The Role of Forests in Water Flow Regulation

Forests and forested wetlands impact the timing and magnitude of water runoff and water flows. Some forest ecosystems act as sponges, intercepting rainfall and absorbing water through root systems. Water is stored in porous forest soils and debris, and then is slowly released into surface waters and groundwater. Through these processes, forests recharge groundwater supplies, maintain baseflow stream levels, and lower peak flows during heavy rainfall or flood events.

Forests, however, typically result in lower surface flows to nearby waterways because of infiltration and the transpiration of water into the atmosphere through leaves. Therefore, reducing forest cover and density generally increases surface water yield from watersheds, although these changes can be short-lived and depend on climate, soil characteristics, and the percentage and type of vegetation removal. For instance, streamflows increased 28 percent following a clear-cutting experiment in a southern Appalachian watershed. The ability of forests to absorb and store runoff can be approximately 20 times greater than that of an impervious parking lot and nearly six times greater than a residential lawn.

The water flow regulation services that forests provide can yield economic benefits to communities. By reducing water runoff during rainstorms, forests reduce the volume of water that a municipal stormwater containment facility or retention pond must store. Communities, therefore, do not need to invest as much in constructing stormwater control infrastructure. Based on this avoided cost of stormwater storage, one assessment estimated that forests near Atlanta, Georgia, saved the city $420 per acre per year.