Evaluation of water use efficiency and optimal irrigation quantity of spring maize in Hetao Irrigation District using the Noah-MP Land Surface Model

Hetao Irrigation District (HID), one of the main grain-producing areas in China, is facing the severe challenge of agricultural water shortages. Improving water use efficiency (WUE) is the most appropriate way to achieve sustainable agricultural development and obtain high returns. However, farmers knew little about efficient irrigation strategies for flood-irrigated fields in the HID. In this study, the Noah-MP Land Surface Model (LSM) with dynamic crop and irrigation schemes is used to jointly simulate the maize yield and irrigation amount in the HID from 2003 to 2012. A sensitivity test for ten sets of irrigation setup with different soil moisture (SM) triggering thresholds (CRI=0–1) is conducted, and the corresponding WUE and irrigation water use efficiency (IWUE) are evaluated and analyzed. In addition, irrigation at different crop growth stages affects total irrigation amounts and crop yield. The results show that under rain-fed conditions, the severe SM deficit during the growing season significantly decreases the maize yield in the HID, with almost no harvest in dry years. With traditional flood irrigation, the irrigation amount per unit area exceeds 1000 mm, leading to low WUE and IWUE, while more than 18–25% of irrigation water is wasted as runoff. With the increase of irrigation intensity, both WUE and IWUE show a significant diminishing marginal effect, usually occurring when SM is greater than 0.24–0.27 m³/m³. The optimal WUE is obtained near CRI= 0.6, while the worst WUE is obtained at CRI= 1. In a normal year, efficient irrigation amount is about 390–480 mm, 53–63% less than that in flood irrigation. In this scenario, most irrigation water becomes SM and transpiration, and less than 5% becomes runoff. With effective irrigation in the reproductive stage, only a small amount of irrigation is needed to promote yield formation. Fully considering the maize yield, WUE, ecosystems, and the available water resources in the HID, the recommended irrigation amount is about 600–650 mm to achieve the optimal maize yield and WUE.