A parallel algorithm for 1D free-surface flow simulations in irrigation canals is shown. The model is based on the Hartree method applied to Saint-Venant equations. Due to the close-to-steady flow nature in irrigation canals, external and internal boundary conditions are linearized to preserve the parallel character. Gate trajectories, off-take withdrawals, and external boundary conditions are modeled as piece-wise functions of time, so there are discontinuities. To achieve a fully parallelized algorithm, an explicit version of the Hartree method is chosen, and external and internal boundary conditions are linearized around operation point. This approach is used to build a computer simulator, written in C-CUDA language. Two tests by ASCE Committee on Canal Automation Algorithms have been used to evaluate accuracy and performance of the algorithm. The Maricopa Stanfield benchmark has been used to prove its accuracy, and the Corning Canal benchmark to evaluate performance in terms of processing time. Surprisingly, solving a 12 hr-long prediction horizon with a cell size of about Δx= 10 m is less than 1 s on a Nvidia K40 card. Results were compared with a serial and a multi-CPU version of the same algorithm. The implementation that showed the best performance on different platforms is the one that uses GPU.