Wetlands Characteristics and Boundaries (1995) / Chapter Skim
Currently Skimming:
8 MAPS, IMAGES, AND MODELING IN THE ASSESSMENT OF WETLANDS
8 MAPS, IMAGES, AND MODELING IN THE ASSESSMENT OF WETLANDS
Pages 190-206
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 190... ...
Offsite methods should be used only when their inherent limitations are recognized, as described here. AERIAL PHOTOGRAPHY AND SATELLITE IMAGING Aerial photography has been used to map wetlands for at least three decades (Olson, 1964; Anderson and Wobber, 1973; Seher and Tueller, 1973; Cowardin and Myers, 1974; Hardy and Johnston, 1975; Gammon and Carter, 1979; Johnston, 1984J.
|
From page 191... ...
This methodology also was tested for delineation of wetlands in the prairie pothole region of North Dakota (FGDC, 19921. Maps produced from TM images were adopted by NRC S for the Yazoo River Basin in Mississippi, but not for North Dakota, because their accuracy was considered too poor in the drier western half of the state (North Dakota State Conservationist, presentation at the National Interagency Memorandum of Agreement Meeting, St.
|
From page 192... ...
The increasing accuracy and decreasing cost of global positioning systems are improving the georeferencing of ground locations, but this technology is not yet widely available at the fineness of resolution needed for wetland delineation. WETLAND DELINEATION UNDER THE FOOD SECURITY ACT In contrast to the field-intensive methods used to identify wetland boundaries for Clean Water Act Section 404 permits, wetland delineations required by FSA are done primarily by offsite methods.
|
From page 193... ...
Determinations Before 1994 After enactment of FSA, each state developed mapping conventions for indirect wetland determinations on agricultural lands, with technical guidance from the four NRCS National Technical Centers (Midwest, 1988; Northeast, 1989; South, 1989; West, 1988~. Mapping conventions vary slightly by state because of regional differences in wetland characteristics and in the availability of data, but the general methodology is the same except where satellite remote sensing has been used.
|
From page 194... ...
FSA delineations are not subject to public review, and the wetland maps are not published. In addition to making wetland determinations, NRCS also prepares wetland inventory maps by use of ASCS 35 mm slides, aerial photographs, soil surveys, and NWI maps.
|
From page 195... ...
All wetland determinations done after Jan. 6, 1994, become certified on agricultural lands if approved mapping conventions were used.
|
From page 196... ...
Because NWI maps depict wetlands that were present on the date of photography, wetland extent can be estimated incorrectly if atypical expansion or contraction of vegetation was occurring at the time of photography. Interannual variation causes fewer errors where ground water maintains wetland hydrology or where vegetation is resistant to interannual variation (such as in forested wetlands)
|
From page 197... ...
MAPS, IMAGES, AND MODELING IN THE ASSESSMENT OF WETLANDS 197 FIGURE 8.1 NVVI map (reduced) prepared from 1:58,000 color infrared aerial photographs.
|
From page 198... ...
. Its utility as an ancillary data source varies regionally; it is least useful in areas where broad expanses of mineral soils with facultative vegetation and little topographic variation complicate wetland delineation.
|
From page 199... ...
The solution procedure is developed interactively by the user, who creates a series of alternative solutions and then selects the most viable. This approach could be used with a wetland delineation decision tree and digital information on soils, vegetation, and topography.
|
From page 200... ...
200 ~ D:STR:BUT:ON CAPS ABLE ^ . ~ ~ .# ~ ~ FIGURE 8.3 HI digital data as of J=u~ and Febm~ 1994.
|
From page 201... ...
NWI maps are being digitized, and NRCS plans to digitize county soil survey maps under its Soil Survey Geographic Data Base program, which should facilitate identification of hydric soils (Reybold and TeSelle, 1989~. EPA's North American Landscape Characterization program, which was developed in collaboration with the USGS EROS Data Center and the NASA Landsat Pathfinder program, will produce digital land cover maps from 1991 Landsat Multispectral Scanner images.
|
From page 202... ...
, and Cohen and Miller (1983~. Although analytical models can describe drainage processes and water table responses for steady rainfall and short-term drawdown, they are not generally applicable to describing water table fluctuations caused by the combined effects of rainfall, drainage, and evapotranspiration over long periods.
|
From page 203... ...
Functional models usually assume that the wetland behaves as a relatively uniform soil system with well-defined boundary conditions, although layers with different hydraulic properties can be considered. The water budget for the wetland is reduced to algebraic equations, and analytical algorithms are used to calculate flux as a function of water table position, depth, and spacing of natural or constructed drains, potential evapotranspiration, and the like.
|
From page 204... ...
Often several months or even years of field hydrologic data on a wetland are insufficient for classification of a marginal site because of the inherent variability of hydrology (Chapter 5~. In such cases, the field data can be used to test and calibrate a model, which can then be used for simulating water table conditions over a long period of record, providing a good basis for determining whether wetland hydrology exists on the site.
|
From page 205... ...
The moving window technique can be applied to any 2dimensional digital data, including aerial photography scanned with a video digitizer or scanning camera. For example, Johnston and Bonde (1989)
|
From page 206... ...
RECOMMENDATIONS 1. Aerial photography can be useful for wetland delineation and mapping if its timing, frequency, and scale are suited for making wetland determinations.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.