برآورد آب باران قابل استحصال از بام‌های کرمانشاه و شناسایی مکان‌های مستعد ذخیرة آب برای آبیاری فضای سبز شهری

Introduction
Water is considered as an important and determinative factor for the continuity of human, animal and plants living on the earth and biosphere. This vital material resolves the primary requirements of human such as drinking, agriculture, and industry. Nowadays, providing water is regarded as a basic challenge in developing countries, like Iran. Given the climate tensions and excessive consumption in most major cities, the problem of water scarcity can be minimized by harvesting rainwater. Since Rainwater Harvesting of roofs in some regions in Iran is considerably remarkable, it can provide some non-drinking requirements of citizens. Basic concept of rainwater harvesting is simple; it is the water can be collected from the roofs and appropriate surfaces, and conveyed downwards by pipes for saving. This water is saved in an appropriate place which can be used in proper time. If the saved water is considered for non-drinking consumption, no filtering will be needed; desalination is enough. It should be noted that rainwater needs to be filtered for drinking. Therefore, for arid and semi-arid regions such as Kermanshah, Rainwater Harvesting from the building roofs to supply water requirement of urban green places can solve a major problem of dehydration.
Material and method
The necessary climatic data for this study are precipitation statistic of synoptic station in Kermanshah during a 65-year period, from 1951 to 2016. They were used as annually and monthly average. Due to the climate of Kermanshah, precipitation period lasts from October to June. The urban data used includes the area of building roofs, the area of urban green places and trees, and thr volume of water required for irrigation. The area of building roofs (in meter) of Kermanshah city was extracted from aerial photos in Google Earth.  
To calculate the volume of harvesting rainwater, we used three components including precipitation, the area of building roofs and run-off coefficient. Run-off coefficient of building roofs is 0.7 by default. Moreover, the required water for irrigation of the green places of Kermanshah city is 12 liters per m² during 24 hours. Although there is rainfall during cold season of the year (January, February and March) with decrease in plant growth, irrigation is stopped. The water consumed for irrigation is on average 4 liters per m² in fall and spring mainly due to moderate weather and rainfall conditions. In the present study, WLC and AHP methods were applied to determine the appropriate location of rainwater saving. The evaluated variables to determine the place are including green places, unused places, agricultural lands in the city, suburb agriculture lands, slope and the proximity to the buildings which were gathered by questionnaire through experts of municipality officials, parks organization and urban green places of Kermanshah city. Moreover, it should be noted that some land uses are considered as the unsuitable places to save rainwater such as asphalt surface, roof surface, and all kinds of service land uses across the city.
Results and discussion
The built area of Kermanshah city is 108365238 m². The maximum precipitation usually occurs in March and April, 70.6 mm, while the minimum is occurs in July and August. The average annual precipitation is 441 mm. Besides, the maximum temperature of Kermanshah city is 38C in August, while the minimum is -3.6 C in January.
The biggest value is recorded for zone 3 in Kermanshah, 21.28 Km², and the greatest area of building roofs is recorded for district 4, 23.8 % with the highest building density. Moreover, the greenest places are related to zone 5, 6.4%. The highest water requirement is also related to zone 5, due to the green places and higher number of trees (8110 ones), 14.7%. The highest ratio of harvesting rainwater is calculated for zone 4 (due to more building density), while the least ratio is calculated for zone 8 (due to less building density). The ratio of the area of urban green places is 4.3% and the ratio of water requirement is 9.8%. The most percent reserve of rainwater requirement is related to zone 4 located in the southeast of the city, 76.5%; zone 3, in the east of the city. It is regarded in the second class. The least percent of reserve rainwater requirement is recognized for zone 8, located in the southwest Kermanshah city. As a matter of fact, it is important to recognize these regions for green places may be of the far regions to gather rainwater which require the transition and saving according to the effective variables. The total volume of water requirement is about 10620000 m³ and the volume of harvesting rainwater is 6200000 m³ which can provide 58.4% of water requirement.   
The highest effectiveness of locating is related to the variable of the locations with green places.  Unused places and agriculture lands in the city are in the next priority for rainwater saving, due to easier land use conversion. In fact, the areas of lower slope are more appropriate for rainwater saving. Besides, the proximity to the place for gathering rainwater is economically efficient. Totally, 42000 m² of the total area of Kermanshah city is more suitable for saving rainwater which is mostly located in district 5 in the northeast of the city. Building roofs, passages, medical centers, service centers, refinery and airports are not appropriate enough to save rainwater, they are approximately 42000000 m².
Conclusion
Rainwater is changed into run-off and flood-water in most of the cities in Iran which is moved out of the city by water channels. Rainwater Harvesting is of considerable importance due to the conservation and management of water sources. The present study is carried out to determine the amount of harvesting rainwater from the building roofs of Kermanshah city to supply water requirement of urban green places. The suitable places for rainwater harvesting were determined by WLC models. The findings show that 6200000 m³ (58.4%) out of total 10620000 m³ of annually water requirement of Kermanshah green places, is provided by rainwater harvesting of the building roofs. According to different environmental variables, about 42000 m² of urban area and suburb of the city are recognized as more appropriate regions for the saving rainwater. Zone 5 of Kermanshah city has better conditions compared with other zones.