Coupling of irrigation depth and nitrogen rates on onion (Allium cepa L.) yield and irrigation water productivity in Northern Ethiopia

Onion (Allium cepa L.) is one of the major vegetable crops grown in Ethiopia. Although its productivity depends on optimization of irrigation water depth and fertilizer application rates, there is limited scientific evidence on the optimal nitrogen rate and irrigation depth in our study area. Therefore, this study investigated the interactive effects of different irrigation depths and nitrogen fertilizer rates on onion yield and resource use efficiency. A field experiment was conducted over two dry seasons (2019–2020) using nine treatment combinations, consisting of three irrigation depths and three nitrogen rates, arranged in a 3 × 3 factorial randomized complete block design with three replications. Prior to analysis, data were tested for normality and homogeneity. Subsequently, data were analyzed using a general linear model, and means were compared using LSD at P < 0.05. Results demonstrated that irrigation depth was the dominant factor (P < 0.001), followed by nitrogen level (P < 0.05). Although marketable yield increased consistently with the combined increase of both inputs, rising from 15.94 ± 0.72 ton ha⁻¹ in T₁ to 31.30 ± 1.20 ton ha⁻¹ in T₉, this trend alone does not determine optimal management. Based on multi-criteria analysis, treatment T₆ (100% of crop water requirement combined with 125% nitrogen) was identified as optimal, recording the highest irrigation water productivity (4.85 kg m⁻³) and economic irrigation water productivity (121.27 ETB m⁻³), along with improved cation exchange capacity (11.433 ± 0.968 meq/100 g soil), reflecting enhanced nutrient retention. Accordingly, T₆ is recommended as the most suitable strategy for improving sustainable onion productivity in the study area and in regions with similar agro-ecological conditions.