Factors affecting rainwater resource management for the present and future include population growth, urbanization, and climate change. Rainwater harvesting (RWH) allows multiple urban water-related issues to be mitigated. In this study, a spreadsheet-based daily water balance model was developed to analyze the existing laws and regulations regarding the storage tank size of RWH facilities. Six buildings at different locations were selected for this study. Two are office buildings, two are school buildings, and two are sports buildings. The term “RWH facility evaluation criteria” is collectively used for rainwater supply satisfaction rate, rainwater guarantee rate, and rainwater utilization rate. A green roof can hold the rainwater for some time, reducing the peak flow and runoff volume. The results provide evidence that, among the selected studied buildings, buildings having a combination of a green roof and RWH facility score the highest in terms of RWH facility evaluation criteria, even though the actual tank size is much smaller than the standard tank size. This is the case with the Yesan County Office, in which a green roof connected to a small (66 m³) rainwater storage tank is installed. As a green roof can decrease the runoff volume, the rainwater can be managed efficiently with less pumping energy and only a small storage tank.

The increasing intensity of rainfall resulting from climate change is projected to worsen the inundation of urban areas by floods. As a viable alternative, stormwater harvesting presents an opportunity to enhance water supply reliability while reducing pressure on both water resources and urban stormwater drainage systems. The stored rainwater can be supplied with a radius of one kilometer around the storage facility, so less pumping energy is consumed compared to supplying the stored rainwater far away from the storage facilities. To visually depict the characteristics of the selected storage facility, land cover maps were created using ArcGIS. These maps were created for a circular region with a radius of one kilometer around each studied storage facility. In this study, a daily water balance model was formulated using spreadsheets to assess the potential of harvesting stormwater and rainwater for various pre-existing storage facilities. Five different types of storage facilities were selected for this study. The term “SRWH facility evaluation criteria” as a whole is used for the storm or rain (SR) water supply satisfaction rate, the SR guarantee rate, and the SR utilization rate. The results provide evidence that, for each selected studied storage facility, the SR water guarantee rate can potentially surpass 70% under conditions of low water demand. Moreover, we investigated the potential of the existing storage facilities to work as multifunctional resources, while the original purpose for which each facility was constructed remains un-affected.

Stormwater harvesting (SWH) is emerging as a vital adaptive strategy for urban climate resilience. In South Korea, different types of storage facilities have been constructed under different regulations and laws. Each type of storage facility has its own original purpose of construction. Although these facilities have better outcomes, we aim to investigate the potential use of these facilities as additional water resources. In this study, we assess the stormwater harvesting (SWH) potential of different types of already-constructed storage facilities. Five different types of storage facilities and three different cases are considered in the present study. Case 1 excludes SWH volume during the flood and winter seasons, while in Case 2, only winter season SWH volume is excluded. In Case 3, the winter season and combined sewer overflows (CSOs) facilities are excluded. The Rainwater Utilization Facility is considered as a baseline for comparison in the present study. The results show that, in Case 2, the Sewage Storage Facility, Stormwater Runoff Reduction Facility, Nonpoint Pollution Reduction Facility, and Buffer Storage Facility has 53.5, 4, 2.4, and 1.2 times more stormwater average annual usage potential, respectively. The findings suggest that these facilities can be utilized as additional water resources. It should be mentioned that the primary objective for which each facility was constructed will remain unaffected. Nevertheless, forthcoming research should focus on a detailed exploration of the quality of the collected stormwater and the energy required to supply the stormwater for the end usage.

Conventional centralized drainage systems exacerbate urban flooding, pollution, and water stress. Low-impact development (LID) is a decentralized alternative; however, its multifunctional benefits, which go beyond the control of stormwater, are often undervalued in planning. This study fills this gap by developing an integrated benefit valuation framework to systematically quantify and estimate the economic value of the co-benefits of five widely implemented LID facilities (vegetated swale, green roof, in-filtration ditch, infiltration trench, and permeable pavement) in Seoul, South Korea. The framework combines annual benefits in four key sectors: water management (runoff reduction), energy savings (building cooling/heating demands), air quality (pollutant deposition and avoided emissions) and climate change (carbon sequestration and mitigation). Applying a transparent, localized spreadsheet model, the results indicate significant multifunctional value for LID systems. While water management provides the primary benefit, there is substantial added value in energy, air quality, and climate co-benefits. In the case of green roofs, such ancillary benefits can exceed hydrological values. The analysis further reveals a consistent scale-benefit relationship and a clear trade-off between the magnitude of benefits and the cost of implementation. This provides evidence of the need for context-sensitive, portfolio-based LID planning. The proposed framework is a practical decision support tool for urban planners and policymakers to consider LID not only as a stormwater solution but also as multifunctional green infrastructure that simultaneously promotes urban water security, energy efficiency, environmental quality, and climate resilience.

Permeable pavement systems are a key low-impact development strategy for mitigating rainfall runoff in urban environments. However, their long-term effectiveness is often compromised by clogging from particulate matter, which progressively reduces permeability. This study evaluates long-term permeability of PPs installed at 13 sidewalk locations in Seoul, South Korea, over 38 months (May 2018–June 2021) using in-situ tests (KS F 2394). Results show that by the end of monitoring, eight sites had declined to Grade 5 (lowest classification), while five sites maintained Grade 3. According to Seoul guidelines, the minimum recommended permeability is Grade 3. These findings indicate that more than half of the sites fell below the recommended standard within 3 to 5 years of installation, highlighting the critical need for regular maintenance. The study aligns with recent research demonstrating that pressure washing can effectively restore permeability in clogged pavements. Based on these results, a guideline revision is proposed: adding a new top category (Grade 0) for superior permeability (≥1.5 mm/sec) and raising the minimum recommended permeability to Grade 2 to ensure longer-term performance before maintenance is required. Future research should develop more efficient cleaning techniques to sustain the hydrological function of existing permeable pavement installations in urban areas.

Climate change and socioeconomic pressures are intensifying water scarcity in arid regions, yet the translation of hydrological deficits into agricultural production losses remains insufficiently quantified. This study develops a hybrid WEAP–machine learning (WEAP–ML) framework coupling process-based hydrological simulation with policy emulation to project irrigation deficits and yield losses for five major crops (wheat, rice, maize, sugarcane, and cotton) from 2015–2100 under four Shared Socioeconomic Pathways (SSP126, SSP245, SSP370, and SSP585). Agricultural water demand is simulated using the WEAP Soil Moisture Method driven by CMIP6 projections, while Long Short-Term Memory and Random Forest models emulate policy-driven demand reductions without altering land-use assumptions. Results reveal a near-linear relationship between irrigation deficit and yield loss, consistent with the FAO-56 crop water production function. By 2100, projected irrigation deficits range from 21% to 32% across SSP scenarios, with aggregate yield losses of 22% to 34%. However, individual crop losses are substantially more severe—maize, sugarcane, and cotton exceeding 40% under SSP370 by 2098—approximately 30% higher than basin-scale averages. Although policy interventions reduce agricultural water demand, their effectiveness progressively declines under high-emission scenarios, indicating that adaptation alone may be insufficient to offset long-term climate impacts. The proposed framework provides a transferable, physically consistent decision-support approach for integrating hydrological processes, adaptive management, and agricultural productivity assessments under climate uncertainty, with applicability to other climate-vulnerable basins.