A novel irrigation canal scheduling approach without relying on a prespecified canal water demand process

Abstract

The creation of an efficient and environmentally friendly irrigation canal schedule strongly depends on the accuracy of the prespecified water demand (PSWD) process in existing scheduling practices and modeling studies. Overestimating water demand would result in spillage losses from over-irrigation, while underestimating water demand would lead to crop yield losses due to deficit water supply. However, prespecifying an accurate water demand often poses various challenges for both farmers and canal administrators, as it involves the temporal variability in agrometeorological conditions, the spatial diversity of soil properties, and the sensitivity of crops to water stress at different growth stages. Therefore, to reduce the uncertainty of prespecifying water demand and avoid the adverse effects of an inaccurate water demand on canal water allocation, this paper proposes an alternative canal scheduling model that maximizes canal scheduling service quality by targeting the ultimate objectives of the irrigation system (UOIS) without considering the prespecified water demand as the model input. The UOIS canal scheduling (UOIS-CS) model aims to optimize the canal water delivery schedule via simulation of soil water and salt migration using the Simultaneous Heat and Water (SHAW) model, to provide the most suitable soil moisture content and soil salinity for crops. The UOIS-CS model is used for the Hetao Irrigation District, Inner Mongolia, a typical agricultural watershed in China, where supplementary irrigation is performed after the autumn harvest (i.e., autumn irrigation) to leach excessive soil salt and preserve a moderate amount of soil moisture for the beginning of the next spring sowing period. To evaluate the irrigation performance of the UOIS-CS model further, a comparative analysis between the UOIS-CS model and the widely used PSWD-based canal scheduling (PSWD-CS) model is conducted. Compared with the PSWD-CS model, the proposed UOIS-CS model exhibits a superior irrigation performance in terms of leaching soil salt and preserving soil water, and a lower risk of canal overload operation during a higher water demand period. The improved performance of the UOIS-CS model is attributed to its greater adaptability to the spatial diversity of soil properties and the temporal variability in agrometeorological conditions, which originates from the full use of the equifinality of model performance metrics and model inputs. The UOIS-CS model provides managers with an effective approach to formulate a canal water delivery schedule with improved irrigation efficiency and lower overload risk. The proposed framework is generic and can be applied to non-crop growing seasons.