Water scarcity in arid and semi-arid metropolitan areas has increased the need for accurate estimation of urban green-space water requirements and improved irrigation efficiency. This study assessed the uncertainty of the LCM-WUCOLS approach and evaluated its applicability under Tehran conditions by comparing its estimates with those obtained from i-Tree Eco for Plot 48 of Tehran City Park. Annual ET0, calculated from Mehrabad Station data using CROPWAT, was 1,286 mm. To assign WUCOLS species factors, the climatic similarity between Tehran and selected California stations was evaluated, and WUCOLS climate region 2 was identified as the most appropriate analogue. Considering the existing vegetation, approximately 98% of the plot area was covered by turfgrass, the annual landscape water requirement estimated by LCM-WUCOLS was 1,225 mm, while irrigation efficiency based on applied water was 45.5%. For direct comparison with i-Tree Eco, turfgrass was excluded from the analysis. Under this condition, the baseline LCM-WUCOLS estimate was 772 mm yr⁻¹, whereas i-Tree Eco estimated 1,043 mm yr⁻¹. Uncertainty analysis showed that varying the species and density factors within their allowable ranges changed the LCM-WUCOLS estimate from 566 to 1,003 mm yr⁻¹, corresponding to approximately 27% below to 30% above the baseline estimate. The proximity of the upper uncertainty bound to the i-Tree Eco estimate highlights the decisive influence of coefficient selection on model accuracy. The findings emphasize the need for local calibration, improved irrigation scheduling, reduced use of water-intensive vegetation, and field validation before broader application across Tehran.