variations of water demands and unknown disturbances and guide us to the best water-supply hydrograph from the main water source.