Physical Hydrology

Lowland Forest Hydrology Procedure

Hydrology of the forest ecosystem plays a critical role not only in ecosystem structure and functions but also in regulating the water cycle and pathways.

Although the hydrology and hydrologic function of forested wetlands in the lower coastal plain have been characterized in general, the hydrologic processes of runoff generation, flow pathways including loss through evapotranspiration (ET), a major component of forest hydrologic balance, for headwater watersheds have not been well studied or described.

Similarly, understanding the hydrologic function of headwater systems is particularly important because they are critical to the hydrology and water quality of the drainage basin, and susceptible to development. These understandings can then be used to support informed watershed management and planning, and to design effective best management practices (BMPs).

Furthermore, both the hydrologic responses to forest management for timber and recently bioenergy production and impacts of land use and climate change on forest hydrology are increasing becoming a societal concern.

Hydrologic processes on relatively low-gradient poorly drained forested landscapes of the lower coastal plain are usually dominated by shallow water table positions.

Most of the outflows (surface runoff and subsurface drainage) from these watersheds in fact drain from saturated areas where the water is at the surface or a shallow water table is present. This means that total outflow depends on the frequency and duration of flooding and on the dynamics of the water table (hydroperiod), which are driven by rainfall and evapotranspiration (ET).

Although potential ET is primarily controlled by solar energy, ET is also dependent on soil type, vegetation type, and seasonal dynamics. Outflow processes on drained pine plantations consist primarily of subsurface flow to the lateral ditches and then channel flow to the watershed outlet.

The ridge and valley micro-topography created by bedding for aerated microsites for plant seedlings enhances surface detention storage capacity and thus precludes overland surface runoff, except for the highest rainfall events. This complicates the task of quantifying the water budget components of these wetland systems, and the need to account for the use of various water and silvicultural management practices, and interactions with surrounding uplands makes the job even more challenging.

Only a few studies have documented hydrology and water budgets for depressional wetlands, bottomland hardwoods, pocosins, pine flatwoods, drained pine plantations, mixed pine and hardwood forests and their interactions with surrounding uplands.

Selected Publications

Mixed Stands

Plym Wetland

Forested wetlands exist as a result of saturated soil conditions. While that notion of wetlands is readily acknowledged, there is a poor understanding of the hydrologic conditions that cause soil saturation for periods prolonged enough to cause anoxia and hence limit the site to hydrophytic species. Precipitation, ground water, or flooding are the sources of water that can cause soil saturation. Soil properties and geomorphic setting interact with those sources of water to yield a complex matrix of conditions that control wetland functions.

The most widely acknowledged hydrologic functions attributed to wetlands include groundwater recharge, water purification, and flood control. These functions, however, are not universal among wetland types. In fact, the hydrologic function of a wetland may change according to landscape position.

Important forest resource management and conservation issues are contingent on wetland hydrology. These issues include:

  • productivity
  • buffer zones
  • surface and ground water quality
  • habitat
  • flood control
Selected Publications

Plantations

Plym Wetland

A vast area of the forests in the southeastern U.S. is on pine plantation forests.

Among that about 1 million hectares of plantation pine in the coastal plain region are drained to improve soil trafficability for harvesting and planting operations and to improve soil moisture growth conditions throughout the year.

These lands on poorly drained soils are being intensively managed with pine forests for maximum timber production.

Intensive forest management activities include access, drainage, harvest, site preparation, regeneration, fertilization, tending, protection, and utilization. Based on Southern Forest Resource Assessment Report (Forest Service Southern Research Station, 2002) timber harvests in the South are expected to increase over the next 20 years, and it is likely that impacts to forested wetlands as a result of intensified forestry will continue.

Timber harvesting reduces ET, thus elevating the ground water level and increasing the water yield from a forested site until the canopy is regenerated. These increases in water volume can change both storm and base flow, and without proper management of both volume and flow rate, may increase flow energy throughout the harvested watershed.

High-energy water can move farther, transport more sediment and nutrients to downstream water bodies, and even affect the sensitive brackish habitats essential for juvenile development of many marine fish and shellfish species.

The Center, as a major cooperator, has been studying the hydrology and water quality of drained pine plantation forests and the water and silvicultural management impacts on hydrology and water quality in Carteret County, NC for more than twenty years.

Selected Publications

South Eastern United States

Uruguay

Map of Uruguay

Uruguay is located in the eastern part of South America (S.A.) between latitudes 30° and 35° South. It is in a zone of humid subtropical to temperate climate.

The country is characterized ecologically and physiographically by native grasslands (savannah) and topography ranging from plains to rolling hills with elevations up to 500 m.

About 85% of Uruguay’s land mass (176,000 km2) is in pasture/agriculture, the highest percentage in the world.

Historically, most of the grasslands have been used for livestock grazing while some of the better soils have been used for row crop farming. Only about 3.3% of the land remains as native forest.

Uruguay’s forest industry began with the Forestry Act of 1988. In 1989, the Uruguayan government instituted financial incentives for the establishment of tree plantations in an effort to diversify the rural economy. In response, national and multinational timber corporations have purchased land and planted trees (primarily eucalyptus, loblolly pine, and slash pine) over significant portions of the landscape.

Approximately 600,000 ha of grasslands were planted to trees between 1990 and 2003. Due to the magnitude of these land use changes, local stakeholders have expressed concerns regarding the impact of converting grasslands to tree plantations on water resources. Of particular concern are the effects of the tree plantations on water yield and downstream water supply, as well as the impact on base flows in the receiving streams and rivers.

In the fall of 1999, a long-term collaborative research and demonstration project began at the El Cerro La Corona ranch site of the Tacuarembo river basin in Uruguay, S.A. It was sponsored by funds from the Weyerhaeuser foundation and Instituto Nacional de Investigación Agropecuaria (INIA). The project was planned and lead by the North Carolina State University (NCSU) team led by Dr. R. Wayne Skaggs, Dr. George (Chip) Chescheir, Dr. J. Wendell Gilliam of Soil Science, and Dr. D.M. Amatya from the U.S. Forest Service.

Other cooperators in the project were Colonvade S.A., INIA scientists, and researchers from the Universidad de la republica. The main goal of the study was to evaluate the long-term impacts of land use conversion from grassland to pine plantation on the hydrologic regime and water quality. The field study employed a long term paired watershed approach to evaluate the effects of afforestation.

Two watersheds were monitored for a three-year pretreatment period during which the land use in both the control and treatment watersheds was grassland with livestock grazing. The treatment watershed was subsequently planted with loblolly pine (Pinus taeda L.) in July 2003, and both watersheds have been continuously monitored to date and monitoring will continue through tree maturation and harvesting.

Selected Publications