## 1 Introduction
### The Context of the Water Energy Nexus in Abu Dhabi  
AD has embarked on a massive United States (US) five-billion-dollar program, based on the aquifer storage and recovery approach [11]. This program is to build capacity so that local aquifers can be used as strategic reserves for desalinated water [12]. The peak demand forecast for water will more than double by 2030 [13]. The Emirate of AD's annual water consumption is estimated to be 2.49 billion m\({}^{3}\) and is expected to reach 5.86 billion m\({}^{3}\) by 2020 [14]. In the UAE, groundwater has been extracted at over 13 times the annual recharge rate [15] in [16]. However, water use varies between the emirates. In AD, 72% of the groundwater is mainly used for agriculture irrigation, followed by desalination (24%) in residential and commercial buildings as domestic water. Plus, four percent is used for treated sewage effluent (TSE), residential district cooling, public park LI and in WFs such as fountains, but not for crop cultivation [16]. In 2012, desalinated water demand in AD exceeded local production, where 201.3 million m\({}^{3}\) had to be imported from Fujieirah, a northeastern emirate (ibid.). Different factors have made the UAE dependent on desalination water. First, the increased water demand of a fast-growing population; second, the low (less than 100 mm/year) average precipitation; third, a very high evaporation rate (two to three m\({}^{3}\) per year) (ibid); and fourth, the limited recharge of the groundwater [4; 17]. For this reason, AD has no choice but to supply domestic water from seawater desalination [17] using techniques such as multi effect distillers (MED), reverse osmosis (RO), and multistage flash (MSF) [18; 19]. The main technology used in the UAE is thermal desalination with MSF (57%) because it is more energy efficient and can produce higher quality water (ibid.). MED are used at 6% for desalinating water and RO at 37% [19; 20]. However, as shown in Table 1, the technique of desalination is still highly energy intensive: RO and MSF technologies use between four-kilowatt hour per cubic meter (kWh/m\({}^{3}\)) to 7.5 kWh/m\({}^{3}\) average and up to 15.40 kWh/m\({}^{3}\), respectively [20]. In addition, MSF and RO technologies also have a very high cost of 1.10-1.25 United States dollars (USD) per m\({}^{3}\) and 0.68-0.82 USD/m\({}^{3}\), respectively, in comparison to 0.05 USD/m\({}^{3}\) for fresh water sources [21]. The water supplied to AD Island (the location of the MFCS) is produced at the Arabian Power Company (AMPC), Taweelah Asia Power Company (TAPCO), Gulf Total Tractebel Power Company (GTTPC), and Emirates CMS Power Company (ECPC), as detailed in Table 2[18]. Desalination plants, which mainly use MSF thermal desalination technology (74%) consume energy at a rate of 15.40 kWh/m\({}^{3}\)[20], bringing the cost of water production between 1.10 to 1.25 USD/m\({}^{3}\) (ibid). Figure 2: Medical facility case study (MFCS) total building water use against LI consumption based on 2016 Abu Dhabi Distribution Company (ADDC) water bills [6; 10]. This provides evidence that the value of water production is not tied to the subsidized (50% discount) commercial and residential AD water tariff, which is AED 4.11/m\({}^{3}\)[10] after discount, which is equivalent to USD 1.12/m\({}^{3}\)[22]. Desalination plants have been optimized since 1990-2000, although the process efficiency is still under investigation through pilot programs [11] such as an adsorption desalination system producing pure water with no need for fossil fuel use (ibid.). Thus, in 2017, the process of desalination was still highly energy intensive [13; 14; 21], hence impacting on the natural environment, and contributing ultimately to climate change. As highlighted in Table 1, it takes 4 m\({}^{3}\) of saline feed water for the MSF technology to generate 1 m\({}^{3}\) of distillate water [20]. Energy consumption and potential effects on the marine environment of brine disposal are the two largest aspects of environmental impact [14; 23; 24; 25; 26].