Hydrologic monitoring of stormwater control measures commonly use pressure transducers to detect and record water level. There are documented difficulties in measuring reference datums needed for calculating flow based on water level and a primary device (LeFevre, et al., 2009; Voyde, 2011).

This project monitored the water balance of four bioretention flow-through planters at Stevens Institute of Technology from July through November 2019. The planters provided a nearly-closed system to investigate the water balance. Pressure transducers and weirs measured inflow and outflow. A weir-specific rating curve was developed to determine flow rates. Despite the calibrations and controlled tests performed, the weir datum after each storm varied several centimeters. This variation in reference height causes large changes in calculated flow rates, that significantly affect the calculated total water volume for each event. A closed water balance was obtained for only 25% of the observations within the estimated measurement uncertainties. The difference between inflow and outflow was larger than the maximum possible volume stored in the planter media for 43% of the observations. The reason for the changing datum may be that while monitoring these systems, operation is at the extreme end of the measuring range. Shallow water is ubiquitous in GI monitoring. Additionally, the asymptotic approach to calculating the datum makes it objectively hard to determine as it changes between every storm. These challenges produce error larger than error propagation estimates and needs to be addressed in future monitoring efforts.

Pressure Transducer Reference Height Variation in Green Infrastructure Monitoring

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