Land development brings about a fundamental shift in the natural water balance. As vegetated land is replaced with the impervious surfaces of buildings, roads, parking lots and other municipal infrastructure, the opportunity for infiltration and evapotranspiration of rainfall is dramatically reduced and surface runoff becomes the dominant drainage mechanism in the new urban water balance.  Rainfall that previously evaporated and seeped into the ground to replenish groundwater aquifers becomes surface runoff, ultimately contributing to increased channel erosion, flooding risk and contaminant loading in our rivers and streams.

One of the most effective ways of minimizing these impacts is to adopt a water balance approach to stormwater management. The water balance approach is best defined as stormwater management planning that mimics the natural water balance by focusing on practices that promote increased infiltration and evaporation, and lower surface runoff volumes and flow rates.

Practices emphasized in the water balance approach are those applied to minimize runoff at source, where rain falls. Examples include increased pervious cover, disconnection of impervious areas from the storm sewer system, cisterns for collection of roof runoff, grassed swales, green roofs, and bioretention areas. Existing stormwater management models (e.g. QUALHYMO) have been adapted to incorporate more of these practices in order to promote widespread adoption of the water balance approach by engineers, planners and other practitioners in the development industry.