عنوان مقاله [English]
The origin of the concept of carrying capacity is unknown. A number of biologists believed strongly that carrying capacity concept is rooted in biology. They define carrying capacity as a maximum population of a particular species that can be supported indefinitely in a specific environment without any sustainable damage on its life support systems` productivity and performance or reducing future capacity to support them. Looking at biological sciences, urban carrying capacity is equal to maximum load and pressure on theurban ecosystem that is mainly focused on the relationship between constraints and supply and demand pressure. Hence there is always a limit to the growth of biological populations that in thecase of its violation, both human and natural environment will be incredibly hazardous. As in retrogressed ecosystems like today metropolises, initial ecosystem structure and ecosystem process is impaired. Nowadays, in all cities, there is a very deep concern of nonrenewable resources, negative external effects of pollution and serious threat of irreversibility global ecosystems. Now, high volume of the urban population and severe metabolism in urban ecosystem forced thecity to tolerate a pressure more than refining capacity and its absorption capacity. Mashhad city is no exception for this rule. Today, the rapid growth of urban population in the metropolitan city of Mashhad affects largely not only urban making policies but also its consequences in the intensification of economic, social, management and biology issues and its concerns and worries make planners adopt measures in order to solve or control issues and crises raised from this phenomenon. Unfortunately, principles that are seen in setting management policies and planning in metropolitan of Mashhad are about meeting beneficiaries’ needs and requests and therefore what is unseen for this issue is environment tolerance thresholds and platform ecosystem capacity in facing with a great volume of material and energy and land usage change. In different studies, different approaches are presented for defining and determining urban carrying capacity. In this study, urban carrying capacity is formed by mixing two basic concepts support capacity and environment strength and it is a level of human activities and urban development density that while is guaranteed for human presentation threshold and different activities in the space includes the minimum damaging feedbacks. In this article, urban carrying capacity is determined as a range of minimum pressure that makes the least change and interference in platform ecosystem to the crisis pressure amount that disturbance in ecosystem performance is beyond that. It aims to integrate the concepts of environmental tolerance thresholds and platform ecosystem capacity with urban development management in order to control and survey urban ecosystem safety in constant management and planning process of Mashhad metropolises. 2 - Methodology In this paper, urban carrying capacity is formed by two basic concepts support capacity and critical capacity. Support capacity is a total sum of environmental facilities without regarding their interaction with each other and critical capacity is maximum load and pressure on theurban ecosystem that is measured based on pollution and in thecase of transition, biology community will face resistance. In order to determine the carrying capacity at first we estimate potential of productivity of each resource and performance loading of each urban facility in response to the real needs of urban dwellers and then based on the standards of environment and pollution-allowed thresholds that are gained through experiences, scientific evidence and legal requirements desired density of development is determined. Determinants of bed carrying capacity of this study will focus on municipal infrastructure capacity and seven factors of water supply, sewage treatment, waste treatment, energy, green areas, public transportation systems and roads. Population density index due to its direct relationship with the amount of pressure on each factor was used to determine intervals of the range and classification of pressure on the land, in which the upper bound and lower bound of each interval was determined based on production optimal per capita or consumption as well as based on targeted levels of services and actual demand for resources. This article study district isthe 9th region of Mashhad municipality located in the West of the city. 3 - Discussion According to the findings of this study, at this time, carrying capacity of more than 80 percent of city blocks of understudy region is passed allowable thresholds in terms of green areas, water supply, and wastewater treatment and they are in critical pressure range. In terms of energy, waste treatment, public transportation and roads, themajority of city blocks are in desirable and allowable thresholds and they are in good condition. Average of floor area ratio in terms of each seven factors of green areas, water supply, sewage treatment, waste treatment, energy, public transportation system and roads were calculated 119.4%, 90.6%, 450.8%, 310.3%, 243.8% and 216.5%, respectively that each of the components may vary depending on the area and so on. To assess the region carrying capacity, the results of the seven factors were combined and integrated by using GIS and based on Liebig’s Law of The Minimum. Results showed that more than 80% of city blocks in under study region were passed allowable thresholds, and they are in critical pressure range now. Later, floor area ratio was achieved equal to 90.6% for the region based on the minimum value obtained from among the determinants factors of carrying capacity that is very different with the situation in the region. 4 - Conclusion In this paper, the optimal thresholds of urban density were studied based on carrying capacity in 9th District of Mashhad and the results showed that the distribution of population and allocation of density development in this region is not based on the carrying capacity and no serious paying attention was made about it. In this study, unlike previous studies, two fundamental points will be focused: first, the number of people that can be sustained in a given area of land depends on their cultures because, in any land, it is people culture that determines their needs and lifestyles, and secondly, in accordance with change in technological levels and consumption over time, the goals and the demand for resources and also environment capabilities will be changed to meet the actual needs of the urban dwellers. As carrying capacity approach is only useful when the thresholds are set ahead of time so it is suggested that we need to use the concept of environmental tolerance capacity as a comprehensive tool for planning before negative effects of population pressure in urban` planning and management process and especially new development in Mashhad.
Keywords:environment, floor area ratio, sustainable development, urban carrying capacity.