The agrihood design: Valuation of ecosystem services for NbS visions in Peri-Urban Bangkok, Thailand

Nature-based Solution (NbS) designs increasingly are being implemented to reduce environmental impacts of urban development and enhance community resiliency to disruptions ranging from floods to climate change to Covid-19. Ruangpan et al. (2020) characterize NbS as taking ...participatory, holistic, integrated approaches, using nature to enhance adaptive capacity, reduce hydro-meteorological risk, increase resilience, improve water quality, increase the opportunities for recreation, improve human well-being and health, enhance vegetation growth, and connect habitat and biodiversity. Clearly, water sensitive urban design (WSUD) (or Sustainable Urban Drainage Systems (SUDS), or Low Impact Development (LID), or Sponge City) concepts are related to NbS, although in general NbS has a broader design focus that includes recreational and community wellbeing and could be applied to non-urban development, such as coastal shoreline protection. Frequently, however, NbS designs are implemented with the assumption that ecosystem service benefits will accrue, but there is limited or no evaluation of the pre-construction design to ensure such benefits are optimized, or post-construction follow-up monitoring to verify benefits. The objective of this study was to develop and trial an assessment approach for the valuation of ecosystem services in a peri-urban area of Bangkok, Thailand. We used the general InVEST (Integrated Valuation of Ecosystem Services and Tradeoffs) framework from Stanford’s Nature Capital Project (https://naturalcapitalproject.stanford.edu/software/invest) to guide our assessment. In our evaluations we considered the ecosystem service benefits of: i) water yield; ii) sediment yield; iii) nutrient yield; iv) carbon sequestration; v) urban heat island mitigation; vi) crop production; vii) habitat quality; and viii) aesthetics, although only the first six have been quantified at this point and are discussed in this presentation. However, InVEST is a more general modeling approach that is better suited for evaluation of larger regional spaces than feature-specific designs and neighbourhood-scale systems. As such, for the water, sediment, and nutrient yield assessments we employed PCSWMM, which provided the added benefit of explicitly representing water quantity and quantity processes that can improve decision-making related to performance of the designed system.

Our ecosystem services valuation approach was tested using three case studies in peri-urban Bangkok, Kluong Luong, Pathum Thani: i) a 34.9 ha area in its currently undeveloped, open and scrub forest state (Khlong 3); ii) the same area as (i), but developed using an Agrihood design concept; and iii) an existing 7.3 ha typical new single detached housing development in the area (Siam HighVille).  The Agrihood design concept uses an integrated community garden-domicile approach that focuses on natural management of water to enhance liveability and community wellbeing. Importantly, it includes ample garden space intended to build a sense of community and wellbeing, integrate better into the surrounding peri-urban landscape, and provide increased food security.

PCSWMM modeling was done using a two phase approach. The first phase was to model each site using 24-hour SCS Type II design storms ranging between 30 and 150 mm to explore the hydrologic and water quality dynamics in a controlled approach. The second phase employed continuous modeling driven by local meteorologic data for the month of October, 2021. PCSWMM results indicated that there would be no surface flooding at Siam HighVille for the 90 mm, 24-hour design storm and minor flooding at the 120 mm, 24-hour design storm, which is consistent with local observation for the site. PCSWMM results also suggested that Khlong 3 (current conditions) and the Agrihood design would experience no surface flooding at the 150 mm, 24-hour design storm. Water quality sampling conducted in Khlong (canal) 3 on 9 sample dates between September, 2021 and January, 2022 showed the mean concentrations of TSS, TP, and TN to be 29.1 mg/L; 0.2 mg/L; and 5.9 mg/L, respectively. PCSWMM results using a combination of MUSLE for pervious surfaces and buildup/washoff for impervious surfaces were able to accurately estimate the concentrations. A series of constructed wetlands were included in the Agrihood design and these were represented as storage nodes in PCSWMM.  Model results indicated that the wetlands should be effective in reducing mean nutrient levels to below eutrophic conditions.

For the ecosystem services valuation, we used infiltrated water volume (mm) as the indicator for water yield, which reflects aquifer recharge for potable water; and TSS, TN, and TP loadings (kg/ha) and event mean concentrations for a design storm as the sediment and nutrient yield indicators. Carbon sequestration (tons/ha) and urban heat island mitigation (based on a cooling index that considered shading, albedo, and evapotranspiration) were assessed based on the InVEST numerical approaches. Current Khlong 3 conditions had the best infiltration indicator due to low percent imperviousness, followed by the Agrihood design and then Siam HighVille. The Agrihood design had lower TSS and nutrient yields and concentrations compared to Siam HighVille. None of the sites had a high carbon sequestration result, although not surprisingly the current Khlong 3 conditions reflected the best value, followed by the Agrihood design and then Siam HighVille. Khlong 3 current conditions reflected the best cooling index value, but the Agrihood design closely followed the Khlong 3 current conditions. Crop production for the Agrihood design would be in the range of $3,859 USD/ha/year, with the other sites having negligible crop production.

This study represents a first effort in developing a practical application framework that would facilitate assessment of NbS ecosystem services to compare different peri-urban development trajectories. Results of the ecosystem services assessment showed that the Agrihood design could provide a positive alternative compared to recent suburban design, but also indicated where design could be enhanced (e.g. urban cooling index). The next steps in this research will be to expand the analysis to other urban designs (current and proposed) and monetizing the ecosystem services.

Reference

Ruangpan, Laddaporn, Zoran Vojinovic, Silvana Di Sabatino, Laura Sandra Leo, Vittoria Capobianco, Amy MP Oen, Michael E. McClain, and Elena Lopez-Gunn. "Nature-based solutions for hydro-meteorological risk reduction: a state-of-the-art review of the research area." Natural Hazards and Earth System Sciences 20, no. 1 (2020): 243-270.

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