Description of hydrologic characteristics requires detailed knowledge of the duration and timing of surface inundation, both yearly and long-term, as well as an understanding of groundwater fluctuations. Because such information is seldom available, the Water Regimes that, in part, determine characteristic wetland and deepwater plant and animal communities are described here in only general terms. Water Regimes are grouped under three major headings, Tidal Salt, Nontidal, and Tidal Fresh. 


Tidal Salt


Tidal Salt Water Regime Modifiers are used for wetlands and deepwater habitats in the Marine and Estuarine Systems, where ocean-derived salinity equals or exceeds 0.5 ppt. These Water Regimes are primarily a function of oceanic tides. 


Subtidal: Tidal salt water continuously covers the substrate. 


Irregularly Exposed: Tides expose the substrate less often than daily. 


Regularly Flooded: Tides alternately flood and expose the substrate at least once daily.


Irregularly Flooded: Tides flood the substrate less often than daily. 


The periodicity and amplitude of tides vary in different parts of the U.S., mainly because of differences in latitude and geomorphology. On the Atlantic Coast, two nearly equal high tides are the rule (semidiurnal); on the Gulf Coast, there is frequently only one high tide and one low tide each day (diurnal); and on the Pacific Coast there are usually two unequal high tides and two unequal low tides (mixed semidiurnal). 


Tides range in height from about 9.5 m (31 ft) at St. John, New Brunswick (NOAA 1973) to less than 1 m (3.3 ft) along the Louisiana coast (Chabreck 1972). Tides of only 10 cm (4.0 inches) are not uncommon in Louisiana. Therefore, although no hard and fast rules apply, the division between Regularly Flooded and Irregularly Flooded Water Regimes would probably occur approximately at mean high water on the Atlantic Coast, at the lowest level of the higher high tide on the Pacific Coast, and just above mean tide level of the Gulf Coast. The width of the intertidal zone is determined by the tidal range, the slope of the shoreline, and the degree of exposure of the site to wind and waves.


Nontidal


Nontidal Water Regime Modifiers are used for all nontidal parts of the Palustrine, Lacustrine, and Riverine Systems. Although not influenced by oceanic tides, Nontidal Water Regimes may be affected by wind or seiches in lakes. Nontidal Water Regimes are defined in terms of the growing season which, for the purposes of this classification, begins with green-up and bud-break of native plants in the spring and ends with plant dieback and leaf-drop in the fall due to the onset of cold weather. During the rest of the year, which is defined as the dormant season, even extended periods of flooding may have little influence on the development or survival of plant communities. 


Permanently Flooded: Water covers the substrate throughout the year in all years. 


Intermittently Exposed: Water covers the substrate throughout the year except in years of extreme drought. 


Semipermanently Flooded: Surface water persists throughout the growing season in most years. When surface water is absent, the water table is usually at or very near the land surface. 


Seasonally Flooded: Surface water is present for extended periods (generally for more than a month) during the growing season, but is absent by the end of the season in most years. When surface water is absent, the depth to substrate saturation may vary considerably among sites and among years. 


Seasonally Flooded-Saturated: Surface water is present for extended periods (generally for more than a month) during the growing season, but is absent by the end of the season in most years. When surface water is absent, the substrate typically remains saturated at or near the surface. 


Seasonally Saturated: The substrate is saturated at or near the surface for extended periods during the growing season, but unsaturated conditions prevail by the end of the season in most years. Surface water is typically absent, but may occur for a few days after heavy rain and upland runoff. 


Continuously Saturated: The substrate is saturated at or near the surface throughout the year in all, or most, years. Widespread surface inundation is rare, but water may be present in shallow depressions that intersect the groundwater table, particularly on a floating peat mat. 


Temporarily Flooded: Surface water is present for brief periods (from a few days to a few weeks) during the growing season, but the water table usually lies well below the ground surface for the most of the season. 


Intermittently Flooded: The substrate is usually exposed, but surface water is present for variable periods without detectable seasonal periodicity. Weeks, months, or even years may intervene between periods of inundation. The dominant plant communities under this Water Regime may change as soil moisture conditions change. Some areas exhibiting this Water Regime do not fall within our definition of wetland because they do not have hydric soils or support hydrophytes. This Water Regime is generally limited to the arid West. 


Artificially Flooded: The amount and duration of flooding are controlled by means of pumps or siphons in combination with dikes, berms, or dams. The vegetation growing on these areas cannot be considered a reliable indicator of Water Regime. Examples of Artificially Flooded wetlands are some agricultural lands managed under a rice-soybean rotation, and wildlife management areas where forests, crops, or pioneer plants may be flooded or dewatered to attract wetland wildlife. Neither wetlands within or resulting from leakage from man-made impoundments, nor irrigated pasture lands supplied by diversion ditches or artesian wells, are included under this Modifier. The Artificially Flooded Water Regime Modifier should not be used for impoundments or excavated wetlands unless both water inputs and outputs are controlled to achieve a specific depth and duration of flooding. 


Tidal Fresh


The Tidal Subsystem of the Riverine System and tidally influenced parts of the Palustrine and Lacustrine Systems are unique because the hydrology of their habitats is driven primarily by nontidal inputs and outputs, but influenced by tides as well. In these habitats, ocean-derived salts measure less than 0.5 ppt. The hydrologic regimes of Tidal Fresh habitats are described through the use of four Nontidal Water Regime Modifiers, coupled with the suffix ‘-Tidal Fresh’, as in Seasonally Flooded-Tidal Fresh, and an additional Modifier based on tidal action. Each of the first four Modifiers reflects the relative duration of substrate inundation during the growing season. It also indicates that, in response to oceanic tides, the water level at each site rises and falls daily. The fifth Modifier focuses specifically on the daily flooding and exposure of the substrate by tides at the water’s edge. Use of certain Modifiers is limited to certain Systems. 


Permanently Flooded-Tidal Fresh: Tidal fresh water covers the substrate throughout the year in all years. This Modifier is used for Riverine, Lacustrine, and Palustrine habitats. 


Semipermanently Flooded-Tidal Fresh: Tidal fresh surface water persists throughout the growing season in most years. When surface water is absent, the water table is usually at or very near the land surface. This Modifier is used for Riverine, Lacustrine, and Palustrine habitats. 


Seasonally Flooded-Tidal Fresh: Tidal fresh surface water is present for extended periods (generally for more than a month) during the growing season, but is absent by the end of the season in most years. When surface water is absent, the depth to substrate saturation may vary considerably among sites and among years. This Modifier is used for Palustrine habitats only.


Temporarily Flooded-Tidal Fresh: Tidal fresh surface water is present for brief periods (from a few days to a few weeks) during the growing season, but the water table usually lies well below the ground surface for the most of the season. This Modifier is used for Palustrine habitats only. 


Regularly Flooded-Tidal Fresh: Tides alternately flood and expose the substrate daily for variable periods (from a few weeks to several months) during the growing season. This Modifier is used for Riverine and Lacustrine habitats.

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