Improved understanding of crop water use is vital for aiding water-saving and sustainable production practices, particularly for water-restricted regions, where limited observations exist. This study investigated the standard evapotranspiration and crop coefficients (single and dual) for drip-irrigated mature orange trees (Citrus sinensis L. cv. Washington navel) for the semi-arid climate of southern Iran. Forty-five Washington navel trees in a clean-cultivated orchard were subjected to five irrigation levels (100%, 90%, 75%, 60%, and 45% of reference evapotranspiration) for two consecutive seasons (2016, 2017). Crop physiological responses including stomatal conductance (g(s)) and leaf water potential (Psi(leaf)) were measured, and the agronomic effects in terms of plant yield (i.e., fruit number and weight) and evapotranspiration water productivity (WPET) were evaluated. The average standard evapotranspiration rate was measured as 5.11 mm day(-1) with the seasonal amount of 1814 mm (partitioned as 84.9-86.5% of transpiration and 13.5-15.1% evaporation), and the crop coefficient ranged from 0.67 in January to 0.96 in June. During periods of high evaporative demand, the non-stressed and moderately stressed trees (100%, 90%, 75% treatments) reduced their g(s) (0.107-0.075 mol m(-2) s(-1)) to maintain a relatively constant Psi(leaf), whereas in severely stressed trees (60% and 45% treatments), Psi(leaf) significantly reduced when g(s) dropped below 0.067-0.077 mol m(-2) s(-1). Considering the current water deficiency in the region, irrigating at 60% ETo ((similar to)67-70% standard crop demand) is recommended for sustainable citrus production.