Stormwater detention ponds are designed to reduce the risk of downstream flooding caused by an increased amount of impervious surfaces from urbanization. These detention ponds slowly release stormwater runoff to decrease the outflows to rates similar to pre-development flows. However, as these ponds detain stormwater, the water level in the pond tends to fluctuate rapidly. There are two main mechanisms responsible for this rapid water level change: the extreme fluctuations that occur during and after large storms, and the more gradual release of water through restrictive outlet structures, seepage, and evaporation. Over time, the water level regime in detention ponds can lead to shifts in the ecological state of the ponds. Our research has two primary goals: 1) to determine the relative extent to which the watershed and engineering design characteristics influence the rapid water level fluctuations in stormwater detention ponds, and 2) to relate those water level fluctuations to in-pond and downstream ecosystem service indicators. To do this, we instrumented 20 detention ponds in Madison, WI with water level loggers to estimate water level, outflow, and inflow. We also used WinSLAMM to model how water level and flood attenuation responded to changes in watershed and engineering design parameters for each pond. In general, detention ponds with smaller areas and larger watersheds with more impervious surfaces experience greater water level fluctuations after a storm. Furthermore, the design of the outlet structures control the character (magnitude, duration, shape) of water level fluctuations that occur in each pond. When water level fluctuations are related to ecosystem services provisioning, detention ponds can be designed in the future to maximize both in-pond and downstream ecosystem services.
