ارزیابی پایداری و تعیین اندازۀ بهینۀ جمعیت شهر مشهد بر اساس وضعیت منابع آبی

Introduction
By 2050, 70% of all population is expected to live in urban areas. Rapid urbanization will lead to the serious water scarcity and contradiction between water demand and supply as well. Also, increase in urban water demand due to population growth has turned to a major concern. Planning and water resource management must be supported by national executives since water resources are facing with the serious issues as a result of the threats of the population growth and climate change.
With the increase of size and density of population, required water resources must be supplied from the out-of-city resources and this raises the question that: “Is there a real capacity for the urban development based on water resources?” Thus, it is inevitable that we must move towards urban planning, sustainable development and optimal population depending on the water resources.
Following the presentation of sustainable development concept, the sustainable city models were presented. In the models, the planners should focus on creation of the cities with less input of energy, materials and less output of pollution. The studies of Haughton on sustainable city models were very helpful and each model presents different strategies for the water and sewage network. Some of the models are including the models of redesigning cities and independent cities. Proposing suitable patterns for the form and shape of cities, also specifying the optimal population size can be considered as steps towards protection of natural resources and reaching sustainable development.
Mashhad, as the center of Razavi Khorasan Province, is located in the Mashhad plain with an arid climate. This is considered as the second most populated metropolitan of Iran and, also, is twelve times as long as the second city of the province. Thus, this province has a mono-central structure and it is also the second greatest religious metropolitan of the world. In addition to the population dwelling there, it annually welcomes about 20 million of pilgrims. One of the most important facilities is drinking water which faces innumerable problems specifically in the summer when the hot weather and the large number of pilgrims increase the need for water. Thus, water shortage becomes more and more obvious. All this is happening while in 1966 in Mashhad plain to provide water was forbidden.
Therefore, addition to keeping the use of this plain to provide the needed water depends on the neighboring realms and to achieve a sustainable management of water resources, Mashhad should be directed toward a sustainable form.
Hence, the ultimate purposes of this study are at first evaluation of the sustainability of Mashhad in terms of water resources by employing the sustainable urbandevelopment models proposed by Haughton and the second determination of the optimal population of the city in 2016 according to the two options of the Water and Sewage Organization of Mashhad to provide the needed water and consumption capitation in 2016.
Methodology
Due to its purposeful nature, this study has used mixed hybrid methodology to consider supportive qualitative data for quantitative analyses. Thus, in order to assess Mashhad’s sustainability in connection with water resources, the forms were scrutinized through library research. Then, Haughton’s studies on urban water and sewage network were identified. In the studies, reduction of permeation basin, sewage treatment, reduction of city size, and supply management were investigated. Also, using qualitative approach, province’s water resources condition, Mashhad plain, and Mashhad itself were calculated through analysis of related documents. Finally, Mashhad’s water condition was analyzed based on current documents and the necessity of application of procedures to gain sustainable development.
At this point, we analyzed the most significant plans for supplying Mashhad’s water resources studied and applied in recent years.  These studies are including substitution of backwater of the sewage refinery, and building Dousti and Ardak dams. The research attempts to specify the optimal population using quantitative approach, the average need of water for the resident population, for consumption  per capita, for pilgrim per capita, and the average usual urban need of water in 2016. The data have been obtained from the minutes of the Water and Sewage Organization of Iran.
Results and Discussion
The results indicate that Mashhad plain is among the critical plains in water resources. From total underground water resources, only 2% is remained while 65.97% of urban population of the province living in Mashhad plain. Mashhad City is the biggest urban spot on the plain. In the first 5 months of 2014, from total provided water of the city, 42% were related to surface resources, so that about 92.7% from was Dousi dam, 3.1% from Karde dam, and 4% from Toroq dam. This matter shows the dependence of Mashhad on outside of permeation basin. Dousti dam is located in 220 km of Mashhad and until the end of 2012; nearly 14217 billion Rials were spent. Evident show that Dousti dam supply is about to end and it cannot be dependent on for long. The results also show that approximately one third of produced sewage has been recycled and great effort should be made in this regard. Plan of water transfer from Ardak dam is under implementation but due to delays it won’t be utilized yet and hence Mashhad would face water shortage in 2016 even with Ardak dam supply.
Conclusion
Therefore, based on the water supply resources, the condition of recycling sewage and comparison with the guidelines of Haughton’s sustainable city models, it can be said that Mashhad’s model totally depends on outside resources and Mashhad City is not a sustainable urban center in regard to water resources. According to two options of Water and Sewage Organization of Mashhad for providing the required water in 2016, the city has surplus population. With the number of pilgrims, based on independence from permeation basin (without Ardak and Dousti dams) and on the dependence on permeation basin, the city has surplus population of 1,192,660 and 550,459 people, respectively. The optimal population is 1,933,340 for the first option and 2,575,541 people for the second option with surplus population even with dependence on outside the permeation basin. Proposed solution for moving towards the sustainability and sustainable management of water resources is to decentralize the population of the city and to convert the mono-central structure of the city into multi-central structure through Haughton‘s models.