Effect of Deficit Irrigation and Mulch Application on Onion (<i>Allium cepa L</i>.) Bulb Yield and Water Productivity Under Drip Irrigation at Ambo, West Shoa, Ethiopia

Drip irrigation, combined with mulch and deficit water application, can significantly improve the water efficiency of irrigated agriculture. Study at Ambo Agricultural Research Center during the 2021/22 and 2022/23 irrigation seasons aimed to determine the most suitable deficit levels and mulch for drip-irrigated onion. The experiment used a randomized complete block design with two factors: irrigation levels (at 55%, 70%, 85%, and 100% of the Evapotranspiration of the crop) and mulch types (un-mulched, plastic, and straw mulch), resulting in a total of twelve treatment combinations replicated three times. The results showed that irrigation levels and mulch significantly affect bulb yield, water productivity, and yield-attributing parameters. Applying 100% ETc level and straw mulch resulted in the highest yields of 39450 kg/ha and 41038 kg/ha, respectively. On the other hand, the lowest yields were obtained with 55% ETc levels and un-mulched treatments, with respective values of 30913 kg/ha and 33969 kg/ha. Water productivity was also higher for irrigation levels at 55% and 70% ETc compared to 85% and 100% ETc, with values of 10.1 kg/m<sup>3</sup> and 9.7 kg/m<sup>3</sup>, respectively. Additionally, straw mulch application resulted in significantly higher water productivity. Economic analysis indicated that straw mulch had a higher net return with 369% MRR (Marginal Rate of Return) and a benefit-cost ratio of 32.8, while applying a 70% ETc level resulted in a 125% MRR. Based on the results, 70% ETc level with straw mulch for onion production is recommended for the study area, considering bulb yield, water productivity, and economic viability.