Interactions of Soils and Hydrology in Wetland Management.
Richard Lowrance1, George Vellidis2, and Randy Williams1. (1) USDA-ARS, 2379 Rainwater Road, Tifton, GA 31794, (2) University of Georgia, Biological & Agricultural Engineering Department, Tifton, GA 31793-0748
Historically, wetlands were managed to remove excess water as quickly as possible. The goal was to convert wetlands and wetland soils to non-wetland uses or to convert them to farmed wetlands such as flooded rice. Wetland management has changed because of a scientific consensus that loss of wetlands adversely affected water quality, flood storage, and wildlife habitat. Today, although some wetland conversion and filling continues, wetlands are more likely to be managed to improve water quality, increase flood storage, and enhance wildlife habitat. Best Management Practices (BMPs) for wetlands include wetland restoration, wetland enhancement, wetland creation, and wetland construction. Thus wetland management as a conservation practice can range from building completely new wetlands for wastewater treatment to increasing the ecological functions of existing wetlands. These management approaches are all recognized as separate but often interacting practices and can be used to achieve a suite of conservation and environmental quality objectives. We will review the existing literature on these management approaches drawing examples from agricultural and urban settings, with an emphasis on the roles of soils. Although urban and agricultural wetlands share certain characteristics, urban wetland soils are often more degraded and have been subject to more extremes of hydrologic changes. Extreme fluctuations from very dry to flooded high energy water can limit the wetland functions and lead to physical and biological changes in wetland soils. We will discuss the effects of historical conditions, especially long term sedimentation and sediment borne nutrients and other chemicals on wetland functions. Many wetlands have received high loads of sediment and sediment borne chemicals and the nature of these wetlands has changed due to the historic and ongoing sedimentation. In some landscapes, the extent of riverine wetlands has increased while in others, sedimentation has reduced the frequency of inundation and created new soil conditions. We will contrast the use of wetland management to achieve watershed scale water quality objectives versus wetland management to achieve site specific objectives. In many cases in highly altered landscapes, managed wetlands may be most effective at outlets of watersheds rather than scattered around throughout the basin. The effects of wetlands as BMPs in agriculture will be discussed particularly the interactions among hydrologic loading, pollutant loading, and soil properties as they influence the water quality functions of managed wetlands. Hydrologic loading is of special importance because the relationship between hydrologic fluxes and storage in wetlands determines the residence time of water. Residence time is critical in nutrient removal by wetlands and riparian zones. Even in ideal conditions for using wetlands as denitrification reactors where high nitrate water is put into a wetland environment, there can low efficiency of nitrate removal due to low residence time of water. The Riparian Ecosystem Management Model (REMM), a model of riparian wetland function, will be used to address the effects of varying N loadings on denitrification in wetland soils. In wetland soils with high denitrification potentials, REMM shows that very high loadings will be denitrified and not reach receiving waters. In soils with lower denitrification potentials, a higher proportion of the nitrogen load is passed through the wetland. Possible trade-offs between water quality improvement and green house gas production in managed wetlands will be discussed. The use of wetlands for large scale N removal will be discussed relative to what is known about N removal and N gas emissions in wetlands. We will conclude the discussion of wetland management with examples of specific management applications and wetland management programs that are applied and available for landscape management. Examples will include water conservation areas, pocket wetlands for nitrate removal, and riparian zone restoration.