WETLAND HYDROLOGY

WETLAND HYDROLOGY

A. The delineation of wetlands are based upon three environmental parameters:

      1. hydrology - the wetland is inundated permanently or periodically by either groundwater, surface water and/or precipitation
      2. vegetation - hydrophytic plants are present
      3. soils - hydric soils are identified

B. Wetlands in landscapes

1. Closed depressions with a fluctuating water table

2. Closed depressions with a perched water table

3. Floodplain morphology

4. Wetlands on slopes

C. Water balance in wetlands

Inputs = Outputs + D Storage

P + Ho + GWd = GWr + So + ET + D Storage

          P = precipitation
          Ho = runoff
          GWd = groundwater discharge
          GWr = groundwater recharge
          So = surface outflow
          ET = evapotranspiration
          D Storage = change in water storage

1. The terms groundwater recharge and groundwater discharge are frequently used when describing the hydrology of wetlands - always use these terms in relation to the groundwater
a. areas of groundwater recharge indicate that water is moving down through the soil/or and geologic strata and contributing to the groundwater supply - a recharge wetland means that water is moving from out of the wetland and into the groundwater
b. areas of groundwater discharge indicate the the groundwater is contributing water to an above ground water body such as a river, lake, stream, wetland, etc - a discharge wetland means that water is moving from the groundwater and feeding into the wetland

D. Wetlands Delineation Manual - 1987 - US Army COE - requirements for jurisdictional wetlands (wetlands are regulated according to federal and state laws)

1. Wetland hydrology encompasses all hydrologic characteristics of areas that are periodically inundated or have soils saturated to the surface at some time during the growing season. Areas with evident characteristics of wetland hydrology are those where the presence of water has an overriding influence on characteristics of vegetation and soils due to anaerobic and reducing conditions. Such characteristics are usually present in areas that are inundated or have soils that are saturated to the surface for sufficient duration to develop hydric soils and support vegetation typically adapted for life in periodically anaerobic conditions.

2. Guidance on duration of saturation for wetland hydrology - greater than 5% of the growing season in most years. The growing season is the portion of the year when soil temperatures (measured at 20 inches below the surface) is above biological zero (5^o C). The growing season in Minneapolis is from about May 1 - October 1 (153 days) - 5% of the growing season would be at least 8 days.

3. The duration criteria also includes the requirement that the conditions of saturation or inundation be present in most years. The COE manual indicates that most years means 51 out of 100 years. Climatic data is used instead of the actual and experimental field evidence of the duration requirement. Currently WETS tables are used to evaluate normal rainfall. How representative of normal conditions are hydrologic data collected in a particular year or season of the year? WETS tables use monthly precipitation data collected from ~ 8000 weather stations. Normal rainfall is then based upon a standard 30 years of weather data. The WETS tables provide monthly and annual thresholds for below normal rainfall (lowest 3 years in 10) and above normal rainfall (highest three years in 10). Other kinds of hydrologic data include tide and stream gages, groundwater monitoring wells, and aerial photography. Some sources of hydrologic data include COE district offices, USGS, NOAA, NRCS, state, county and local agencies, local landowners, etc.

4. Field methodology - examination of the hydrologic parameter requires digging a soils pit to depth of 16 inches and observing the level at which water stands in the hole after sufficient length of time has been allowed for water to drain into the hole. This level represents the depth to the water table. The depth to water table will always be nearer to the surface due to the capillary fringe. For saturation to impact vegetation it must occur within a major portion of the root zone (usually within 12 inches of the surface). With a water table at this shallow depth (within 12 inches of the surface) it is generally assumed that saturation to the surface will occur due to water table fluctuations or capillary action.

The capillary fringe exists directly above the water table. The soil above the water table is saturated and water is held under tension. The capillary fringe is of varying thicknesses depending upon the average pore size (which is related to texture).

Using constants and units in centimeters the equation above reduces to:

Hc = 0.15/r

Therefore, the height of rise of the capillary fringe is inversely proportional to the average radius of the pores.

5. Because hydrologic data is often unavailable for project sites, the Corps of Engineers Manual allows a number of primary indicators to provide sufficient evidence of wetland hydrology. When combined with a hydrophytic plant community and hydric soils, these indicators can be used to verify the presence of wetland hydrology:

a. sediment deposits

b. watermarks

c. drift lines

d. visual evidence of soil saturation

e. visual evidence of inundation

There are also a list of secondary indicators. At least two of these secondary indicators are required for the presence of wetland hydrology when combined with hydrophytic plants and hydric soils. Examples of secondary indicators are:

a. oxidized root channels

b. water stained leaves

c. local soil survey data

E. Guidance on the delineation of hydrologic alteration of wetlands

Hydrophytic vegetation and hydric soils (soil morphology) may persist for decades following the alteration of hydrology that may render an area a non-wetland. Onsite evidence of drained soils includes: (1) presence of ditches or canals of sufficient depth to lower the water table below the major portion of the rooting zone (2) presence of dikes or levees that obstruct inundation or divert surface runoff (3) presence of a tile system to promote subsurface drainage. When hydrology has been altered or is suspected to have been altered, the wetland specialist should perform an assessment of the hydrologic parameter using the following procedures for jurisdictional wetland delineation: (1) describe the type of alteration - the effects of urbanization and agricultural use that may result in the overall decline in the health and magnitude of the wetland resource (2) describe the effects of the alteration - a decline in water table depth due to drainage recorded by monitoring wells; an increase in storage and an enlarged wetland resulting from stormwater inputs from urbanization; efficient removal of flood flows by stream channelization resulting in riparian wetlands becoming drier and (3) characterize the preexisting condition - use existing aerial photography, soil survey maps and any other available soils/vegetation documentation.

F. Darcy's Law

The equation is used to quantify groundwater flow:

                  v = K (dH/dL)
                      v = velocity of flow
                      K = hydraulic conductivity
                      dH = difference in hydraulic head
                      dL = length of the flow path
                     (dH/dL) = hydraulic gradient

G. Piezometers and water table wells

H. Cone of depression

I. Slope morphology and landscape elements

A. Slope length and slope gradient influence the topographic location of wetlands

B. Divergent and convergent slopes

J. The soil's hydrologic cycle

K. Effect of layering and contrasting textures on groundwater flow