The arid forests of Abu Dhabi provide a variety of valuable provisioning, regulating and cultural ecosystem services. They require irrigating. And groundwater is the source. About 95% of groundwater consumption is for agriculture and forestry. Over-extraction threatens groundwater resources. The Government of Abu Dhabi recently passed Law 5 to restrict groundwater abstraction. We have determined the minimum allocation required for the irrigation of two tree species. We carried out experiments at Madinat Zayed in the western desert on two arid-forest species: Al Ghaf (Prosopis cineraria) and Al Sidr (Ziziphus spina-christi), both planted on a 7m×7m grid. We measured the actual evapotranspiration (ETc) using heat-pulse equipment in the trees. Saline groundwater with an electrical conductivity of about 8–10 dSm−۱ was used for irrigation. Current practice is to drip irrigate with 60 L d‐۱. Both species displayed distinct, and different, summer ‘deciduous behaviours’ that determine their seasonal pattern of ETc. A single crop-factor approach, using ETc predicted from Kc.ETo, where Kc is the crop factor and ETo is the reference evaporation, would not provide appropriate irrigation allocations. From our hourly measurements of ETc, made over 3 years, we quantified the seasonal pattern in Kc. For Al Ghaf, Kc ranged from 0.1 during February–July, to 0.15 in November–December; for the Sidr, Kc was at a minimum of 0.06 in May, and rose to 0.16 in December. Daily irrigation requirements were provided for Law 5. With a 25% factor-of-safety, and a 25% salt-leaching requirement, irrigation requirements for Al Ghaf ranged from 24.4 L d−۱ in January to 52.8 L d−۱ in July. For Al Sidr the range was from 33.8 L d−۱ in April to 53.5 L d−۱ in September. These are a 40% saving on current practice.