Evaluating the cumulative effects of alteration on New England wetlands

In New England, patterns of glacial deposition strongly influence wetland occurrence and function. Many wetlands are associated with permeable deposits and owe their existence to groundwater discharge. Whether developed on deposits of high or low permeability, wetlands are often associated with streams and appear to play an important role in controlling and modifying streamflow. Evidence is cited showing that some wetlands operate to lessen flood peaks, and may have the seasonal effect of increasing spring discharges and depressing low flows. Wetlands overlying permeable deposits may be associated with important aquifers where they can produce slight modifications in water quality and head distribution within the aquifer. Impacts to wetlands undoubtedly will affect these functions, but the precise nature of the effect is difficult to predict. This is especially true of incremental impacts to wetlands, which may, for example, produce a change in streamflow disproportionate to wetland area in the drainage basin, i.e., a nonlinear effect as defined by Preston and Bedford (1988). Additional research is needed before hydrologic function can be reliably correlated with physical properties of wetlands and landscapes.A model is proposed to structure future research and explore relationships between hydrologic function and physical properties of wetlands and landscapes. The model considers (1) the nature of the underlying deposits (geologic type), (2) location in the drainage basin (topographic position), (3) relationship to the principal zone of saturation (hydrologic position), and (4) hydrologic character of the organic deposit.     

Evaluating cumulative effects on wetland functions: A conceptual overview and generic framework

This article outlines conceptual and methodological issues that must be confronted in developing a sound scientific basis for investigating cumulative effects on freshwater wetlands. We are particularly concerned with: (1) effects expressed at temporal and spatial scales beyond those of the individual disturbance, specific project, or single wetland, that is, effects occurring at the watershed or regional landscape level; and (2) the scientific (technical) component of the overall assessment process. Our aim is to lay the foundation for a research program to develop methods to quantify cumulative effects of wetland loss or degradation on the functioning of interacting systems of wetlands. Toward that goal we: (1) define the concept of cumulative effects in terms that permit scientific investigation of effects; (2) distinguish the scientific component of cumulative impact analysis from other aspects of the assessment process; (3) define critical scientific issues in assessing cumulative effects on wetlands; and (4) set up a hypothetical and generic structure for measuring cumulative effects on the functioning of wetlands as landscape systems.We provide a generic framework for evaluating cumulative effects on three basic wetland landscape functions: flood storage, water quality, and life support. Critical scientific issues include appropriate delineations of scales, identification of threshold responses, and the influence on different functions of wetland size, shape, and position in the landscape.The contribution of a particular wetland to landscape function within watersheds or regions will be determined by its intrinsic characteristics, e.g., size, morphometry, type, percent organic matter in the sediments, and hydrologic regime, and by extrinsic factors, i.e., the wetland's context in the landscape mosaic. Any cumulative effects evaluation must take into account the relationship between these intrinsic and extrinsic attributes and overall landscape function. We use the magnitude of exchanges among component wetlands in a watershed or larger landscape as the basis for defining the geographic boundaries of the assessment. The time scales of recovery for processes controlling particular wetland functions determine temporal boundaries. Landscape-level measures are proposed for each function.     

A conceptual framework for assessing cumulative impacts on the hydrology of nontidal wetlands

Wetlands occur in geologic and hydrologic settings that enhance the accumulation or retention of water. Regional slope, local relief, and permeability of the land surface are major controls on the formation of wetlands by surface-water sources. However, these landscape features also have significant control over groundwater flow systems, which commonly play a role in the formation of wetlands. Because the hydrologic system is a continuum, any modification of one component will have an effect on contiguous components. Disturbances commonly affecting the hydrologic system as it relates to wetlands include weather modification, alteration of plant communities, storage of surface water, road construction, drainage of surface water and soil water, alteration of groundwater recharge and discharge areas, and pumping of groundwater. Assessments of the cumulative effects of one or more of these disturbances on the hydrologic system as related to wetlands must take into account uncertainty in the measurements and in the assumptions that are made in hydrologic studies. For example, it may be appropriate to assume that regional groundwater flow systems are recharged in uplands and discharged in lowlands. However, a similar assumption commonly does not apply on a local scale, because of the spatial and temporal dynamics of groundwater recharge. Lack of appreciation of such hydrologic factors can lead to misunderstanding of the hydrologic function of wetlands within various parts of the landscape and mismanagement of wetland ecosystems.     

