Optimizing irrigation scheduling in apple orchards: soil moisture thresholds balance yield, fruit quality, and water use efficiency.

Water scarcity necessitates efficient irrigation strategies in high-value crops like apple. While regulated deficit irrigation (RDI) is known to influence apple yield and quality, the optimal soil moisture threshold for automated systems in region-specific contexts remains poorly defined. This two-year study (2024-2025) evaluated the effects of four precise, soil moisture-based irrigation thresholds on the yield, fruit quality, and water productivity (WP) of 'Super Chief' apples in the northwestern Himalayas. An automated irrigation system using a Human-Machine Interface (HMI) and calibrated soil moisture sensors maintained Volumetric Water Content (VWC) at four thresholds: 100% (I100, 45% VWC), 80% (I80, 36% VWC), 60% (I60, 27% VWC), and 40% (I40, 18% VWC) of field capacity (θfc). Full irrigation (I100) maximized yield (70.80 t ha⁻¹) and fruit size (165.60 g). Severe deficit (I40) significantly enhanced fruit firmness (9.56 N) and soluble solids content (13.95 °Brix) but at the cost of a 20% yield reduction. Crucially, the I80 treatment achieved a near-optimal yield (66.24 t ha⁻¹) while using 20% less irrigation water than I100, resulting in a 31% higher water productivity (WP). Multi-criteria TOPSIS analysis objectively ranked I80 as the best compromise strategy. For 'Super Chief' apples in the Himalayan region, maintaining soil moisture at 80% θfc via sensor-based automation is the superior strategy, effectively balancing marketable yield, enhanced fruit quality, and water conservation.