Soil Water‐Salt Dynamics and Crop Yield Response to Combined Irrigation and Drainage in Maize‐Tomato Rotation Farmland

ABSTRACT The integration of subsurface drainage with drip irrigation under plastic mulch has become a key strategy for mitigating soil salinity in arid regions. However, the mechanisms driving their combined effects on soil moisture, salt redistribution, and crop yield remain poorly understood. This study investigates a rotational cropping system of maize and tomato over 2 years (2019–2020), employing four irrigation treatments: full irrigation (W1) and deficit treatments at 90% (W2), 80% (W3), and 70% (W4) of total capacity. Three drainage depths: 0.8 m (D 0.8 ), 1.1 m (D 1.1 ), and 1.4 m (D 1.4 ), with a control (CK) for comparison were evaluated. Results reveal that increased irrigation enhances soil moisture content in the 0–100 cm profile, while deeper drainage consistently reduces moisture levels. Notably, fields with subsurface drainage exhibited significantly lower soil moisture than CK, with greater drainage depths leading to further reductions. Desalination was observed in the 0–60 cm layer at irrigation levels above 500 mm, while lower irrigation primarily affected the upper 0–30 cm and 0–40 cm depths. Subsurface drainage markedly improved the desalination rate in the 0–40 cm layer compared to CK ( p < 0.05). Both maize and tomato yields increased with higher irrigation and drainage depths, and subsurface drainage significantly enhanced irrigation water productivity (IWP). The first combination of multi‐variable assessment of IWP was made. Relative to CK and D 0.8 , IWP increased by 5.4% to 28.9% under the D 1.4 treatment, which could effectively alleviate soil salinization and increase crop yields. Thus, utilizing IWP as a key evaluative metric, this study concludes that a drainage depth of 1.4 m is optimal for the studied region.