HYDROLOGY OF TWO SMALL WETLAND BASINS IN EASTERN MASSACHUSETTS1

ABSTRACT: A hydrologic budget was prepared for two geologically different wetland basins in eastern Massachusetts for the 1971 water-year. Water table conditions prevailed at one wetland underlain by peat while an artesian system functioned at the other wetland which was underlain by muck. Hydrologic responses were generally similar at both wetlands, although each functioned differently in detail. Both wetlands exhibited high spring discharges and depressions of low flow. Ground water accounted for an estimated 93% of the total annual discharge from both wetlands; in late summer the peat deposit recharged the regional ground water body. Evapotranspiration in the spring was retarded in probable consequence of the extreme wetness of the wetland soils.     

Specific Yield Functions for Estimating Evapotranspiration from Diurnal Surface Water Cycles

The White method has been routinely used to estimate evapotranspiration using diurnal variations in groundwater levels. Applications to surface water systems (e.g., wetlands) are less common. For applications to surface water systems, a stage‐dependent specific yield function must be defined. This is especially important for small wetlands formed in topographic depressions with bowl shaped bathymetries. Existing formulations of the specific yield function include weighting factors that impact the relative importance of the soil and open water specific yields on the composite value. Three formulations of the specific yield function from the literature were compared and found to produce varied results. Based on a comparison with empirical estimates of specific yield based on observed ratios of net precipitation to water level rise, one of the existing formulations is generalized and recommended for general use. The recommended function is dependent on wetland bathymetry, magnitude of the diurnal fluctuation, spatial extent of the equilibration area, and soil‐specific yield. A sensitivity analysis was conducted to examine the relative importance of these variables. The specific yield function is independent of wetland size and is strongly dependent on the basin profile coefficient (p), an indication of wetland shape. For most natural wetlands, bathymetry strongly influences specific yield.     

Strategies for assessing the cumulative effects of wetland alteration on water quality

Assessment of cumulative impacts on wetlands can benefit by recognizing three fundamental wetland categories: basin, riverine, and fringe. The geomorphological settings of these categories have relevance for water quality.Basin, or depressional, wetlands are located in headwater areas, and capture runoff from small areas. Thus, they are normally sources of water with low elemental concentration. Although basin wetlands normally possess a high capacity for assimilating nutrients, there may be little opportunity for this to happen if the catchment area is small and little water flows through them.Riverine wetlands, in contrast, interface extensively with uplands. It has been demonstrated that both the capacity and the opportunity for altering water quality are high in riverine wetlands.Fringe wetlands are very small in comparison with the large bodies of water that flush them. Biogeochemical influences tend to be local, rather than having a measurable effect on the larger body of water. Consequently, the function of these wetlands for critical habitat may warrant protection from high nutrient levels and toxins, rather than expecting them to assume an assimilatory role.The relative proportion of these wetland types within a watershed, and their status relative to past impacts can be used to develop strategies for wetland protection. Past impacts on wetlands, however, are not likely to be clearly revealed in water quality records from monitoring studies, either because records are too short or because too many variables other than wetland impacts affect water quality. It is suggested that hydrologic records be used to reconstruct historical hydroperiods in wetlands for comparison with current, altered conditions. Changes in hydroperiod imply changes in wetland function, especially for biogeochemical processes in sediments. Hydroperiod is potentially a more sensitive index of wetland function than surface areas obtained from aerial photographs. Identification of forested wetlands through photointerpretation relies on vegetation that may remain intact for decades after drainage. Finally, the depositional environment of wetlands is a landscape characteristic that has not been carefully evaluated nor fully appreciated. Impacts that reverse depositional tendencies also may accelerate rates of change, causing wetlands to be large net exporters rather than modest net importers. Increases in rates as well as direction can cause stocks of materials, accumulated over centuries in wetland sediments, to be lost within decades, resulting in nutrient loading to downstream aquatic ecosystems.     

