Water requirements for irrigation with saline groundwater of three datepalm cultivars with different salt-tolerances in the hyper-arid United Arab Emirates

Direct measurement of sap flow enabled determination of the seasonal pattern of water use, ETc, of three date (Phoenix dactylifera L.) varieties irrigated with groundwater at different salinities: S1 at 5 dS m−1 and S3 at 15 dS m−1. The ETc at the higher salinity was 43–46% lower across all varieties. The crop factor, Kc, was computed from ETc / ETo, where ETo is the reference evapotranspiration. By proximal sensing using a light stick, we measured the fraction of light intercepted, LI, by the trees’ canopies. For all varieties and salinities, we found the ratio Kc LI−1 to be about 0.95, which enables proximal sensing to be used to predict ETc for all varieties and across salinities. These predictions can then be used to schedule irrigation the recommended rate of 1.5 ETc, which accounts for a 25% factor-of-safety and a 25% salt leaching fraction. For S1, the salt-tolerant ‘Lulu’ used 50 kL tree−1 y−1, the moderately tolerant ‘Khalas’ consumed 43.1 kL tree−1 y−1, and the salt-intolerant ‘Shahlah’ transpired 57.3 kL tree−1 y−1. Whereas the drop in ETc across all varieties was similar between S1 and S3, there were large differences in the drop in date production. Date production between S1 and S3 dropped 29% for ‘Lulu’, 43% for ‘Khalas’, and 52% for ‘Shahlah’. Analysis of the consumed water productivity, CWP (kg-dates kL−1) provides insight into the impacts of salinity on date yield. For the tolerant ‘Lulu’ the CWP for S3 was higher (2.21 kg-dates kL−1) than that for S1 (1.78 kg-dates kL−1), although production was higher with S1 (89.1 kg tree−1) than S3 (62.9 kg tree−1). The CWP for ‘Khalas’ was the same for both treatments (≈ 1 kg-dates kL−1). For the salt intolerant ‘Shahlah’, CWP dropped between S1 (1.5 kg-dates kL−1) and S3 (1.34 kg-dates kL−1). Based on the price of dates, the CWP can also be used to assess the economic value of irrigation water by variety and salinity.