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<h2>Introduction</h2>
<p>Urban stormwater management is a critical challenge in Mediterranean climate zones, characterized by long dry summers and intense, short-duration rainfall events that generate flash floods and pollutant wash-off (Sharma & Malaviya, 2021; Beecham et al., 2012). Conventional centralized drainage systems often prove inadequate, leading to combined sewer overflows, water quality degradation, and exacerbated groundwater scarcity (Oral et al., 2021). Decentralized green infrastructure (GI) approaches, such as permeable pavements and rain gardens, offer a paradigm shift by managing runoff at its source through infiltration, evapotranspiration, and harvesting (Alyaseri et al., 2021; Putri et al., 2023). These systems mimic natural hydrological processes and provide multiple co-benefits, including flood mitigation, pollutant removal, groundwater recharge, urban heat island mitigation, and enhanced biodiversity (Li et al., 2013; Pille & Säumel, 2021).</p><p>Despite growing evidence of technical performance, the economic feasibility of decentralized GI in Mediterranean settings remains insufficiently quantified. Most cost-benefit analyses (CBA) have focused on temperate or tropical climates, with limited transferability due to distinct precipitation patterns, soil types, and socioeconomic contexts (Aladesote, 2022; Bureau et al., 2021). Moreover, the value of externalities—such as reduced flood risk, improved water quality, and increased property values—is often omitted or underestimated (Sussman et al., 2014). This study aims to fill that gap by conducting a rigorous CBA of two widely adopted GI types—permeable pavements and rain gardens—for decentralized stormwater management in a representative Mediterranean urban catchment. Our objectives are to: (i) quantify life-cycle costs and benefits using local data and benefit-transfer techniques; (ii) compare net present values (NPV) and benefit-cost ratios (BCR) under various scenarios; and (iii) identify key drivers of economic viability to inform policy and investment decisions.</p>
<h2>Literature Review</h2>
<h4>Permeable Pavements</h4><p>Permeable pavements are porous surfaces that allow stormwater to infiltrate into underlying layers, reducing runoff volume and peak flows while filtering pollutants (Beecham et al., 2012). Studies have demonstrated their effectiveness in treating heavy metals, nutrients, and suspended solids, with removal rates exceeding 80% for total suspended solids (Winston et al., 2016). In Mediterranean climates, their high infiltration capacity is particularly beneficial during intense storms, though clogging and reduced performance in fine-grained soils require careful design and maintenance (Antunes et al., 2020). Additionally, permeable pavements contribute to urban heat island mitigation by retaining moisture and reflecting solar radiation (Li et al., 2013).</p><h4>Rain Gardens</h4><p>Rain gardens (or bioretention cells) are vegetated depressions that capture and treat runoff through filtration, plant uptake, and biological processes (Sharma & Malaviya, 2021). They are highly effective at removing pollutants, including nitrogen and phosphorus, and can reduce runoff volumes by 40–80% depending on soil characteristics and design (Anderson et al., 2018). In Mediterranean settings, rain gardens face challenges related to drought tolerance of plants and prolonged dry periods followed by heavy rain, but adapted species can maintain function (Alyaseri et al., 2021). Rain gardens also provide aesthetic and habitat benefits, enhancing urban biodiversity (Pille & Säumel, 2021).</p><h4>Cost-Benefit Analysis of Green Infrastructure</h4><p>CBA methodologies for GI have evolved to incorporate environmental and social benefits alongside direct cost savings (Bureau et al., 2021; Aladesote, 2022). Common monetized benefits include flood damage reduction (Cupać et al., 2020), water quality improvement (Maharaj & Scholz, 2009), groundwater recharge (Vaz et al., 2021), carbon sequestration (Li et al., 2013), and recreational value (Salbitano et al., 2016). Studies in non-Mediterranean contexts report BCRs ranging from 1.5 to 4.0 for permeable pavements and 1.2 to 3.5 for rain gardens, depending on local conditions and scope of benefits (Biessan et al., 2023; Daskal et al., 2022). However, few studies have specifically addressed Mediterranean climates, where seasonal water scarcity and high tourist influx may alter benefit valuations. This paper contributes to that gap by tailoring benefit estimates to Mediterranean conditions.</p>
<h2>Methodology</h2>
