Impact of Supplemental Irrigation on Yield, Water Productivity, and Economic Profitability of Broad Bean (Vicia faba L., var. minor)

This paper investigates the effects of irrigation and plant density on the yield and water productivity of broad bean in central Italy over two growing seasons (2020–2021 and 2021–2022). The study emphasizes the role of legumes in promoting sustainable and competitive farming systems, with broad bean (Vicia faba L., var. minor) being a promising crop due to its low production costs and resource requirements, particularly water. Optimizing water use and increasing yield are crucial to enhancing the economic viability of broad bean cultivation. We present crop water assessments and evaluate two supplemental irrigation (SI) strategies, a machine learning -based decision support system (ML-DSS) and the crop evapotranspiration estimation approach (ETC), under three plant densities of 40, 50 and 60 plants m−1. A simplified economic analysis based on water efficiency indicators was implemented to assess the economic profitability of SI on broad bean. Irrigation significantly increased growth and yield by 40 % on average, compared to rainfed conditions (4.54 Mg ha−1 vs. 2.71 Mg ha−1). No significant differences were found in yield (4.42 Mg ha−1 vs 4.66 Mg ha−1) and water productivity between the ETC and the ML-DSS system (7.57 vs 8.00 kg ha−1 mm−1 on average), although both irrigated treatments significantly outperformed rainfed conditions (5.27 kg ha−1 mm−1 on average). Under irrigation, higher plant density improved total yield and total yield water productivity (TYWP), particularly in the second year, marked by severe drought (from 3.21 to 4.70 Mg ha−1, increasing from 40 to 60 plants m−2). Results showed that a plant density of 60 plants m−2 maximized outputs under irrigated conditions, while a plant density of 50 plants m−2 was optimal for rainfed conditions. Supplemental irrigation led to higher and more consistent yields over time. The analysis of economic profitability showed a variation between the two experimental years, with an average gross margin (GM) increase of 42 % and 27 % for irrigated treatments compared to rainfed ones. However, the high incidence of irrigation water cost (IWC) in the second year significantly reduced economic water productivity, making the irrigated treatments with low plant density economically unsustainable.