A sustainable agricultural system requires increasing water use efficiency and enhancing knowledge of crop water use. This prerequisite is more pronounced in the regions with inadequate water resources and limited observational data such as southern Iran. Therefore, this study aimed at finding the water requirement of mature orange trees (Citrus sinensis (L.) Osbeck, cv. Tarocco Ippolito) by identifying standard evapotranspiration rate and crop coefficients (single and dual). Seventy-two orange trees in a drip-irrigated orchard with loam soil were classified into six treatments and irrigated at 100%, 90%, 80%, 70%, 60%, and 50% of reference evapotranspiration rate during 2017 and 2018. Soil moisture variability and crop physiological responses, including stem water potential (Ψ_stem), net photosynthesis (A_n), and stomatal conductance (g_s) were measured. Our results showed that irrigating at 90% ETo provided the full water requirements of the trees. The mean crop evapotranspiration rate was calculated as 5.2 mm day^−1, with the crop coefficient ranging from 0.65 to 0.95. The average irrigation rate performed traditionally in the region was 19% higher than the actual requirement. Analysis of physiological response highlighted the controlling role of stomata in regulating transpiration and maintaining leaf turgor. During the peak water-stress, g_s ranged from 0.11 to 0.12 molm⁻² s⁻¹ in fully irrigated trees, to 0.04–0.08 molm⁻² s⁻¹ in highly stressed trees. Our findings will provide a useful guideline for the local growers and agencies to achieve better irrigation scheduling and higher water productivity for the region.

可持续的农业系统需要提高用水效率并增强对作物用水的了解。在水资源不足和观测数据有限的地区（例如伊朗南部），这一先决条件更为明显。因此，本研究旨在通过确定标准的蒸散速率和作物系数（单株和双株）来发现成熟橙树（柑桔（Osbeck），塔罗科伊波利托（Tarocco Ippolito））的需水量。在2017年和2018年期间，将滴水灌溉果园中的72棵橙树分为壤处理和六种处理，分别以参考蒸散量的100％，90％，80％，70％，60％和50％进行灌溉。水分变异性和作物生理响应，包括茎水势（Ψ_茎），净光合作用（A _n）和气孔导度（g _s）被测量。我们的结果表明，以90％的ETo灌溉可提供树木的全部水分需求。计算的平均作物蒸发蒸腾量为5.2 mm day ^-1，作物系数为0.65至0.95。该地区传统上的平均灌溉率比实际需要高出19％。生理反应分析突出了气孔在调节蒸腾作用和维持叶片膨大中的控制作用。在峰值水分胁迫期间，全灌树的g _s从0.11到0.12 molm ^-2 s ^-1到0.04-0.08 molm^-2 s ^-1在高应力树中。我们的发现将为当地种植者和机构提供有用的指导，以实现该地区更好的灌溉计划和更高的水生产率。