QUANTIFYING SOIL MOISTURE FLUXES IN AN URBAN RAIN GARDEN IN BUFFALO, NY

Buffalo, New York, like many great lakes cities, experiences combined sewer overflows (CSOs) during high precipitation storm events. These overflows occur when stormwater enters the sewer system above its capacity to treat wastewater. This results in a combination of untreated sewage, and stormwater runoff discharged directly to local waterways. In 2021, the city experienced ~2.6 billion gallons of CSO. One of the city’s responses to this is to build green infrastructure. A form of this is rain gardens installed along city streets. These gardens consist of sandy drainage soil and plants with the goal of delaying discharge to the sewer system and removing water in the form of evapotranspiration. Research has been conducted into rain gardens like these, but more needs to be known about their hydrologic effectiveness. We installed a network of ONSET soil moisture sensors and a logging box in one rain garden and monitored soil moisture at 10-minute intervals from May through August. The sensors were installed at 15, 30, and 45 cm below the surface in two locations. This data was correlated with weather data from a USGS station ~265 meters south of the rain garden. The results are measurements of soil moisture at depth used in a 1D wetting front model. The model assumes a homogeneous soil texture, ponding at the surface, and downward suction occurring at the wetting front to produce an infiltration rate dependent on the intensity of a storm. Trends in evapotranspiration cycles can also be seen in one of the 15 cm sensors. The fluctuations in soil moisture appear to follow a 24-hour trend and may delineate the depth of the root-water uptake zone. Increases in soil moisture at a 45 cm sensor without observed increases at shallower sensors indicate lateral flow of water possibly originating from the street inlet point. This research links rain garden engineering parameters to hydrologic behavior in response to storm events. We hope that these findings can be used to right-size future rain gardens to accommodate runoff from intense storms while minimizing their ground footprint.