The blue and grey water footprints of date production in the saline and hyper-arid deserts of United Arab Emirates

Dates are economically and culturally important in the United Arab Emirates (UAE). The Emirates has a hyper-arid climate, such that rainfall (green water) is irrelevant for date production. Date farmers rely on irrigation with groundwater (blue water) that is brackish to grow the dates. However, the quantity of groundwater left in the underground reserves is diminishing. Leaching of polluted water (grey water) containing nitrogen and salts from the rootzone of the date palms is compromising the quality of the remaining groundwater reserves. Quantification of the water footprints of WFgreen, WFblue and WFgrey, in L kg(-1) of dates produced can be used to assess the magnitude of the impacts of date production on the quantity and quality of the UAE's valuable groundwater resources. Our water-use experiments on dates near Dubai were used to determine these footprints. We measured the tree transpiration, ETc (m(3) year(-1)) of three date varieties of 'Lulu' (salt-tolerant), 'Khalas' (moderately tolerant), and 'Shahlah' (salt-intolerant), irrigated with water at two rates of salinity with electrical conductivities (EC) of 5 and 15 dS m(-1). The WFgreen = 0, because of the negligible rainfall. Our recommendation is for irrigation to be at 1.5 x ETc to enable the leaching of salts. So with the transpiration loss of ETc there is drainage of 0.5 x ETc back to groundwater. The WFblue is, therefore, ETc/Y, where Y is the yield of dates (kg). We found WFblue = 646.6 L kg(-1). The grey water footprints were WFgrey = 523 L kg(-1) for nitrogen and 970 L kg(-1) for salt. The salt WFgrey had the largest magnitude. The economic benefit-cost ratio (BC) of the prior dilution of the brackish groundwater with desalinated water for irrigation was found to be 1.4. However, the externality of the environmental impact of the disposal of the rejected brine from desalination will need to be addressed.