need of study. Several important principles have been established as a framework for hydrologic assessment of wetlands.

The duration and frequency of saturation or inundation of a site vary according to the site' s hydrogeologic setting, and they depend on regional differences in physiography and climate and on antecedent moisture conditions (Skaggs et al., 1991; Winter, 1992; Brinson, 1993a; Mausbach and Richardson, 1994). The duration of saturation or inundation can be depicted for a wetland's hydroperiod, on a graph that shows the position of the water table or standing water in the area over time. A wetland's hydroperiod integrates all aspects of its water budget (rainfall, evapotranspiration, runoff from adjacent areas, flooding, net seepage of ground water). A major technical challenge is to determine an average or characteristic hydroperiod for sites on which there are no hydrologic data, or for which hydrologic data cover only a short interval.

Figure 2.3 shows hydroperiods for selected wetlands. The elevation of the water surface is shown relative to the elevation of the land surface, which is arbitrarily set at zero. As shown by Figure 2.3, water levels in some wetlands (for example, a marsh maintained by ground water, or a tidal marsh) are always above or close to the surface; In contrast, water levels in bottomland hardwood forest might come close to the surface only during specific periods of the year. Water levels in a tidal salt marsh can fluctuate dally. The water levels in a fen, which is maintained mainly by continuous ground water discharge, fluctuate the least. In many wetlands that are wet only seasonally, direct evidence of wetland hydrology might not be obvious for relatively long periods.

The hydrologic boundary of a wetland is different from the hydrologic boundary of the watershed that contains it. The wetland is that locus of points in which the water balance produces enough saturation to maintain substrate and biota that are characteristic of wetlands. In contrast, the watershed that contains the wetland typically includes upland areas that share a common drainage pathway with the wetland. The wetland boundary might change over time as a complex function of factors that control the balance of terms in the water budget for the entire watershed. Climate change would be the most basic natural cause of change in the boundary of a wetland, but other factors—for example, sedimentation in channels, earthquakes, the activities of beavers, and land management practices—can alter hydrology and change the size of a wetland.

Because particular hydrologic conditions are essential requirements for wetlands, it is logical that hydrology be evaluated when wetlands are identified or delineated. This is now the case: All wetland delineation manuals require direct

FRONT MATTER (R1-R20)

EXECUTIVE SUMMARY (1-12)

1 INTRODUCTION AND BACKGROUND (13-19)

2 ECOLOGY OF WETLAND ECOSYSTEMS (20-42)

3 WETLAND DEFINITIONS: HISTORY AND SCIENTIFIC BASIS (43-64)

4 WETLAND DELINEATION: PAST AND CURRENT PRACTICE (65-89)

5 WETLAND CHARACTERIZATION: WATER, SUBSTRATE, AND BIOTA (90-148)

6 ESPECIALLY CONTROVERSIAL WETLANDS (149-167)

7 REGIONALIZATION (168-189)

8 MAPS, IMAGES, AND MODELING IN THE ASSESSMENT OF WETLANDS (190-206)

9 REGULATION OF WETLANDS: ADMINISTRATIVE ISSUES (207-214)

10 FUNCTIONAL ASSESSMENT OF WETLANDS (215-226)

REFERENCES (227-252)

APPENDIXES (253-296)

INDEX (297-306)

LIST OF PUBLICATIONS (307-308)