ارزیابی خدمت اکوسیستمی کاهش خطر سیلاب شهری با رویکرد دوره بازگشت بلندمدت 25 ساله، مطالعه موردی: کلان‌شهر تبریز

ABSTRACT
Urban districts, especially metropolises, increase impervious surfaces, leading to large volumes of rainwater runoff on the ground surface rather than penetrating underground. This can result in urban hazards such as urban floods and the negative, irreparable consequences that follow. In the meantime, urban green infrastructure plays a significant role in mitigating urban flood risk due to its low cost and its provision of ecosystem regulation services. Therefore, this study aims to evaluate the ecosystem service of mitigating urban flood risk in Tabriz metropolitan with a 25-year return period. In this research, data from Landsat satellite images, land use/land cover, meteorology, a biophysical table, GIS, and the InVEST software were used. The findings showed that in 1984, during 15-, 30-, and 45-minute rainfall events, the volume of water absorbed and retained was 6.77, 5.47, and 4.72 million cubic meters, respectively, and the ecosystem service benefit of mitigating urban flood risk was 17.19, 13.89, and 11.99 million dollars, respectively. In 2002, during 15-, 30-, and 45-minute rainfall events, the volumes of water absorbed and retained were 7, 5.62, and 4.83 million cubic meters, respectively, and the ecosystem service benefits of mitigating urban flood risk were 32.08, 25.78, and 22.16 million dollars, respectively. In 2022, during 15-, 30-, and 45-minute rainfall events, it was 7.85, 6.15, and 5.20 million cubic meters, respectively, and the ecosystem service benefit of mitigating urban flood risk was 56.08, 43.96, and 37.14 million dollars for the entire Tabriz metropolitan area. The results showed that land use/land cover had a greater role in the potential for runoff absorption and retention in Tabriz than other factors, including soil hydrological group. In Tabriz metropolitan, across all three time periods, wasteland, agricultural land, and low residential density land uses had the highest runoff absorption and retention, while water and pasture land uses had the lowest. The results also showed that in Tabriz metropolis, during all three time periods, districts 6, 5, and 7 had the highest runoff absorption and retention, while districts 9 and 8 had the lowest
Extended Abstract
Introduction
Urban districts, especially metropolitan areas, with increasing impervious surfaces, cause stormwater runoff from rain to flow on the ground surface rather than penetrate underground, thereby posing urban hazards, including urban floods and their negative, irreparable consequences. In the meantime, gray stormwater infrastructure, due to its age and cost, lacks the capacity to comprehensively address urban flooding and its negative consequences, whereas green infrastructure, due to its low cost and provision of ecosystem regulation services, plays a significant role in mitigating urban flood risk. Therefore, the purpose of this study is to evaluate the ecosystem service of mitigating urban flood risk in Tabriz metropolitan with a 25-year return period.
 
Methodology
The current research is descriptive-analytical in terms of method and has a developmental-applicative nature. The required information was collected using library, documentary, electronic sources, surveys, and field observations. In this research, the urban flood risk mitigation model from the InVEST 3.12.0 software package has been used. This model is one of the models that mitigates the risk of urban flooding based on the vector map of the study area/watershed map of the study area, rainfall (in millimeters), land use/land cover map, soil hydrological group raster map, the biophysical table, vector map of built infrastructure, and table of damage caused by urban flood estimates. Finally, this model estimates the result through the following raster and vector files:
1) It calculates the amount of runoff in the form of raster data.
2) It estimates the amount of absorption and retention of runoff (in millimeters) in the form of a raster file that shows the relative amount of rainfall (expressed as a percentage of rainfall).
3) It calculates the volume of runoff retention (in cubic meters) through a raster file.
4) It calculates the flood risk in the form of a descriptive table and the field of vector files and raster data that help to identify spatial changes in the specified local limits through the calculated values.
5) It estimates the damage caused by the flood through a monetary assessment.
6) It estimates the monetary value of the ecosystem service of urban flood risk reduction using the avoided damage cost method in the form of a descriptive table.
 
