Water scarcity severely limits horticultural productivity in semi-arid regions of Ethiopia, making improved irrigation efficiency critical for sustaining crop yields. This study evaluated the effects of bed and furrow width configurations on onion ( Allium cepa L. ) yield and water productivity (WP) under surface irrigation at the Woleh irrigation scheme during two consecutive growing seasons (2022–2024). A randomized complete block design with ten selected furrow–bed treatment combinations and three replications was employed, chosen from a total of 18 possible configurations derived from the factorial combination of three furrow widths (20, 30, and 40 cm) and six bed widths (20, 30, 45, 60, 75, and 90 cm). The selection of the ten treatments was based on agronomic feasibility, farmer practice, and field layout constraints. Soil and crop parameters were measured to quantify bulb yield and WP, while crop water requirements were estimated using the FAO CROPWAT model to support irrigation scheduling. Partial budget analysis assessed economic performance. Treatment effects were highly significant ( p < 0.001) for plant height, bulb diameter, bulb yield, and WP, while year and treatment × year interaction effects were not significant, indicating stable performance across seasons. The 30 cm furrow × 45 cm bed configuration ( T ₅ ) produced the highest bulb yield (24.51 t ha ⁻¹ ) and WP (4.10 kg m ⁻³ ), corresponding to approximately 39.42% and 39.00% increases in bulb yield and WP, respectively, compared with the conventional 20 × 20 cm used as the control. This configuration was associated with improved yield and WP; however, the underlying mechanisms were not directly measured in this study and are therefore inferred from evidence reported in previous studies. Economic analysis confirmed its superiority, yielding the highest net benefit (USD 5606.61 ha ⁻¹ ) and marginal rate of return (483.74%). The findings indicate that the 30 cm furrow × 45 cm bed configuration can improve onion productivity, WP, and economic returns under semi-arid conditions. Adoption of the 30 cm furrow × 45 cm bed design provides a practical, low-cost approach for smallholder farmers to improve WP and farm profitability under water-limited conditions.