Modeling and simulation of water systems over time allows us to see the dynamic changes in these systems and empowers stakeholders to make informed decisions to maximize the adaptive capacity of the resource [
27]. Because the water and energy systems are connected and coevolving [
8], especially in the case of Qatar, it is essential to choose a method that can be used to develop and analyze the impacts between the two systems together, particularly in the light of the region’s seasonal electricity and relatively stable water consumption patterns. A system dynamics model can be used to analyze multi-scenarios and multi-attributes of the water–energy interaction over time [
7]. It can be used to put together both physical and socio-economic behavioral facets of a given matter in a holistic, flexible and transparent way [
28] and its ability lies in modeling the behavior of a system which has not been developed and estimated before [
29]. The process of developing the system dynamics model involves the interrelated activities of articulating the problem, proposing a dynamic hypothesis, building a simulation model, testing that simulation model and finally designing and evaluating different policy measures [
30].
The methodology allows for qualitative or conceptual modeling as well as quantitative or numerical modeling [
31], and has been extensively used in developing a water systems. Zarghami and Akbariyeh [
32] developed a system dynamics (SD) model to study the water system of Tabriz, a city situated in the west of Iran. The author’s incorporated supply and demand resources as well as water management and conservation tools and estimated the impacts of five different scenarios on the water shortage of the city. Sharawat et al. [
33] used system dynamics to model sustainable development of water resources using the temporal projections of population growth, for district headquarter city Rohtak in North India. The projections were done from 2016 to 2041 for six population scenarios, to study policies using a mass balance model of the water system in the city. Chen et al. [
34] modeled the supply and demand water system of Shanshan Country in northwestern China and focused on the water resource management techniques. The tool integrated the operational management of the water system, sources of water supply and the water demand from different users. The impacts of climate change were considered and several strategies were simulated to test water policies on water sources, irrigation land, irrigation efficiency and water demand. Chang et al. [
35] developed a model for the city of Urumqi (an arid area), and investigated the urban water resource security with the help of water supply demand pressure and urban expansion index for the years 2011 to 2030. The authors also evaluated the carrying capacity of the system while considering the effects of climate change, population growth and industrial development for the duration between 2006 and 2030.
Sun et al. [
36] constructed a comprehensive national-scale water assessment and management system through system dynamics by developing five subsystems (economy, population, water supply and demand, land resources, and water pollution and management) that affect the sustainable utilization of water resources. Further studies such as that of Alvi et al. [
37] developed a hybrid agent-based and system dynamics household model to estimate the water consumption for an urban area. Chhipi et al. [
7] developed a system dynamics model as a decision support tool for the urban water system of Penticton, British Columbia, Canada. Duran-Encalada et al. [
28] estimated the quantity and quality of water across the US–Mexican transborder communities of the Rio Grande/Rio Bravo Water Basin. Chen and Wei [
38] conducted an extensive theoretical literature review on the application of system dynamics for the past 20 years, with the review focusing on research related to flood control, disaster management, water resource security and water environment security. A more recent literature review by [
39] assessed the application of system dynamics in the WSDNs. The authors found that the literature addresses the supply side of the network. However, there is a lack of research related to water distribution networks (WDNs).
2.1. Water Policies and Methods to Reduce Energy Consumption
Policies to assist in developing sustainable water systems require a fine balance between addressing social, economic and environmental issues. The right pricing, reliability and accessibility are imperative, as decision makers face problems including water resource scarcity, environmental pollution, high subsidy and high transmission and distribution losses [
40]. The supply side practices often involve disrupting the entire systems through development of new projects or requiring extensive changes to the current structures. Demand side policies on the other hand depend on the changes from individual entities (people, households, companies, etc.) through changes in equipment and behavior, as well as pricing and are considered efficiency measures that involve improvements in technology, human conduct and a combination of both [
41].
In terms of energy use, wastewater treatment and reuse is much more efficient than desalination [
8]. Recycling and reusing lowers the water demand and extends the life of the existing water supply stock [
42]. Urban water systems can be improved significantly through water reuse, rainwater harvesting, dual pipe systems, reduced water losses and water conservation policies [
43]. One primary solution in addressing water issues is the transition away from centralized water systems towards decentralized and integrated or multifunction systems, with measures involving water reclamation, gray water recycling and rain and storm water harvesting [
44]. These systems can work independently or in tandem with the existing water infrastructure [
45,
46]. The use of water multiple times from higher to lower quality needs, is an important method of water resource management and reuse [
33]. For example, lower quality water can be used in toilets, as toilet flushing uses an estimated 20 to 30% of household water consumed [
33].
However, reuse and efficiency measures need to be addressed in holistic ways, as some measures may look to be more efficient, but may cause alternate cost such as in the case of drip irrigation, which saves water but can sometimes use more energy as compared to flood irrigation systems [
47]. Storage systems can also be used to reduce the strain on the system during peak times and provide the utility at lower pricing. Pumped hydraulic storage continues to be one of the most efficient methods of storing both water supply and energy (electricity) [
48].
