Journal of Watershed Management Research

Introduction and Objective: Water management in Iran is crucial due to resource scarcity and population growth. Urban runoff, an alternative and less-utilized water source, can turn into floods with increased urban impervious surfaces. Utilizing ponds to store this runoff and supply water needs, especially for green spaces, is an effective solution. This study aimed to evaluate the potential for runoff harvesting in Mahmoudabad city, Mazandaran, assess the feasibility of using existing ponds, and identify optimal locations for storage reservoirs using the SWMM model and AHP method
Material and Methods: The study area in this research is the city of Mahmoudabad in Mazandaran province, with an area of approximately 510 square kilometers. Due to its geographical location and annual rainfall (around 1000 mm), this city has good potential for runoff harvesting. Two main water reservoirs (abbandans) named Ghellaben and Karbalayi Mirzaali, with areas of 12 and 11 hectares respectively, exist within the study area, and the agricultural lands covered by them are mainly rice paddies.To achieve the research objectives, a four-stage approach was considered, including simulating the current situation with the SWMM model, rehabilitation and improvement of the existing water reservoirs, evaluating the conditions after improvement, and locating new ponds using the AHP method if necessary. Initially, the urban watershed of Mahmoudabad was divided into 5 urban hydrological units and 2 units related to the agricultural lands of the water reservoirs. The required parameters for the SWMM model, including land use, slope, imperviousness percentage, and drainage network characteristics for each sub-catchment, were determined using GIS software and field data. The SWMM model was calibrated and validated using rainfall data. To design the input rainfall for the model, intensity-duration-frequency curves of the region were used. To estimate the harvestable runoff, simulations were performed for return periods of 2, 5, 10, 25, and 50 years. To comply with environmental considerations, the environmental flow, equivalent to the flow before urban development, was subtracted from the total runoff. To calculate the water requirements for green spaces and agricultural lands, the FAO Penman-Monteith method and Cropwat 8.0 software were used. The water requirement for rice in the lands under the command of the water reservoirs and the water requirement for green spaces in the urban hydrological units were calculated. In the location finding section, the Analytic Hierarchy Process (AHP) method was used. The hierarchical structure included the goal (determining suitable locations for runoff storage), criteria (land use, rainfall amount, generated runoff volume, water demand for green space irrigation, drainage network status, land slope, water demand, runoff to demand ratio, spatial distance between the runoff source and the consumption location), and options (potential runoff storage locations). Pairwise comparisons of the criteria were performed using Saaty's 9-point scale, and the final weight of the criteria was calculated using the normalized geometric mean method. The consistency of the pairwise comparison matrix was checked by calculating the Consistency Ratio (CR). Finally, suitability levels for different classes of each criterion were determined, and the options were ranked based on their final score.
Results: The results showed that the total runoff volume generated in the city of Mahmoudabad is about 1,182,606 cubic meters. After deducting the environmental flow, the harvestable runoff volume was estimated to be 713,337 cubic meters. Table 3 shows the values of total runoff, environmental flow, and harvestable runoff for different return periods in each sub-catchment. The calibration of the SWMM model was performed using HEC-HMS software and the indices of Relative Error (RE) and Root Mean Square Error (RMSE), which showed acceptable results.
The results of the assessment of the existing water reservoirs indicated that their current capacity is not sufficient to store a significant volume of runoff. By carrying out rehabilitation operations, it is possible to increase the storage volume of Ghellaben water reservoir to 228,000 cubic meters and Karbalayi Mirzaali water reservoir to 484,000 cubic meters. This increase in capacity will be achieved by increasing the depth of the water reservoirs to at least 4 meters.  The annual water requirement for rice in the lands under the command of the water reservoirs was estimated to be approximately 1227 mm per hectare. The water requirement for green spaces also varied in different urban sub-catchments. The results showed that urban runoff harvesting can supply about 20% of the water requirement for green spaces in the city of Mahmoudabad. Table 5 presents the water requirement for rice and green spaces in each sub-catchment. In the site selection section using the AHP method, the final weights of the effective criteria in determining suitable locations for runoff storage were calculated. "Rainfall amount" with a weight of 0.3074 and "Generated runoff volume" with a weight of 0.2056 were the most important criteria in this regard. "Land use" and "Drainage network status" also had moderate importance. Table 7 shows the pairwise comparison matrix of the criteria, and Table 8 shows the final weights of the criteria. The suitability levels for different classes of each criterion were also determined, indicating that areas with higher rainfall and runoff, high water demand for green spaces, good drainage network, low slope, and low soil permeability have higher suitability for runoff storage. Table 9 provides the suitability levels for different classes of the criteria. Finally, out of 8 examined nodes, 2 nodes were selected as suitable locations for runoff harvesting and storage.

Conclusion: The study demonstrated that urban runoff harvesting in Mahmoudabad has significant potential for water resource management and reducing pressure on groundwater. The harvestable runoff volume is estimated at about 713,337 cubic meters. Rehabilitating existing ponds, especially Ghalban and Karbalai Mirza Ali, significantly contributes to collecting this runoff and meeting some agricultural and green space water needs (up to 20% of green space demand). The SWMM model and AHP method are effective tools for evaluating this potential and optimizing storage reservoir locations. Implementing comprehensive urban runoff management plans can contribute to sustainable water management in Mahmoudabad.