THE VULNERABILITY OF WETLANDS TO CLIMATE CHANGE: A HYDROLOGIC LANDSCAPE PERSPECTIVE1

ABSTRACT: The vulnerability of wetlands to changes in climate depends on their position within hydrologic landscapes. Hydrologic landscapes are defined by the flow characteristics of ground water and surface water and by the interaction of atmospheric water, surface water, and ground water for any given locality or region. Six general hydrologic landscapes are defined; mountainous, plateau and high plain, broad basins of interior drainage, riverine, flat coastal, and hummocky glacial and dune. Assessment of these landscapes indicate that the vulnerability of all wetlands to climate change fall between two extremes: those dependent primarily on precipitation for their water supply are highly vulnerable, and those dependent primarily on discharge from regional ground water flow systems are the least vulnerable, because of the great buffering capacity of large ground water flow systems to climate change.     

Simulated wetland conservation-restoration effects on water quantity and quality at watershed scale

Wetlands are one of the most important watershed microtopographic features that affect hydrologic processes (e.g., routing) and the fate and transport of constituents (e.g., sediment and nutrients). Efforts to conserve existing wetlands and/or to restore lost wetlands require that watershed-level effects of wetlands on water quantity and water quality be quantified. Because monitoring approaches are usually cost or logistics prohibitive at watershed scale, distributed watershed models such as the Soil and Water Assessment Tool (SWAT), enhanced by the hydrologic equivalent wetland (HEW) concept developed by Wang [Wang, X., Yang, W., Melesse, A.M., 2008. Using hydrologic equivalent wetland concept within SWAT to estimate streamflow in watersheds with numerous wetlands. Trans. ASABE 51 (1), 55–72.], can be a best resort. However, there is a serious lack of information about simulated effects using this kind of integrated modeling approach. The objective of this study was to use the HEW concept in SWAT to assess effects of wetland restoration within the Broughton's Creek watershed located in southwestern Manitoba, and of wetland conservation within the upper portion of the Otter Tail River watershed located in northwestern Minnesota. The results indicated that the HEW concept allows the nonlinear functional relations between watershed processes and wetland characteristics (e.g., size and morphology) to be accurately represented in the models. The loss of the first 10–20% of the wetlands in the Minnesota study area would drastically increase the peak discharge and loadings of sediment, total phosphorus (TP), and total nitrogen (TN). On the other hand, the justifiable reductions of the peak discharge and loadings of sediment, TP, and TN in the Manitoba study area may require that 50–80% of the lost wetlands be restored. Further, the comparison between the predicted restoration and conservation effects revealed that wetland conservation seems to deserve a higher priority while both wetland conservation and restoration may be equally important.     

Evaluating cumulative effects of disturbance on the hydrologic function of bogs,fens, and mires

Few quantitative studies have been done on the hydrology of fens, bogs, and mires. Consequently predicting the cumulative impacts of disturbances on their hydrologic functions is extremely difficult. For example, few data are available on the role of bogs and fens with respect to flood desynchronization and shoreline anchoring. However, recent studies suggest that very small amounts of groundwater discharge are sufficient to radically modify mire surface-water chemistry, and consequently, vegetation communities and their associated surface-water hydrology. Bogs and fens are, in a sense, hydrobiologic systems, and any evaluation of cumulative impacts will have to (1) consider the complicated and little understood interactions among wetland hydrology, water chemistry, and biota, and (2) place the effect of individual wetland impacts within the context of the cumulative impacts contributed to the watershed from other geomorphic areas and land uses.It is difficult to evaluate the potential cumulative impacts on wetland hydrology because geologic settings of wetlands are often complex and the methods used to measure wetland streamflow, groundwater flow, and evapotranspiration are inexact (Winter 1988). This is especially so for bogs, fens, and mires underlain by thick organic soils. These wetlands, found in the circumboreal areas of North America, Europe, and Asia, are major physiographic features in eastern North America, northern Europe, and Siberia (Kivenen and Pakarinen 1981, Gore 1983, Glaser and Janssens 1986). Their very scale makes it difficult to quantify the hydrologic function accurately. The hydrology of small bogs and fens found elsewhere is just as poorly understood because of conflicting conceptual models of pertinent hydrologic processes.This article (1) reviews our current understanding of the hydrologic function of bogs, fens, and mires at different scales and in different physiographic settings and (2) presents hypotheses on potential cumulative impacts on the hydrologic function that might occur with multiple disturbances.     

