Integrated Agronomic, Hydraulic, and Economic Assessment of Subsurface Drip Irrigation Under Deficit Irrigation for Greenhouse Cucumber in Arid Saudi Arabia

Improving irrigation efficiency is essential for sustaining greenhouse cucumber production in Saudi Arabia, where agriculture relies heavily on limited non-renewable groundwater resources. This study evaluated the combined effects of subsurface drip irrigation (SSDI) and regulated deficit irrigation on hydraulic performance, soil-water distribution, cucumber productivity, water-use efficiency, physiological response and economic return. A split-plot experiment with three replicates was conducted during a single growing season in greenhouses without supplemental lighting, using sandy soil containing 88.4–95.0% sand. Three irrigation levels (100%, 80%, and 60% of crop evapotranspiration, ETc) and three dripline types (XFD, UNIRAM, and REIN) were evaluated. Irrigation water had an ECw of 0.91 dS m−1 and a SAR of 4.5. Seasonal irrigation depths were 363, 294, and 226 mm, saving 18.8% and 37.7% water. Marketable yield declined from 99.5 t ha−1 at 100% ETc to 80.6 and 65.1 t ha−1 at 80% and 60% ETc, respectively, whereas irrigation water-use efficiency reached 72.0 kg m−3 at 60% ETc. XFD showed the highest hydraulic uniformity and generally maintained better crop performance. Leaf proline responses were dripline-dependent, with the highest concentration under XFD at 60% ETc. Under the experimental conditions, 80% ETc offered a practical compromise between yield maintenance and water saving. Under a fixed-water-budget scenario, reallocating water saved at 60% ETc increased the potential cultivated area and improved overall economic return, although this benefit was conditional on the availability of additional production capacity.