Over the past 60 years, rapid urbanization in Seoul has significantly altered the water cycle. This has led to various urban water management issues, ultimately posing direct threats to the sustainability of the city and the safety of its residents in the form of climate crises. In response, Seoul has been institutionalizing water cycle management and implementing policy initiatives for over 20 years. However, the ambiguity in setting management goals and evaluation methods has resulted in a decline in the sustainability and effectiveness of water cycle management policies. To revitalize water cycle recovery policies, it is essential to establish management indicators that can quantitatively assess the state of the water cycle using relatively simple methods. Additionally, setting recovery goals based on these indicators and conducting evaluations of implementation plans and projects will strengthen the sustainability and effectiveness of water cycle policies.
This study establishes the basic criteria for evaluating the state of the water cycle based on the conditions that allow sufficient water to be retained within a watershed. It develops indicators and calculation formulas for evaluating the state of the water cycle that can quantitatively assess the state of the water cycle by utilizing widely used simplified formulas for key water cycle elements (evapotranspiration, surface runoff, soil maximum potential retention, etc.). The proposed method evaluates the state of the water cycle based on the quantitative ratio of elements that increase and decrease the water retention in the watershed (basic water cycle rate). Furthermore, recognizing that the state of the water cycle is significantly influenced not only by natural watershed characteristics but also by human activities (artificial water cycle elements), the evaluation method incorporates these factors. In the case of Seoul, significant changes in water retention within the watershed are influenced by factors such as rainwater utilization and management facilities, groundwater use, and outflow of groundwater, which are reflected in the proposed Seoul water cycle indicator.
To calculate the water cycle indicator for each district in Seoul, data on runoff coefficients, average annual rainfall, monthly average temperatures, groundwater use, outflow of groundwater, and water cycle management flow were collected and analyzed by district. The runoff coefficient, which indicates the runoff characteristics based on land use and purpose, is the most influential factor affecting changes in the water cycle indicator. By incorporating the watershed's imperviousness and the slope, the runoff coefficients were detailed for over 70 land use types. Average annual rainfall was estimated using five years of data (2020-2024) from 38 AWS observation points within and around Seoul, employing Thiessen polygon analysis to calculate the average rainfall for each district. Monthly average temperatures were analyzed using environmental data from 834 S-Dot sensors installed throughout Seoul. Data on rainwater utilization and management flow, as well as groundwater use and outflow, were analyzed using publicly available information from the Seoul Water Cycle Information System.
The analysis of the basic water cycle indicator (a metric reflecting only natural water cycle elements) for each district in Seoul revealed that some districts, including Nowon-gu (0.99), Seocho-gu (0.98), Yongsan-gu (0.96), Eunpyeong-gu (0.89), Dobong-gu (0.88), Gangbuk-gu (0.88), and Gwanak-gu (0.83), which include mountainous areas, show water cycle rates above 0.9, indicating a relatively balanced state of water inflow and outflow. In contrast, districts such as Jung-gu (0.57), Yeongdeungpo-gu (0.60), Geumcheon-gu (0.61), Gwangjin-gu (0.63), and Dongdaemun-gu (0.64) exhibit very high runoff coefficients, leading to a state of water cycle imbalance where outflow significantly exceeds inflow.
When analyzing the Seoul water cycle indicator by additionally considering groundwater use and outflow, as well as rainwater utilization and management capacity, the general trends between districts with high and low basic water cycle rates were similar. However, the overall distribution showed a decrease of between 0.06 and 0.42 compared to the basic water cycle indicator. Notably, in districts such as Seongdong-gu (0.70→0.28), Dongdaemun-gu (0.64→0.28), Jung-gu (0.60→0.29), Yeongdeungpo-gu (0.57→0.23), and Songpa-gu (0.73→0.45), the Seoul water cycle indicator significantly decreased compared to the basic water cycle indicator. These districts are characterized by a high concentration of subway lines and stations, as well as apartment complexes and large buildings, making them the areas with the highest occurrence of outflow groundwater in Seoul. The analysis indicates severe distortions in the water cycle due to artificial water cycle elements in these districts.
Based on the analysis of the basic and Seoul water cycle rates for each district, several proposals for recovering the water cycle in Seoul are made. First, it is necessary to implement institutional improvements to suppress the increase in runoff coefficients and manage them gradually through the expansion of eco-friendly land use conversions during redevelopment and reconstruction projects. Second, to reduce the occurrence of outflow groundwater and enhance soil retention, it is essential to establish groundwater retention promotion areas and create a regulatory framework for managing outflow groundwater. Third, for regions that require improved water cycle indicator, it is recommended to add a weighting factor for water cycle management flow, thereby promoting the installation of rainwater utilization and management facilities and expanding water cycle management infrastructure.
Seoul must establish a water cycle recovery strategy that considers the characteristics of the water cycle, such as high impervious surface rates and complex urban structures composed of extensive underground spaces and facilities, as a result of high urbanization. Specifically, areas where human activities have intensified distortions in the water cycle (characterized by significant differences between basic and Seoul water cycle rates) must minimize distortions caused by artificial water circulation. Additionally, in areas with high impervious surface rates where water cycle imbalance is exacerbated, efforts should focus on transitioning to eco-friendly land use with low imperviousness through long-term urban planning approaches to improve water cycle imbalances.
The proposed water cycle indicator in this study can quantitatively assess the state of the water cycle using just three measured factors, making data collection and analysis straightforward. This simplicity and ease of understanding make it a valuable tool for the establishment of water cycle recovery strategies in Seoul.