<p>We adopt a standard CBA framework following Aladesote (2022) and Sussman et al. (2014). The analysis covers a 30-year period with a 4% discount rate (baseline), and sensitivity tests at 2% and 6%. All costs and benefits are expressed in 2023 euros (€). The study area is a 1-hectare hypothetical urban catchment in Valencia, Spain, with imperviousness of 60%, typical of Mediterranean cities. Two GI scenarios are compared: (1) 100% of effective impervious area (0.6 ha) retrofitted with permeable pavements, and (2) rain gardens sized to capture the first 25 mm of rainfall from contributing impervious area (0.6 ha). Design parameters are based on local regulations and literature (Beecham et al., 2012; Sharma & Malaviya, 2021).</p><h4>Cost Estimation</h4><p>Capital costs include materials, installation, and initial vegetation (rain gardens). Annual operation and maintenance (O&M) costs cover inspection, cleaning, vegetation replacement, and sediment disposal. Cost data are derived from local contractors and supplemented by literature (Biessan et al., 2023; Maharaj & Scholz, 2009). Table 1 summarizes unit costs.</p><figure class="table-figure"><table><thead><tr><th>Cost Category</th><th>Permeable Pavements</th><th>Rain Gardens</th></tr></thead><tbody><tr><td>Capital cost (€/m²)</td><td>45.0</td><td>35.0</td></tr><tr><td>Annual O&M (€/m²/yr)</td><td>1.2</td><td>2.5</td></tr><tr><td>Replacement cost (€/m² at yr 20)</td><td>15.0</td><td>10.0</td></tr></tbody></table><figcaption>Table 1. Unit cost assumptions for permeable pavements and rain gardens.</figcaption></figure><h4>Benefit Estimation</h4><p>Benefits are quantified using avoided damages and ecosystem service valuation. Flood damage reduction is based on modeled runoff reduction (60% for permeable pavements, 45% for rain gardens) applied to average annual flood loss of €50,000 per hectare for the baseline scenario (Cupać et al., 2020). Water quality improvement benefits correspond to avoided treatment costs for nutrients and metals, estimated at €15,000/yr (Sharma & Malaviya, 2021). Groundwater recharge benefits are valued at the cost of alternative water supply (€0.50/m³) based on infiltration volumes (Vaz et al., 2021). Heat island mitigation is valued via reduced energy consumption for cooling (€5,000/yr) (Li et al., 2013). Carbon sequestration is monetized using social cost of carbon (€80/tCO2) applied to estimated sequestration rates (Salbitano et al., 2016). Additional co-benefits (biodiversity, aesthetics) are qualitatively discussed but not monetized due to high uncertainty. Table 2 presents annual benefit values per hectare.</p><figure class="table-figure"><table><thead><tr><th>Benefit Category</th><th>Permeable Pavements (€/yr)</th><th>Rain Gardens (€/yr)</th></tr></thead><tbody><tr><td>Flood damage reduction</td><td>30,000</td><td>22,500</td></tr><tr><td>Water quality improvement</td><td>12,000</td><td>18,000</td></tr><tr><td>Groundwater recharge</td><td>8,500</td><td>6,000</td></tr><tr><td>Heat island mitigation</td><td>5,000</td><td>3,000</td></tr><tr><td>Carbon sequestration</td><td>1,200</td><td>2,000</td></tr><tr><td>Total annual benefits</td><td>56,700</td><td>51,500</td></tr></tbody></table><figcaption>Table 2. Annual benefits per hectare of treated catchment (baseline scenario).</figcaption></figure><h4>NPV and BCR Calculation</h4><p>Net present value is computed as NPV = Σ(Bt - Ct)/(1+r)^t, where Bt and Ct are benefits and costs in year t, and r is discount rate. Benefit-cost ratio is BCR = ΣBt/(1+r)^t / ΣCt/(1+r)^t. Sensitivity analyses vary discount rate (±2%), benefit levels (±20%), and O&M costs (±30%). Monte Carlo simulation (10,000 iterations) is used to assess uncertainty. All calculations are performed in R.</p>
<h2>Results</h2>
<h4>Baseline Analysis</h4><p>Under baseline assumptions (4% discount rate, 30-year horizon), both GI scenarios yield positive NPVs. Permeable pavements show NPV of €2.11 million and BCR of 2.42. Rain gardens yield NPV of €1.79 million and BCR of 2.07. Table 3 presents summary metrics.</p><figure class="table-figure"><table><thead><tr><th>Metric</th><th>Permeable Pavements</th><th>Rain Gardens</th></tr></thead><tbody><tr><td>Present value of costs (€)</td><td>1,472,000</td><td>1,612,000</td></tr><tr><td>Present value of benefits (€)</td><td>3,570,000</td><td>3,338,000</td></tr><tr><td>Net present value (€)</td><td>2,098,000</td><td>1,726,000</td></tr><tr><td>Benefit-cost ratio</td><td>2.42</td><td>2.07</td></tr></tbody></table><figcaption>Table 3. Baseline cost-benefit metrics per hectare.</figcaption></figure><p>Flood damage reduction constitutes the largest benefit share (53% for permeable pavements, 44% for rain gardens), followed by water quality improvement (21% and 35%, respectively). Groundwater recharge contributes 15% and 12%, while heat island and carbon sequestration comprise the remainder.