Long-Term Effects of Treated Wastewater Irrigation on Soil Properties, Plant Growth, and Drip Irrigation Sustainability

The reuse of treated wastewater for landscape irrigation offers an important strategy for reducing freshwater demand in water-scarce environments, but its long-term effects on drip irrigation systems, soil quality, and plant performance require integrated assessment. This study evaluated a 24-month drip-irrigation experiment using freshwater (FW) and treated wastewater (TW) to irrigate Duranta erecta as a landscape hedge. Hydraulic performance, soil chemical properties, plant growth, and relative leaf chlorophyll were monitored, and an Integrated Drip Irrigation System Sustainability Index (IDISSI) was developed to integrate hydraulic, soil, and plant-performance indicators. Treated wastewater caused greater deterioration in emitter performance, with relative emitter discharge decreasing to 70.286% after 24 months compared with 94.962% under FW, while emitter clogging reached 29.714% under TW compared with 5.038% under FW. Soil salinity also increased under TW, with EC in the 0–30 cm layer increasing from 1.28 to 1.46 dS m−1, whereas it decreased to 1.24 dS m−1 under FW. Despite these changes, plant height reached 100 cm under both irrigation treatments by 9 months, and relative leaf chlorophyll showed no significant difference between water sources (p = 0.7246). After 24 months, IDISSI was 0.802 ± 0.024 under FW and 0.407 ± 0.026 under TW (p < 0.001), with the largest difference arising from the hydraulic component. The findings demonstrate that satisfactory plant performance may coexist with deterioration in irrigation-system and soil indicators, highlighting the importance of integrating hydraulic, soil, and plant responses when assessing the long-term performance of treated-wastewater drip irrigation.