Ecological Landscape Design in Semi-Arid Areas on Basis of Water Sensitive Urban Design Approach (case study: Mohajeran City)

Increasing urbanization has a profound effect on the ecological structures. One of the most important challenges is water shortage. Concurrently, urban green space is major water consuming. The green space dependence on groundwater and drinking water in Iran has led to a decline in groundwater levels and increasing water demand. While, cities embrace impervious surface which not only create run off, but also it determinate recharge and flood in rainy seasons. Mohajeran is one of those cities facing with these problems. It seems that water sensitive urban design (WSUD), beside ecological design frameworks are able to represent appropriate solution. In this study First, the sources and uses of water is identified on basis of water sensitive design then, the strategies for green spaces development in the city of Mohajeran is presented. The results indicated that potable water resources (Runoff, rain and grey water) have a considerable potential to substitution with groundwater resources in watering the green spaces. Therefore, development strategies were proposed to develop drought resilient green space on basis of the best management practices such as urban runoff management, rainwater harvest in residential houses, sewage treatment.
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Keywords: Ecological Design, Water resource, Water Sensitive Urban Design, Mohajeran, Water shortage.
Introduction
Today, cities face a variety of challenges, but also complex, which is called as wicked problems. Climate change with increasing frequency and severity of rainfall, long periods of heat and drought, urban population growth and its consequences, water crisis and its supply challenges, environmental pollution in all its dimensions, and extensive changes in land use are among these issues. Meanwhile, the interaction of climate change, water and urban environments, and the challenges it poses, is a challenging part of the issue. There are different approaches to integrate managing water resources in urban areas. This theme was titled "Integrated drinking water management" between 1960 and 1970 by the Civil Engineering Society. This topic has been introduced as “low impact development” in the United States, “sustainable drainage system” in the UK, water sensitive urban design in Australia and New Zealand, “the sponge cities” in the Netherlands, or generally known as the Green-Blue Infrastructure.
Australia is antecedent in developing of water sensitive urban design (WSUD) approach due to climate change and drought. WSUD is based on decentralizing approach in water resources management which focuses on a local Practice. Its purpose is to Plan and design of urban fabric to manages and protect natural water cycles in the urban environment in a way that ensures the sensitivity of water management to hydrological, natural and ecological processes. This approach seeks to manage two contradictory problems of flood / runoff and water stress which caused by drought. For this reason, it tries to conceive the cycle of water as a multi-layered system and, avoiding isolated and fragmented approaches, manage water system in the artificial environment and ecosystem appropriately. WSUD Approach includes two key dimensions: the first dimension is water sensitivity consideration and design is the second dimension. In the first dimension, the integrity of water management in the urban environment is considered. While, the planning and design landscape in related to the management of water resources is addressed. In hydrology the concept of ecological design is well defined by Water Sensitive Urban Design (WSUD). Ecological design could be considered alongside of WSUD in landscape design which is any form of design that minimizes environmentally destructive impacts by integrating itself with living processes.
Nowadays, more than 500 cities in Iran face with water shortage problems. Mohajeran city is one of them which is the subject of this study. This city suffer difficulty emitting runoff during rainy season and suffers from drought in dry season. It seems that the best manner to curb these problems is following WUCD principles and employing ecological principles concurrently.
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Methods and materials:
Mohajeran city is located at the West of part of Markazi province. Mohajeran's climate is semi dry. The average precipitation of this region is 337 mm rain annually. The city is about 1600 hectares. Total green spaces area is about 160 hectares. Figure 1 shows the location of Mohajeran on the map of Iran, and indicates the study area in Mohajeran which is an urban residential district with an area of 0.19 km2.

Fig. 1 Location of the study area in Mohajeran, Iran
At first this article reviews WSUD literature related to water shortage conditions, collecting rain water, reusing of gray water in urban landscapes, promoting water consumption efficiency, then applies ecological principle to create an strategic plan to promoting urban landscape. The second part of research was conducted based on ecological design principles that expressed by Van der Ryn and Cowan in 1996. At first a review of WSUD literature related to water shortage conditions, collecting rain water, reusing of gray water in urban landscapes, promoting water consumption efficiency and ecological was carried out. Then we tried to use the ecological design and WSUD concepts in order to design more effective green landscape. To achieve this goal the exact volume of rainwater had to be estimated. So equation1 was used:
(1)
Where Q [m3] is the annual volume of collect able rainwater; Ca is the average runoff coefficient; S is the seasonal loss coefficient (the ratio of rainfall in rainy season to annual rainfall); I is the initial split-flow coefficient (the ratio of rainfall rejecting first flush to annual rainfall); A [m2] is the rainwater harvesting area; H [mm] is the rainfall with different occurrence probabilities; Ca was estimated by using Eqs. (2)
(2)
Where Ci is the runoff coefficient of different underlying surfaces; Ai [m2] is the areas of different underlying surfaces
It is believed that runoff is a precious resource and should have been used in Mohajeran city but unfortunately has been neglected and many problems such as inundation and flooding and water shortage in dry seasons have roots in ignoring it. Therefore, runoff must be stored and reused. To calculate cisterns which have capability to store water in rainy season and use it during dry season we use equation 3:
Cistern capacity can be estimated by using Eq4
(3)
Where Ca is the runoff coefficient of different underlying surfaces; Ai [m2] is the areas of different underlying surfaces and h [mm] is the rainfall with different occurrence probabilities
Another uncommon water resource in Mohajeran city is grey water. Passing some stages of purification will help it to be reused in landscape irrigation
Annual precipitation in the study area is 337 mm so by use of equation 1 the volume of the collectible water is 4021.7 m3 annually. The Maximum volume of runoff is for Rooftops and pavement areathe minimum amount of runoff. Figure 3 shows volume of collectible water in different land uses in the study area.