A Complete Water Balance of a Rain Garden

A bioinfiltration rain garden was retrofitted from an existing traffic island at Villanova University in 2001. It has been monitored continuously since 2003 at a 5-min timeseries resolution and with instrumentation that would enable a water balance calculation. This 20-year data set allows for an in-depth analysis of the hydrologic pathways and management in the rain garden. Using physical equations and modeled data (based on real-time measurements), a balance of all influent, stored, and effluent water within the rain garden was constructed. Analysis shows the rain garden captures 73.5% of runoff, resulting in a post-implementation management of 86.2% of all rainfall in its watershed. In comparison to the hydrology of other land covers, implementing the rain garden resulted in the management of 37.6% more rainfall than pre-implementation, producing a hydrological signature similar to that of cultivated land or low development levels (e.g., 30% impervious). Additionally, with the long data record, several statistical techniques were applied to determine the amount of monitoring needed for a certain level of precision in system performance assessment. For 5% uncertainty, approximately 3 years of continuous data is needed to assess performance. This analysis not only facilitates understanding the function of rain garden systems, but also provides conclusions and methodology for understanding the uncertainty associated with the extent of monitoring performed on these green stormwater infrastructure systems. These findings provide practical knowledge as monitoring of stormwater management infrastructures is becoming a more standard part of their operation. Human activity has disturbed the natural hydrology of habited spaces, which are becoming more densely populated, increasing the necessity to further encroach on unaltered land. Construction and infrastructure for the purpose of creating habitable spaces from previously untouched land creates surfaces that water cannot pass through, generating increasingly more stormwater runoff, which degrades the quality of water in streams and rivers. This problem magnifies as populations grow and the need for habitable spaces increases. Hydrological systems, called green stormwater infrastructure, have been developed to combat this issue while minimizing the amount of space they occupy. These systems are designed to collect stormwater and use natural hydrological processes to reduce the amount of water (and consequentially amount of pollutant) that is discharged as overflow, reversing a degree of urbanization. Some studies have quantified this effect, but none with an extensive amount of data, leading to uncertainty and variability over how these systems perform at providing a more natural hydrology. A rain garden in a 50% impervious watershed that has been monitored continuously over 20 years shows that green stormwater infrastructure can achieve a degree of natural hydrology and demonstrates the necessity of long-term monitoring for confidence in this conclusion. A rain garden's ability to restore a developed watershed to a degree of natural hydrology is demonstratedUsing continuously monitored data, implementing the rain garden was found to manage 86.2% of all rainfall in its watershedAt least 3 years of monitoring is needed for confidence in performance and to identify if there is variability in performance over time

Automated summary

This study demonstrates that green stormwater infrastructure, specifically a rain garden, can achieve a degree of natural hydrology and highlights the necessity of long-term monitoring for performance assessment and variability analysis.