Water pricing should include economic costs of production and supply. Moreover, water demand reduction can be achieved through incentive-based billing, conservation campaigns and water saving devices [
49]. “Techniques used in demand management programmes include: intermittent water supply; water loss reduction (including leak detection and repair); comprehensive metering, changes in water pricing concepts, installation of water saving devices (retrofitting), wastewater reuse, institutional development, and public awareness and educational campaigns” [
1].
2.2. Desalination, Wastewater Reuse and Groundwater Studies in the Region
Ibrahim and Shirazi [
50] examine the potential transition of the Energy-Water-Environment nexus towards a circular economy for the country of Qatar. The authors discuss that there is no comprehensive policy towards circular economy despite the enormous potential and that constructed wetlands can play a significant role in wastewater treatment and recycled wastewater usage. Similarly, Tahir et al. [
51] evaluate vulnerabilities in the water networks and desalination plants for the Middle East region, and highlight the advances made to make the systems more resilient. The authors find that oil spills, harmful algae blooms and plant equipment failure are the most significant vulnerabilities in the region, which are being mitigated through mega reservoirs and research in technologies related to solar desalination and pretreatment techniques. Darwish and Mohtar [
52] discuss challenges related to desalination, wastewater reuse and groundwater use for Qatar and recommend a reverse osmosis desalting system to save natural gas usage in the country. The authors also recommend water conservation measures such as the storage of treated wastewater in aquifers for strategic reserves and the use of renewable energy for desalting and wastewater treatment. Ahmad and Al-Ghouti [
53] highlight the groundwater management practices that can be used to achieve sustainable groundwater usage in the state of Qatar. The authors recommend that, for aquifers, there is a need for enhancing rainfall infiltration and recharging through treated sewage effluent. Usage of groundwater treatment techniques, efficient irrigation practices and the development of water-use tariff structure is also recommended.
Multiple other studies have also discussed the potential of desalination, wastewater reuse and groundwater for Qatar. Atilhan et al. [
22] use a systems-integrated approach to optimize the water desalination and distribution networks. Mannan et al. [
54] examine the environmental and human health impacts of multistage flash desalination using life cycle assessment. Jasim et al. [
55] discuss the efficacy of wastewater treatment and discuss the reuse of treated sewage effluent and wastewater in supplementing the growing demand on desalinated water. Lambert and Lee [
56] present the results of a national survey that study the acceptability of greywater reuse and find that framing of greywater reuse as a cost saving measure can increase its acceptance among both Qatari nationals and expatriates. Alsheyab and Kusch-Brandt [
57] examine resources such as nitrogen, phosphorus and sulfide, etc. embodied in wastewater and assess their profitability after recovery. Ahmad et al. [
58] perform a hydrogeochemical characterization and quality evaluation of groundwater to assess its usage for domestic and agricultural use.
2.3. Qatar’s Water Statistics (Production and Consumption Patterns)
Qatar’s National Vision 2030 highlights the importance of the needs of current and future generations, by way of economic growth, social improvement and environmental management. The document envisions the need for a balance between development and the environment, including air, land water and biological diversity. It also calls for action to deal with the dwindling water resource, as well as the impact of climate change on water levels in the country [
59]. The Qatar National Research Strategy [
60] recognizes water security as one of the four grand challenges, and wants to address it through developing, refining and enhancing desalination in addition to waste water re-use capabilities (
Table 1).
Qatar’s focus on water security is due to its unique geographic and demographic characteristics, and because it is one of the poorest countries in terms of natural fresh water resources [
61]. A small country with an area of only 11,627 sq. km, Qatar is a peninsula that is approximately 185 km in length and 85 km in width. It is surrounded by the Arabian Gulf with a coastline of 550 km and has the only land border with Saudi Arabia that is nearly 60 km long [
62,
63]. Topographically, the country can be considered a flat land, with land surface elevations varying from 0 m to around 107 m above mean sea level [
64]. The primary freshwater resource in Qatar can be found in the form of groundwater, however it exists in limited quantity and is brackish in nature [
65] with rainwater being the primary source of recharge [
66]. The annual average rainfall is around 82 mm with high temperatures increasing the evaporation rates to an annual average of 2200 mm resulting in insufficient replenishment of the groundwater [
52]. This means that the country’s main source of freshwater is its ability to desalinate the seawater, with the first desalination plant in the country being commissioned in 1953 [
65]. The energy required for the desalination process in the country is entirely met through natural gas [
54] provided by Qatar Petroleum [
67]. Energy and water have a unique link in the country as both electric power and desalinated water are produced together in most plants known as cogeneration power desalting plants (CPDP), with simple gas turbine cycle or gas turbines combined with steam turbine to form a gas turbine combined cycle (GTCC) [
52]. Although the power generation and water desalination business is deregulated and owned by private entities in the form of IPWPs (independent power and water providers), the country has streamlined its electricity and water distribution network through a government corporation named KAHRAMAA (Qatar General Electricity and Water Corporation) [
67].
