Water use of Al Samr (Acacia tortilis) forests irrigated with saline groundwater and treated sewage effluent in the hyper-arid deserts of Abu Dhabi

Abstract The arid forests planted in Abu Dhabi provide a variety of valuable ecosystem services. The forests need to be irrigated, and currently groundwater (GW) provides the bulk of this water. However GW recharge is very low, and reserves are rapidly dwindling and becoming more saline. In 2016, Law 5 was passed in Abu Dhabi with the objective to set GW extraction limits and define irrigation-usage allowances. Here we sought to define the usage allowance for Al Samr trees, and this work complements our previous research on Al Ghaf and Al Sidr forests. We have measured tree water-use, ET c, using sapflow monitoring of GW-irrigated trees, and trees irrigated with treated sewage effluent (TSE). Maximum rates of tree water-use, ET c, were found to around 10 L d −1 , and there are two distinct deciduous periods where ET c briefly dropped below 2 L d −1 . The total annual water use of the TSE-irrigated trees was 2.2 kL y −1 , which is about 25% higher than the 1.8 kL y −1 for the GW-irrigated trees. For Law 5, we recommend that the irrigation allocation for Al Samr trees be simply based on a constant ET c of 10 L d −1 . So for GW irrigation, allowing for a 25% factor-of-safety, and a 25% salt-leaching fraction, the recommended allocation would be 15 L d −1 . This represents a saving of 75% from the current practice of irrigation 60 L d −1 . For TSE, without the need for salt leaching, the irrigation allocation would only need to be 12.5 L d −1 .