Water Quantity

Kenneth N. Brooks
University of Minnesota

Since the first century AD there has been a fascination about the hydrologic role of forests. Perceptions of forests and water relationships led to forest-water lore suggesting that forests enhance rainfall, store water when rainfall is plentiful, release water during dry periods, and prevent floods. Public perceptions of deforestation effects on clean water supplies in the United States led to federally protected forest reserves in 1891 and to the 1897 Organic Administrative Act stating that these reserves were to “... protect and enhance water supplies, reduce flooding, [and] secure favorable conditions of water flow, ...” (USDA Forest Service, 2000). Given this history, we can now refer to over a century of forest hydrology research that provides a more scientific view of relationships between forests, forest management and water.

Hydrologic Function of Forests

Forests occur where there is abundant moisture but they also consume water luxuriously, transferring large amounts of precipitation on watersheds to the atmosphere through evapotranspiration (ET). Nevertheless, the total quantity of atmospheric moisture contributed by forests pales in comparison with contributions from oceans. Consequently, forests have little effect on regional or global precipitation, but they strongly influence the fate of precipitation on a watershed. The large surface area of forest vegetation catches a portion of rainfall which then evaporates, a process called interception. The amount of interception in any single rainstorm is small, but cumulative forest interception losses can amount to as much as forty percent of annual precipitation, significantly reducing the quantity of water reaching soil surfaces.

Most forest soils are extremely pervious and readily absorb rainfall and snowmelt at the soil surface, a process called infiltration. With high infiltration capacities, surface runoff is rare in forests. Instead, water infiltrates and is either stored (when there are soil moisture deficits), or moves through subsurface pathways in its journey to stream channels or groundwater. Because subsurface flow travels slowly to stream channels, in contrast to surface runoff, peak flows in forested watersheds are lower than with most other vegetative cover conditions. During extended wet periods significant groundwater recharge can occur from forest soils.

Water that is held by the soil is largely extracted by tree roots and returned to the atmosphere through stomata of plant leaves, a process of transpiration. The combination of interception and transpiration by forests is excessive, leaving soils persistently drier than soils under other types of vegetation. Any subsequent rainfall or snowmelt is more likely to be stored in the drier forest soils, reducing the amount of water available for streamflow or groundwater recharge. Such reductions in groundwater recharge can then reduce groundwater flow into streams. The paradox is that forests both promote groundwater flow to streams through rapid recharge during wet periods, but they also can deplete dry season streamflow because of high ET losses. The net effect of forests on dry season streamflow thus depends on the relative magnitude of these two influences over time.

The unique tropical montane cloud forests (TMCF) that occur in the cloud engulfed mountains of tropical Central and South America play a different hydrologic role than the temperate and lower elevation tropical forests that are characterized above. Here, forests intercept the persistent cloud moisture which drips to the soil, adding more moisture annually than is lost through evapotranspiration. As a result, streamflow and groundwater recharge from TCMF forests exceeds that which would be caused by rainfall alone. Although less is known about the hydrologic processes of these forests, removal of these forests appear to reduce water yield, a response much different than most forests as discussed below.

Forest Management Effects on Water Yield

Over 135 watershed studies globally have shown that practices such as thinning, selective harvesting, and clear cutting forests, generally increase annual water yield from watersheds in proportion to the amount of biomass removed. Increases are higher in wet climates (up to 400mm/year) but lower in drier climates (less than 60 mm/year). As forests regrow, water yield returns to natural forest conditions unless species composition changes. If species with high interception (conifers) replace species with low interception (hardwoods), annual water yield will be reduced. If forests are replaced with vegetation having shallower roots and/or a shorter growing season, such as pastures or annual crops, water yield generally increases. Natural phenomena such as fires and insect outbreaks that reduce living biomass also can increase water yield. As some forests age, water yield has been shown to increase even though biomass may not change dramatically.

Opportunities to augment water supplies through forest management appear promising, but there are caveats. Most often increases in water yield occur during the wettest time of the year when streamflow is already high, and reservoirs full. Furthermore, it appears that streamflow cannot be increased significantly, if at all, during dry seasons. Harvesting floodplain forests, such as cottonwood, willow, or salt cedar, can potentially reduce groundwater losses but such practices could be constrained by wildlife habitat degradation and other environmental concerns.

Forest Management Effects on Flooding

Because forest removal adds water to streams, the implication for flooding is apparent. Extensive removal of forest biomass, particularly if accompanied by soil disturbance, can increase runoff volumes and peaks and cause localized flooding. Flooding can be exacerbated by road construction, poor logging practices and other activities that compact soils, reduce infiltration rates and increase surface runoff. In 1998, Hurricane Mitch resurrected the debate about forests and floods, with many blaming the flood devastation in Central America on deforestation. Close examination of this hurricane and other extreme meteorological events suggests that as the amount of rainfall or snowmelt becomes excessively large and widespread over river basins, forests have minimal effects on flooding. Therefore, we conclude that forests cannot prevent major floods, such as the 100-year flood, but they can mitigate flooding from moderate levels of rainfall or snowmelt on watersheds. We can also conclude that forests provide greater hydrologic stability to watersheds, holding streambanks and soils in place, and yielding less variable and lower streamflow per unit of rainfall than other vegetative cover types.

References and Further Reading

Andreassain, V. “Waters and Forests: From Historical Controversy to Scientific Debate.” Journal of Hydrology. 291 (2004): 1-27.

Brooks, Kenneth; Ffolliott, Peter; Gregersen, Hans; and Leonard DeBano. “Hydrology and the Management of Watersheds.” Third Edition. Ames: Iowa State Press, 2003.

Bruijnzeel. L.A. “Hydrological Functions of Tropical Forests: Not Seeing the Soil for the Trees?” Agriculture Ecosystems & Environment. 104. (2004):185-228.

Calder, I.R. “Forests and Hydrological Services: Reconciling Public and Science Perception.” Land and Water Resources Research 2 (2002): 2.1-2.12. http://www.luwrr.com.

Ice, George G., and John D. Stednick., Editors. “A Century of Forest and Wildland Watershed Lessons.” Bethesda: The American Society of American Foresters, 2004.

Lee, Richard. “Forest Hydrology.” New York: Columbia University Press, 1980.

USDA Forest Service. 2000. “Water & the Forest Service.” Policy analysis. P.O. Box 96090, Washington DC.

Posted: 3 August 2006