Enhancing economic irrigation water productivity through sustained deficit irrigation in mandarin under semi-arid conditions

Under increasing water scarcity and rising production costs, the competitiveness of Mediterranean agriculture depends on the efficient and economically sustainable use of irrigation water. Beyond maximizing yield, irrigation strategies would be evaluated in terms of their capacity to generate economic returns. With the aim of relating economic water productivity to irrigation strategies in the production of adult mandarin trees in SE-Spain, three randomly distributed irrigation treatments were applied over three seasons: (i) control (CTL), irrigated at approximately 100% of crop evapotranspiration (ET_c) throughout the entire crop cycle; (ii) Sustained Deficit Irrigation (SDI), irrigated at 70% of ET_c during the entire season; and (iii) Regulated Deficit Irrigation (RDI), irrigated as SDI except during the initial phase of fruit growth stage II, when irrigation was reduced to 35% of ET_c until fruits reached about 70% of their final size. The economic irrigation water productivity (EWP_i) was calculated as the ratio between economic profit and irrigation water use. For this purpose, we considered the parameters reported by Martin-Gorriz et al. (2022). Accordingly, water and energy costs were set at 0.24 € m⁻³, the mandarin price at 0.33 € kg⁻¹, and a fixed subsidy of 325 € ha⁻¹ from the European Union’s Common Agricultural Policy was included. The profit was calculated as the difference between total revenue and the sum of variable, fixed, and opportunity costs. Total variable costs (TVC) included expenses related to machinery, raw materials, irrigation, and labour, while fixed costs (TFC) comprised depreciation, start-up costs, insurance, and taxes; and total opportunity costs (TOC) accounted for land rental value and the interest on both fixed and variable capital. Despite an average seasonal reduction of 1095 m³ ha⁻¹ in irrigation water under SDI compared with the CTL treatment (5387 m³ ha⁻¹), no differences were detected in yield. In contrast, the RDI treatment, which reduced applied water by an average of 2539 m³ ha⁻¹, resulted in a 29.3% yield reduction relative to CTL. The TFC and TOC remained relatively constant across treatments and seasons, at 1185 and 269 € ha⁻¹, respectively. In contrast, TVC showed a wider variation, ranging from 2875 to 3812 € ha⁻¹, and were mainly driven by water and energy for irrigation. On average, TVC accounted for approximately 70% of the total production costs. Over the study period, the irrigation scheduling of SDI increased the EWP_i by 94.3% compared with the CTL (0.68 vs 0.35 € m⁻³), whereas the RDI strategy reduced EWP_i a 22.9% due to the water stress intensity reached and its negative effect on crop yield. The relationship between yield and EWP_i was linear (y₀=−0.97^*** and a=7.49×10⁻⁵^***; R²=0.87) and according to this relationship, a minimum yield of 12.9 t ha⁻¹ is required for irrigation water to generate positive economic value. Finally, SDI proved to be an effective and economically robust irrigation strategy, translating water savings into higher economic returns per unit of water and enhancing the competitiveness of semi-arid Mediterranean citrus production systems.

Funding: Biodiversity-Foundation: “Innovative agricultural practices to contribute to environmental improvement and biodiversity in the Mar Menor area (NEWAGROMARMENOR)”.