Assessment of the Water Productivity of Winter Wheat at Multiple Scales in Well Canal Irrigation Areas of the North China Plain

ABSTRACT Water productivity (WP) reflects crop yield per unit of water input, but its assessment varies with scale due to differing water inputs and reuse patterns. This study investigates the scale dependency of the WP in the Shijin Irrigation District, the largest well–canal irrigation area in the North China Plain, by defining five scales: crop, field, subbranch canal, main canal and irrigation district. Using the HYDRUS‐1D and MODFLOW models, which were employed to simulate water movement in both the unsaturated and saturated zones, considering factors such as soil moisture, groundwater levels and water losses, water budgets were calculated at each scale, and three WP indicators—net inflow WP, irrigation–precipitation WP and net irrigation–precipitation WP—were evaluated. The results show that nearly 93% of outflows are reused or retained at larger scales, increasing the net inflow WP by 25%. However, expanded water loss pathways at larger scales increase irrigation demands, causing the irrigation–precipitation WP to decrease from 1.42 kg m −3 at the crop scale to 1.19 kg m −3 at the irrigation district scale. The net irrigation–precipitation WP improves by 29%–54% across scales because of reduced return flows. Despite increased canal losses, net irrigation WP remains unchanged across scales, as these losses are fully deducted as return flows. These findings highlight the significant impact of scale on WP assessment and the importance of considering water reuse and loss pathways in irrigation planning.