Modeling the Effects of Tile Drain Placement on the Hydrologic Function of Farmed Prairie Wetlands

The early 2000s saw large increases in agricultural tile drainage in the eastern Dakotas of North America. Agricultural practices that drain wetlands directly are sometimes limited by wetland protection programs. Little is known about the impacts of tile drainage beyond the delineated boundaries of wetlands in upland catchments that may be in agricultural production. A series of experiments were conducted using the well‐published model WETLANDSCAPE that revealed the potential for wetlands to have significantly shortened surface water inundation periods and lower mean depths when tile is placed in certain locations beyond the wetland boundary. Under the soil conditions found in agricultural areas of South Dakota in North America, wetland hydroperiod was found to be more sensitive to the depth that drain tile is installed relative to the bottom of the wetland basin than to distance‐based setbacks. Because tile drainage can change the hydrologic conditions of wetlands, even when deployed in upland catchments, tile drainage plans should be evaluated more closely for the potential impacts they might have on the ecological services that these wetlands currently provide. Future research should investigate further how drainage impacts are affected by climate variability and change.     

MULTIVARIATE CLASSIFICATION OF SMALL ORDER WATERSHEDS IN THE QUABBIN RESERVOIR BASIN,MASSACHUSEVFS1

ABSTRACT: A multivariate approach was used to analyze hydrologic, geologic, geographic, and water-chemistry data from small order watersheds in the Quabbin Reservoir Basin in central Massachusetts. Eighty three small order watersheds were delineated and landscape attributes defining hydrologic, geologic, and geographic features of the watersheds were compiled from geographic information system data layers. Principal components analysis was used to evaluate 11 chemical constituents collected bi-weekly for 1 year at 15 surface-water stations in order to subdivide the basin into subbasins comprised of watersheds with similar water quality characteristics. Three principal components accounted for about 90 percent of the variance in water chemistry data. The principal components were defined as a biogeochemical variable related to wet. land density, an acid-neutralization variable, and a road-salt variable related to density of primary roads. Three subbasins were identified. Analysis of variance and multiple comparisons of means were used to identify significant differences in stream water chemistry and landscape attributes among subbasins. All stream water constituents were significantly different among subbasins. Multiple regression techniques were used to relate stream water chemistry to landscape attributes. Important differences in landscape attributes were related to wetlands, slope, and soil type.     

拉萨河流域高寒湿地水质影响因子分析

西藏高寒湿地在生态平衡、生态建设和经济社会发展中发挥着重要作用。本文以西藏拉萨河流域内各个典型高寒湿地为研究对象,通过系统聚类法和综合污染指数法,对流域内各项水质指标进行综合分析和评价。结果表明：各个湿地的总N、Cu元素含量都超出了Ⅰ类水质标准;总P、Zn没有超标;Mn元素含量除了塘嘎郭湿地超标3倍外,其他均小于国家标准;Fe元素含量除了塘嘎郭湿地超标7倍外,其他均小于标准。总N、总P、pH值、Cu、Fe等因子对拉萨河流域内各个高寒湿地水质污染贡献最大。拉萨河水体有机污染较重,其余各个湿地有水体富营养化的趋势,同时流域内湿地独特的自然因素造成该流域内重金属污染偏高。对策建议包括应加大对城市污水的治理,加强流域内及周边矿藏资源的开发管理等。     

WETLAND ECOSYSTEM STUDIES FROM A HYDROLOGIC PERSPECTIVE1

ABSTRACT: Selected studies from the literature were reviewed to determine the extent of knowledge about the relationship between hydrology and wetland ecosystem studies. Wetland studies of chemical input-output relationships have been the most dependent on hydrologic data of all wetland investigations; yet, very few of these studies have attempted to measure all components of a wetland's water balance. Usually, unmeasured components were calculated as the difference between measured inputs and outputs. Ground water frequently was overlooked. Chemical input-output investigations primarily were concerned with determining the amount of input retained in the wetlands. Few studies also included direct measurement of biogeochemical processes within wetlands of elements that were part of simultaneous input-output investigations. The importance of uncertainties in chemical budgets that are due to uncertainties in hydrologic budgets has been addressed in very few wetland investigations. Although many studies have emphasized the importance of hydrology to wetland ecosystem research, few studies have documented this, so that hydrology remains one of the least understood components of wetland ecosystems.     

