Stage - discharge
At PSIC, we measure the volume of water flowing into and out of the park pond system. This information, combined with data on the concentration of pollutants in the water, allows us to calculate the total amounts of pollutants entering and leaving the system. By comparing these two amounts, we can assess how well the park’s green infrastructure is filtering polluted stormwater.
Two key variables in water quantity calculations are stage and discharge. Stage refers to water level (i.e. the height of the water above the pond floor). Discharge refers to streamflow (i.e. the amount of water that passes through a stream in a fixed amount of time). Discharge is what we’re interested in for the next part of our calculations, but it requires manual measurements, while stage is easy to measure continuously with pressure sensors. We therefore graph the two variables against each other to find a mathematical relationship between them. From this relationship, one can easily estimate discharge values for any given stage value.
Nitrogen and phosphorus concentrations can be measured from park water samples using a spectro::lyser sensor. By coupling streamflow and sensor data, we can calculate nitrogen and phosphorus fluxes into and out of the park. If there is a lower nutrient load leaving the park than entering it, then the green infrastructure is likely helping to filter out the pollutants along the way.
The map below shows the two locations that have been identified near the inflow and outflow of the ponds to continuously monitor stage (water level) and associated discharge (streamflow).
Measuring rainfall is an important parameter in estimating the volume of stormwater treated and pollutants removed by stormwater BMPs. PSIC uses a tipping bucket rain gauge to measure rainfall in Roger Williams Park. Having a rain gauge in the park provides better accuracy of precipitation by monitoring localized rain events.