The Soil Water Apportioning Method (SWAM): An approach for long-term, low-cost monitoring of green roof hydrologic performance

• We developed a method to monitor green roof runoff and evapotranspiration, citywide.
• The method was validated for three extensive green roof types in New York City.
• Accurate values were obtained using only one low-cost soil moisture sensor per site.
• SWAM’s cost and labor requirements are lower compared to other monitoring schemes.

As cities increasingly adopt green infrastructure for the decentralized management of stormwater, there is a growing need to develop cost-effective approaches for the long-term monitoring and performance evaluation of these systems. In this study, a water balance approach – termed the Soil Water Apportioning Method (SWAM) – was developed to enable economic assessment of the long-term hydrologic performance of green roofs. SWAM provides estimates of green roof runoff and evapotranspiration (ET) based solely on measurements of local precipitation, substrate moisture, and the substrate maximum water storage capacity. To validate the approach, SWAM generated values of runoff and ET were compared with 30 months of runoff and ET data obtained from an extensive vegetated mat green roof located in New York City. Accurate runoff and ET estimates were obtained using as few as one substrate moisture measurement per day, although various other data logging frequencies were tested – with best results achieved with data logged every 6–24-h (Nash-Sutcliffe efficiency indices of 0.85–0.91 and 0.73–0.78, for runoff and ET respectively). SWAM-generated runoff values were further validated using 14 months of runoff data obtained at two other extensive green roof sites located in New York City, one a built-in-place system and the other a modular tray system. Results from this study indicate that SWAM provides a viable, low-cost approach for the citywide monitoring of green roof hydrologic performance with nominal instrumentation and implementation costs.

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