The HydroPlus swale GI device was modeled after the EPA SWMM swale LID device, but uses a different algorithm.
As of October 6, 2020 the EPA SWMM swale did not maintain a water balance, with in-flowing water (q0) < infiltration (f1) + runoff (q1)
This error with SWMM is attributed to a faulty use of a volumetric water balance, adjusted for different swale areas. 
The HydroPlus code uses volulme to maintain the swale water balance, similar to all other GI devices.
The HydroPlus code allows for the swale to have a storage area below the soil, and to percolate and exfiltrate the infiltrated water. 