With its 10 desalination plants, Qatar produced water close to 2.07 million m
3/day with a total of 605.7 million m
3 desalinated water produced in the year 2017 [
67]. Because of access to only Arabian Gulf water and its characteristics of high temperature, salinity, turbidity and presence of marine organisms [
68], the main desalination technologies used in the country are multi-stage flash distillation (MSF), multi-effect distillation (MED) and reverse osmosis (RO), with MSF supplying 75% of the total capacity [
54]. MSF and MED consume an estimated 20 kWh to produce 1 m
3 of water, whereas RO needs around 5 kWh/m
3 [
52], but the combination of technologies as mentioned above results in energy consumption of between 9 and 15 kWh per distilled m
3 of potable water in Qatar [
69]. Because of the combined power and water production cycles, the region experiences inefficiencies as the water demand stays stable throughout the year, but the electricity demand fluctuates [
70].
Figure 1 shows the breakdown of water use balance with the potentially available resources on top and the use case on the bottom pie chart. A total of 1014.71 million m
3/year of water is available throughout the system (in 2016) with around 55% of the resource being supplied through desalination, 25% through groundwater abstraction and 20% through treated sewage effluent [
63].
Not only Qatar, but also the region (GCC and MENA) in itself lacks sufficient potable water, with water storage for large urban centers being between only 12 h and 3 days [
70]. Some of the excess water that is produced through desalination is conserved in storage systems or is injected in aquifers [
22].
Table 2 shows Qatar’s water reserve capacity as of 2017 with total storage operating capacity of 6.69 million m
3 which provides a storage of around 2 to 3 days of water use. Furthermore, to address the issue of strategic storage for longer periods, the country is constructing man-made “mega-reservoirs”. Also known as the “water security mega reservoirs project”, the aim of the system is to provide 7 days of potable water storage, with the first phase to provide storage of around 10.46 million m
3 for the expected water demand by 2026, and the second stage to provide additional storage for a total of 17.28 million m
3, for the expected demand of 2036 [
71]. The project is set in 5 strategic locations with 40 concrete reservoirs of dimensions 300 × 150 × 12 m
3 set to be built by 2036, with up to 24 being built in the first phase, with each reservoir having a capacity of between 390,900 m
3 and 440,970 m
3 [
71]. Furthermore, a natural form of storage that the country relies on is the groundwater aquifers found beneath the soil. This groundwater is found in 4 main aquifers known as Al Masahabiya, North Qatar, Central Qatar and South Qatar, with a minor fifth aquifer called Doha, found near the capital. All of the facilities above are included in the freshwater transmission network in Qatar, including the functionality of the “mega reservoirs” that are connected through 1440 km of pipeline with the complete distribution network at 8380 km (to reach 10,000 km by end of 2022) [
72].
Complete and secure water and sanitation facilities are provided to nearly all citizens [
73]. Since 2015, nearly 90% of the buildings have been connected to the sewerage system, with the rest being served by tankers transporting the wastewater to treatment plants and sewage lagoons [
74]. There are 24 wastewater treatment plants in the country (2017) with a designed capacity of 827.9 thousand m
3/day with the total amount of wastewater collected in the year amounting to 231.47 million m
3; 99% or 228.67 million m
3 of the wastewater generated was treated [
75]. All of the treatment plants are designed for secondary treatment, with 19 of them achieving tertiary level wastewater treatment and the largest four of these 19 able to remove nitrogen and phosphorus.
Figure 2 shows the details of the water and sewerage distribution network in the state of Qatar in the form of a water balance flow chart. As can be seen, the two main sources of water are the desalinated water from the Arabian sea and the water withdrawn from aquifers, with a third source of water being the treated sewage effluent from the wastewater treatment plants. The literature indicates that desalinated and groundwater networks are not connected. However, the treated sewage water network is coupled with the aquifers as some of the treated water is deposited to replenish the water levels.
Qatar’s water consumption is divided based on the source of water, as ground water is used for agriculture, desalinated water for portable consumption and treated wastewater for irrigation of crops and landscaping [
52]. To date, Qatar’s arable land is estimated at approximately 1.2% [
76] with the value added to the GDP of just 0.2% [
77], while the percentage of water use for irrigation and livestock is as high as 28.75% [
63]. The primary purpose of treated wastewater is landscaping which includes parks and lawns. Treated wastewater is not used for edible agriculture because of social, religious and local marketing views [
65]. However, in recent years according to KAHRAMAA, tertiary treated wastewater is being supplied for agriculture in some instances (for fodder crops).
Using
Figure 2,
Figure 3 and
Figure 4, we can see that the main supply to the household and industry sector is desalinated water, with less than 6% and 2% of the total water supplied through groundwater respectively. The water demand of the commercial sector is entirely met through desalination, whereas the government sector (which includes greenspaces in the country) water demand is met through 60% desalination and 40% treated wastewater. The largest consumer of groundwater is agriculture, with around 78% (of 300 million m
3) consumption with the rest of the demand being met through treated wastewater. Reports and data mention the total amount of urban wastewater collected, but the division is not made between how much wastewater is produced through household, industry and commercial sectors. Furthermore, the statistical reports mention the losses in the water system only in the desalination distribution network.