Results and discussion
The findings showed that in 1984, with a rainfall of 21.93 mm (0.000022 cubic meters) and a 15-minute rainfall and a 25-year return period, the amount of water absorbed and retained was 49 percent, the volume of water absorbed and retained was 6.77 million cubic meters, and the ecosystem service benefit of mitigating urban flood risk was $17.19 million. With a 30-minute rainfall, the amount of water absorbed and retained was 62 percent, the volume of water absorbed and retained was 5.47 million cubic meters, and the ecosystem service benefit of mitigating urban flood risk was $13.89 million. During a 45-minute rainfall event, 70 percent of the rainfall was absorbed and retained, totaling 4.72 million cubic meters, and the ecosystem service benefit of mitigating urban flood risk was $11.99 million for the entire Tabriz metropolitan area. In 2002, with a 15-minute rainfall of 14.07 mm (0.000014 cubic meters), the amount of water absorbed and retained was 51 percent, the volume absorbed and retained was 7 million cubic meters, and the ecosystem service benefit of mitigating urban flood risk was $32.08 million. With a 30-minute rainfall, the amount of water absorbed and retained was 64 percent, the volume of water absorbed and retained was 5.62 million cubic meters, and the ecosystem service benefit of mitigating urban flood risk was $25.78 million. With a 45-minute rainfall, the amount of water absorbed and retained was 72 percent, the volume of water absorbed and retained was 4.83 million cubic meters, and the ecosystem service benefit of mitigating urban flood risk was $22.16 million for the entire Tabriz metropolitan area. In 2022, with a rainfall of 10.78 mm (0.000011 cubic meters) and a 15-minute rainfall, the amount of water absorbed and retained was 57 percent, the volume of water absorbed and retained was 7.85 million cubic meters, and the ecosystem service benefit of mitigating urban flood risk was 56.08 million dollars. With a 30-minute rainfall, the amount of water absorbed and retained was 70 percent, the volume of water absorbed and retained was 6.15 million cubic meters, and the ecosystem service benefit of mitigating urban flood risk was 43.96 million dollars. With a 45-minute rainfall event and a 25-year return period, the amount of water absorbed and retained was 77 percent, the volume absorbed and retained was 5.20 million cubic meters, and the ecosystem service benefit of mitigating urban flood risk was 37.14 million dollars for the entire Tabriz metropolitan area.
 
Conclusion
The results showed that with increased rainfall duration (15, 30, and 45 minutes) across all three periods (1984, 2002, and 2022), the amount of runoff absorption and retention, and consequently the ecosystem service benefit of reducing urban flood risk, decreased. The results also showed that districts with higher population density and consequently higher residential land use had the lowest potential for runoff absorption and retention. In Tabriz metropolitan, due to the small amount of land uses related to green infrastructure, including green space, agricultural lands, and pastures, which play a major role in mitigating urban flood risk, these land uses have not been able to play a significant role in mitigating urban flood risk. However, green infrastructure has played a greater role than water infrastructure in mitigating the risk of urban flooding in Tabriz. Also, land use/land cover has played a greater role than other factors, including the soil hydrological group. Despite the low capacity of Tabriz's runoff absorption and retention, the role of ecosystem services in mitigating the risk of urban flooding in Tabriz remains significant. Without this role, Tabriz would have to bear the high costs of potential flood risk. As the trend of increasing impervious surfaces in the Tabriz metropolitan area continues, which has increased continuously over recent decades, the capacity for runoff absorption and retention in Tabriz will decrease further, and as a result, the volume of possible floods and the resulting economic damage will increase significantly.
 
Funding
There is no funding support.
 
Authors’ Contribution
Authors contributed equally to the conceptualization and writing of the article. All of the authors approved thecontent of the manuscript and agreed on all aspects of the work declaration of competing interest none.
 
Conflict of Interest
Authors declared no conflict of interest.
 
Acknowledgments
We are grateful to all the scientific consultants of this paper.