NEBRASKA'S SANDHILLS LAKES: A HYDROGEOLOGIC OVERVIEW1

ABSTRACT: Lakes, marshes and wet meadows occur in the broad, flat, interdune valleys of the Nebraska Sandhills, a vegetation-stabilized dune field underlain by sediments containing an enormous supply of groundwater. Hydrologic, geologic and possibly climatologic factors influence the chemical quality of lake water. Central and eastern lakes generally are in connection with the groundwater reservoir. The hydrologic nature of western lakes and the cause of their high alkalinity is not fully understood. Lakes in close proximity may vary in both their chemical characteristics and in the degree to which they change in size and depth over time. Climatic, hydrogeologic and natural lake-aging processes of accumulation of organics appear to be primarily responsible for lake-size variations.     

ESTIMATION OF DRAINAGE AREAS IN A HOMOGENEOUS LANDSCAPE1

In areas where there is little or no topographic relief and where soils, vegetation, geologic structure and other factors are essentially uniform, identification of drainage basin boundaries is difficult or impossible. In such a homogeneous landscape, however, assumptions may be made that the hydrologic and geomorphic controls over drainage basin development are constant within the area. If this is true, it is suggested, the drainage area of a stream is related solely to the stream length and factors governing the length-area relationship are also constant. A simple formula based on these assumptions and the gravity model is proposed which can be used to estimate drainage divides in a homogeneous landscape.     

Does Wetland Location Matter When Managing Wetlands for Watershed‐Scale Flood and Drought Resilience?

Wetland protection and restoration strategies that are designed to promote hydrologic resilience do not incorporate the location of wetlands relative to the main stream network. This is primarily attributed to the lack of knowledge on the effects of wetland location on wetland hydrologic function (e.g., flood and drought mitigation). Here, we combined a watershed‐scale, surface–subsurface, fully distributed, physically based hydrologic model with historical, existing, and lost (drained) wetland maps in the Nose Creek watershed in the Prairie Pothole Region of North America to (1) estimate the hydrologic functions of lost wetlands and (2) estimate the hydrologic functions of wetlands located at different distances from the main stream network. Modeling results showed wetland loss altered streamflow, decreasing baseflow and increasing stream peakflow during the period of the precipitation events that led to major flooding in the watershed and downstream cities. In addition, we found that wetlands closer to the main stream network played a disproportionately important role in attenuating peakflow, while wetland location was not important for regulating baseflow. The findings of this study provide information for watershed managers that can help to prioritize wetland restoration efforts for flood or drought risk mitigation.     

A rationale for coastal wetland restoration through spoil bank management in Louisiana,USA

The rationale and outline of an implementation plan for restoring coastal wetlands in Louisiana is presented. The rationale for the plan is based on reversing the consequences of documented cause-and-effect relationships between wetland loss and hydrologic change. The main feature is to modify the extensive interlocking network of dredged spoil deposits, or spoil banks, by reestablishing a more natural water flow at moderate flow velocity (<5 cm/sec). Guidelines for site selection from thousands of potential sites are proposed. Examples of suitable sites are given for intermediate marshes. These sites exhibit rapid deterioration following partial or complete hydrologic impoundment, implying a strong hydrologic, rather than sedimentological, cause of wetland deterioration. We used an exploratory hydrologic model to guide determination of the amount of spoil bank to be removed. The results from an economic model indicated a very effective cost-benefit ratio. Both models and practical experience with other types of restoration plans, in Louisiana and elsewhere, exhibit an economy of scale, wherein larger projects are more cost effective than smaller projects. However, in contrast to these other projects, spoil bank management may be 100 to 1000 times more cost effective and useful in wetland tracts <1000 ha in size. Modest spoil bank management at numerous small wetland sites appears to offer substantial positive attributes compared to alternative and more intensive management at a few larger wetland sites.     