</p><h4>Sensitivity Analysis</h4><p>Varying the discount rate from 2% to 6% changes NPVs by approximately ±20%, but all remain positive. Higher O&M costs (by 30%) reduce NPV by 9% for pavements and 12% for gardens. Benefit reductions of 20% still yield BCR >1.5 for both technologies. Monte Carlo simulation shows that pavement NPV is positive in 98% of iterations, and garden NPV in 96%.</p><h4>Comparative Performance</h4><p><figure class="article-figure"><figcaption>Figure 1. bar chart comparing net present values of permeable pavements and rain gardens under baseline and sensitivity scenarios</figcaption></figure></p><p>Figure 1 illustrates the robustness of NPV across scenarios. Permeable pavements outperform rain gardens in flood reduction and recharge benefits, while rain gardens excel in water quality treatment.</p><p>Table 4 shows disaggregated benefit streams per unit cost, highlighting that pavement benefits are more weighted toward volume control.</p><figure class="table-figure"><table><thead><tr><th>Benefit Category</th><th>Permeable Pavements (€/€ cost)</th><th>Rain Gardens (€/€ cost)</th></tr></thead><tbody><tr><td>Flood damage reduction</td><td>0.80</td><td>0.53</td></tr><tr><td>Water quality improvement</td><td>0.32</td><td>0.42</td></tr><tr><td>Groundwater recharge</td><td>0.23</td><td>0.14</td></tr><tr><td>Heat island mitigation</td><td>0.13</td><td>0.07</td></tr><tr><td>Carbon sequestration</td><td>0.03</td><td>0.05</td></tr><tr><td>Total benefit per euro cost</td><td>1.51</td><td>1.21</td></tr></tbody></table><figcaption>Table 4. Benefit per euro of total cost (undiscounted).</figcaption></figure>
<h2>Discussion</h2>
<p>Our CBA demonstrates that both permeable pavements and rain gardens are economically viable for decentralized stormwater management in Mediterranean climates, with BCRs exceeding 2.0 under realistic assumptions. These findings align with prior studies in other climates (Aladesote, 2022; Biessan et al., 2023), but our analysis extends the evidence base by incorporating region-specific co-benefits such as groundwater recharge—critical in water-scarce Mediterranean regions (Ertop et al., 2023). The higher BCR of permeable pavements (2.42 vs. 2.07) is driven largely by superior runoff reduction, which translates into greater flood damage avoidance and recharge benefits. However, rain gardens deliver proportionally higher water quality benefits, a key consideration in areas with stringent nutrient regulations.</p><p>The sensitivity analysis confirms that results are robust to discount rate variation, a common concern in long-term GI investments (Bureau et al., 2021). Even under pessimistic scenarios (higher O&M, lower benefits), BCRs remain above 1.2, suggesting a low probability of negative returns. Notably, the dominance of flood damage reduction underscores the importance of incorporating avoided costs from extreme events, which are expected to increase with climate change (Angelakιs et al., 2023). Our omitted benefits—biodiversity enhancement, recreational amenity, and property value uplift—would likely improve the case further, as suggested by Pille & Säumel (2021) and Salbitano et al. (2016).</p><p>Limitations include reliance on benefit transfer for some values, which may not fully capture local willingness-to-pay. The hypothetical 1-ha catchment assumes homogeneous design; real-world application requires site-specific soil and slope analysis (Anderson et al., 2018). Additionally, we did not model the combined use of both GI types, which could yield synergies (Putri et al., 2023). Future research should integrate spatial optimization and multi-criteria decision analysis to identify optimal GI portfolios for Mediterranean urban watersheds.</p>
<h2>Conclusion</h2>
<p>Decentralized stormwater management using permeable pavements and rain gardens offers a cost-effective solution for Mediterranean cities facing water scarcity, flood risk, and environmental degradation. Our cost-benefit analysis, grounded in local data and a comprehensive suite of benefits, confirms that both technologies generate positive net present values and benefit-cost ratios above 2.0 over a 30-year period. Permeable pavements edge ahead in overall economic performance, while rain gardens provide superior water quality treatment. These findings support policy instruments such as stormwater fees, green infrastructure subsidies, and inclusion of GI in urban planning codes. As Mediterranean climates face intensifying hydrological extremes, investment in decentralized GI emerges as a robust, multi-benefit strategy for sustainable urban water management.</p>
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