Wetland Silviculture & Water Tables

Maintaining Wetland Hydrology in Ditched Sites & New Tool for Managers

This aerial view of the drained pine forests at the Carteret, North Carolina study site was captured in the late 1980s when the study began. Twenty-one years of water table data were collected here. Image courtesy of Devendra Amatya.

Water tables are everywhere, but their levels fluctuate – especially in the poorly drained clay soils of coastal South Carolina.

Topography also affects water tables, and forested wetlands in the Coastal Plain tend to be flat. “They respond rapidly to rainfall and evapotranspiration,” says Devendra Amatya, a USDA Forest Service research hydrologist.

Amatya is on a quest to understand and model water’s underground movements as well as the water balance in pine plantations and wetlands on shallow coastal plain soils.

He recently contributed to a collaborative project that has resulted in five papers, all published in the journal Wetlands, the official journal of the Society of Wetland Scientists. The project began as invited talks at the society’s 2016 meeting. The research team includes scientists from the Agricultural University of Cracow, Clemson University, the Polish Academy of Sciences, Weyerhaeuser Company, North Carolina State University, and the SRS Center for Forested Wetlands Research.

“We developed a relatively simple model that planners and land managers can use to compute daily water table depths,” says Amatya. The model is designed for poorly drained coastal soils of the southeastern U.S. It involves solving ordinary differential equations and uses daily rainfall and potential evapotranspiration as inputs. It can be used to assess the effects of land management, wetland restoration, and climate change, as discussed in Daily Water Table Depth Computing Model.

Roots grow too, and as trees mature, the roots suck water more powerfully. This changes hydraulic conductivity and transmissivity, as the study shows. Image courtesy of Devendra Amatya.

Water tables are also affected by plant growth. In the southeastern U.S., forests control about 60 percent of regional hydrology. Silviculture interacts with wetland hydrology in several ways. For example, different tree species use groundwater at different rates, and groundwater keeps trees alive during drought.

Climate, geology, and human activities – such as building ditches to drain wetlands – also affect water tables. Through the mid-1900s, people built vast networks of canals and ditches in the southeastern U.S. to drain forested wetlands for agriculture and silviculture.

Drained sites are better for timber production – soil trafficability improves and loblolly pine grows faster. “For wetland silviculture on the southeastern Coastal Plain, drainage is necessary,” says Amatya. “But as the Clean Water Act requires, hydric soil characteristics must also be retained.”

As trees grow, they change the way water moves through the soil. Soil hydraulic conductivity is the rate of water movement laterally in the soil. Taller trees have deeper and denser roots and more sucking power, which increases hydraulic conductivity. However, most studies assume soil hydraulic conductivity, does not change over time, even as trees mature.

Transmissivity is a measure of how water moves horizontally through the soil profile. It is defined as a product of conductivity and the soil depth. Transmissivity also increases as trees mature, as the scientists showed in Effects of Drainage for Silviculture, which was led by R. Wayne Skaggs of North Carolina State University.

A well and well logger that measured the water table depth in between the ditches. Image courtesy of Devendra Amatya.

In a mature loblolly pine plantation, transmissivity was 50 square meters per day – which means that, if the soil depth was 5 meters, the rate of water movement would be 10 meters per day. After harvest, transmissivity fell to 5.5 square meters a day, meaning the conductivity substantially fell. Eight years after replanting, transmissivity had increased to 34 square meters a day.

When water moves through the soil more easily, the soils drain more quickly. The result is a lower water table, which could remove wetland hydrology from drained sites. Amatya and his colleagues tested these findings in a modeling case study, discussed in Effects of Drainage.

The model shows if drainage ditches were three feet deep, they would have to be more than 200 feet apart to sustain wetland hydrology in a young pine plantation. The finding assumes that the site has six inches of surface storage. In a mature plantation, the ditches would have to be more than 950 feet apart to sustain wetland hydrology.

Minor drainage ditches for silvicultural operations are exempt from the Clean Water Act.

The modeling results suggests that drainage ditches, which are typically spaced 350 to 650 feet, will only satisfy wetland criteria for approximately five years after planting. After about eight years, as trees mature and soil properties change, wetland hydrology may be lost.

“In some cases, ditches may need to be allowed to gradually fill with sediment, or ditch outlets need to be completely plugged so that wetland hydrology can be retained,” says Amatya. “There are other methods that can reduce drainage intensity, conserve drainage water, and potentially sustain wetland hydrologic conditions as the trees mature.”

Otherwise, wetland hydrology can be lost, at least temporarily. In a drained pine plantation in North Carolina, none of the sites met wetland hydrology criteria.

However, during heavy rains, the ditched sites began to act like wetlands again. “Whether intensively managed or unmanaged, during extreme drought or heavy rain, they behave very similarly,” says Amatya. “Water tables on all soil types and with all sorts of vegetation behaved similarly during extreme storms.”

Water tables are part of the water cycle, which is complex and dynamic. Visit the U.S. Geological Survey for an interactive version of this graphic.

Amatya and his colleagues discuss these findings in Long-Term Water Table Dynamics of Forested Wetlands. In that study, the scientists examine multiple sites in the humid southeastern Atlantic Coastal Plain. Sites at Santee Experimental Forest in South Carolina are unditched, relatively undisturbed, and meet the hydrologic definition of a wetland. However, the pine plantation had been drained with ditches that were three feet deep or more and around 300 feet apart. “The ditches may have altered the hydrology of the site by increasing the runoff and making them drier,” says Amatya.

In Comparison of Hydrology of Two Atlantic Coastal Plain Forests, Amatya and his colleagues examined two similar sites. One of the sites was ditched and one was not. The sites had different average water tables. However, the effect of drainage on runoff was obscured by large interannual differences.

“Runoff, driven by water table position on these forest systems, is highly variable, depending on soil water storage,” notes Amatya. “Long-term monitoring provides better insights on climate and vegetation management effects on wetland hydrology, flow regime, and model validations, besides others.”

The team included researchers studying forests and those studying wetlands. The two disciplines have much to contribute to each other, as outlined in Silviculture and Forest Wetlands of the Southeastern U.S.: An Introduction to the Special Feature.

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For more information, email Devendra Amatya at devendra.m.amatya@usda.gov.

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