Estimation of evapotranspiration and crop coefficient of drip-irrigated orange trees under a semi-arid climate

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 (psi(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.