Green―Ampt model of a rain garden and comparison to Richards equation model

摘要

Traditional stormwater management, which relies heavily on detention storage, does not mitigate groundwater depletion resulting from groundwater pumping and loss of groundwater recharge. There has been an increasing interest in the use of alternative practices, such as rain gardens, that enhance infiltration of stormwater. A rain garden is a landscaped garden in a shallow depression of relatively small area that receives the stormwater from an impervious surface. We developed a simple numerical model that can be applied in the design and evaluation of rain gardens. Water flow through the rain garden soil is modelled over three layers: a root zone, a middle storage layer of high conductivity, and a lower layer that represents the subsoil at the site. To continuously simulate recharge, runoff and evapotranspiration, a Green―Ampt equation coupled with a surface water balance is used. For the climate of southern Wisconsin, simulation results show that the model yields similar results to a model based on the Richards equation: very high recharge rates are possible in the rainy season (twice the natural annual rates). A rain garden with an area of 10 to 20% of the contributing impervious area maximizes groundwater recharge. Increasing the depression depth increases recharge, but also increases ponding times, potentially affecting plant survival. The feasibility of a rain garden depends heavily on the saturated hydraulic conductivity of the underlying soil. These preliminary results show promise as a screening model and applicable tool for rain garden design.