Richards equation model of a rain garden

Traditional stormwater management does not mitigate groundwater depletion resulting from groundwater pumping and reduction in recharge. Infiltration practices, such as rain gardens, offer a potentially effective approach for addressing groundwater depletion. A rain garden is a landscaped garden in a shallow depression that receives the stormwater from nearby impervious surfaces, focusing recharge. We have developed a numerical model that can be applied in rain garden design and evaluation. Water flow through the rain garden soil is modeled over three layers: a root zone, a middle storage layer of high conductivity, and a subsoil lower layer. To continuously simulate recharge, runoff, and evapotranspiration, the model couples the Richards Equation with a surface water balance. The model was applied to the climate of southern Wisconsin. Simulation results show that very high recharge rates are possible during the non-snowfall season. (The model does not handle snowmelt.) A rain garden with an area of about 10-20% of the contributing impervious area maximizes groundwater recharge. Increasing the depression depth increases recharge and saturation times, affecting plant survival. Rain garden feasibility also depends on the subsoil hydraulic conductivity.