Use of created cattail (Typha) wetlands in mitigation strategies

In order to balance pressures for land-use development with protection of wetland resources, artificial wetlands have been constructed in an effort to replace lost ecosystems. Despite its regulatory appeal and prominent role in current mitigation strategies, it is unclear whether or not created systems actually compensate for lost wetland resources. Mitigation predictions that rely on artificial wetlands must be analyzed critically in terms of their efficacy. Destruction of wetlands due to burial by coal fly ash at a municipal landfill in Danvers, Massachusetts, USA, provided an opportunity to compare resulting growth of created cattail (Typha) marshes with natural wetland areas. Once the appropriate cattail species was identified for growth under disturbed landfill conditions, two types of artificial wetlands were constructed. The two systems differed in their hydrologic attributes: while one had a surface water flow characteristic of most cattail wetlands, the second system mimicked soil and water conditions found in naturally occurring floating cattail marshes. Comparison of plant growth measurements for two years from the artificial systems with published values for natural cattail marshes revealed similar structure and growth patterns. Experiments are now in progress to investigate the ability of created cattail marshes to remove and accumulate heavy metals from polluted landfill leachate. Research of the type reported here must be pursued aggressively in order to document the performance of artificial wetlands in terms of plant structure and wetland functions. Such research should allow us to start to evaluate whether artificial systems actually compensate for lost wetlands by performing similar functions and providing the concomitant public benefits.     

Capturing LiDAR‐Derived Hydrologic Spatial Parameters to Evaluate Playa Wetlands

The digital elevation model data from traditional stereo photogrammetric methods are inadequate in providing accurate vertical parameters to feed hydrologic models for low‐lying, extremely flat areas. High‐resolution light detection and ranging (LiDAR) data provide the robust capability of capturing small variations in low‐relief playa wetlands. The Rainwater Basin in south‐central Nebraska includes a complex of seasonally shallow playa wetlands that attract millions of migratory waterfowl every spring and fall. This research focuses on the development of a procedure with applicable protocols to produce LiDAR‐derived three‐dimensional wetland maps and to extract the critical surface parameters (i.e., watershed boundaries, flow direction, flow accumulation, and drainage lines) for playa wetlands. The topo‐hydrologic conditions of playa wetlands were evaluated at the watershed level. The results show that in the Rainwater Basin, 70.7% of the historic hydric soil footprints identified in the Soil Survey Geographic (SSURGO) database were not functioning as topographically depressional wetlands. This finding was confirmed by a recent five‐year Annual Habit Survey showing that 69.8% of the historic hydric soil footprints did not function during the spring migratory bird seasons between 2004 and 2009. The majority of playa wetlands' topographic conditions have been substantially changed and the SSURGO data cannot fully reflect current topographic reality in the Rainwater Basin.     

LEAKY ACRES RECHARGE FACILITY: A TEN-YEAR EVALUATION1

From 1971-1980, studies were conducted at Fresno, California, to identify and quantify, where possible, the soil and water chemistry, subsurface geologic, hydrologic, biologic, and operational factors that determine the long term (10-year) effectiveness of basin type artificial ground water recharge through alluvial soils. This paper updates previous findings and refers to publications that describe the geology beneath the basins and regional geology that determine the transmission and storage properties for local ground water management and chemical quality enhancement. High quality irrigation water from the Kings River was used for recharge. Construction and land costs for the present expanded facility 83 ha (205.2 ac) using three parcels of land were $1,457,100. The nine-year annual mean costs for only canal water, maintenance, and operation were $110.42/ha·m ($13.62/ac·ft) based on an average recharge rate of 1338 ha·m/yr (10,848 ac·ft/yr) at 86 percent facility efficiency. The measured end of season recharge rate averaged 14.97 ± 0.24 cm/day. The 10-year mean actual recharge rate based on actual water delivered, total ponded area, and total days of recharge was 12.1 cm/day.