Coupled effects of irrigation management and water salinity on date palm cultivars in the hyper-arid environment of the United Arab Emirates : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy (PhD) in Soil and Environment Sciences, School of Agriculture and Environment, Massey University, Palmerston North, New Zealand
Dates, and the farming of date palms (Phoenix dactylifera L.), are culturally, aesthetically and economically important in the United Arab Emirates. In this hyper-arid region, dates require irrigation, as rainfall is virtually non-existent. Groundwater is relied upon as the source of this irrigation water. Yet, the groundwater reserves in the Emirates are expected to run-out in about 55 years. Furthermore they are becoming more saline. In the Emirate of Abu Dhabi, Law 5 has been passed and that will limit the amount of water that can be withdrawn for agriculture, or any other purposes. Thus there are imperatives to minimise the amount of water being used for the irrigation of date palms, and to limit the amount of salt leaching from the rootzone of the date palms. These critical issues provide the underpinning reasons for the research described in this thesis. Environment Agency – Abu Dhabi (EAD) has invested in two research projects to determine the minimum amount of irrigation water, as a function of salinity that needs to be applied to date palms to ensure economic returns from date production. These two projects underpin my doctoral research. Using the Compensation Heat Pulse Method (CHPM) of monitoring sapflow has enabled quantification of palm-tree water use, ETc. This was carried out on three cultivars of differing salt tolerances: the salt-tolerant ‘Lulu, the moderately tolerant ‘Khalas’, and the salt-intolerant ‘Shahlah’. Two salinities of groundwater were considered: 5 dS m⁻¹ and 15 dS m⁻¹. The sustainable daily rate of irrigation was considered to be 1.5 ETc, which accounts for a 25% factor-of-safety, and a 25% salt-leaching fraction. This represents considerable savings over current practices. As well, both proximal and remote sensing were used to extrapolate these findings onto commercial date farms. Finally, an assessment of the green, blue and grey water footprints of date production was made. The grey-water footprint from salt leaching was found to be the largest. A benefit-cost assessment was made of the option of using desalinated water to augment and dilute the brackish groundwater used for irrigation. To dilute 15 dS m⁻¹ groundwater to 5 dS m⁻¹ irrigation water was shown to have a benefit-cost ratio of 1.4. However, the environmental impact of the reject brine will need to be considered.