|Hydrology, the way in which a
wetland is supplied with water, is one of the most important factors in
determining the way in which a wetland will function, what plants and animals
will occur within it, and how the wetland should be managed. Since wetlands
occur in a transition zone where water based ecosystems gradually change to
land based ecosystems, a small difference in the amount, timing or duration of
the water supply can result in a profound change in the nature of the wetland
and its unique plants, animals and processes.
Hydroperiod is the seasonal pattern of the water level that results from the combination of the water budget and the storage capacity of the wetland. The water budget is a term applied to the net of the inflows, all the water flowing into, and outflows, all the water flowing out of, a wetland. The storage capacity of the wetland is determined by the geology, the subsurface soil, the groundwater levels, the surface contours and the vegetation. The hydroperiod of coastal wetlands exhibits the daily and monthly fluctuations associated with tides, whereas inland wetlands tend to show, to a greater degree, the effects of storm and seasonal events such as spring thaw, fall rains and intermittent storm events.
In headwater areas where streams originate, watersheds tend to be small and have shallow soils with low water storage capacity. Hydroperiods of wetlands in headwater areas often show water levels that rise and fall rapidly in response to localized storm events which supply the streams and wetlands with runoff. An exception is areas where soils are dominated by sandy glacial deposits. These areastend to have deeper soils, gentle slopes and more predictable hydroperiods. Sandy glacial deposits also tend to occur in colder climates and to be frozen for a period of the year often providing the opportunity to conduct management operations under frozen conditions.
Larger streams, which receive much of their water from the combined flows of many smaller streams, tend to respond more slowly to precipitation and exhibit the results of the average conditions over the larger combined watershed as opposed to local storm events.
Hydroperiods of wooded swamps associated with larger river systems tend to show water levels that reflect events general to the larger area such as fall rains and spring thaw and are, therefore, more predictable.
The number of times that a wetland is flooded within a specific time period, such as yearly, is known as the flood frequency. Flood duration is the amount of time that the wetland is actually covered with water. It should also be noted that many wetlands are never flooded, but the wetland definition does require the soil to be saturated for at least a part of the growing season. Only hydrophytes, a relatively small group of vascular plants with special adaptations which includes many endangered species, are able to survive in soils that are saturated for more than a short period during the growing season. Therefore, the duration and timing of flooding and or saturation will limit the number of species that can survive in the wetland.
Residence time is a measure of the time it takes a given amount of water to move into, through and out of the wetland. Since chemical processes take time and follow one another sequentially, the degree to which wetlands can change water chemistry is determined to an extent by residence time. This is one reason why it is important not to create a channel across a wetland. in the direction of flow, increasing outflow rate and decreasing residence time.
Wetlands receiving inflow from groundwater are known as discharging wetlands because water flows or discharges from the groundwater to the wetland. A recharge wetland refers to the reverse case where water flows from the wetland to the groundwater. Recharge and discharge are determined by the elevation of the water level in the wetland with respect to the water table in the surrounding area. Riparian wetlands often have both functions, they are discharge wetlands, receiving groundwater inflow from upslope areas and they are recharge wetlands in that they feed lower elevation groundwater through groundwater outflow. The same wetland may be a discharging wetland in a season of high flow and a recharging wetland in a dry season. Seeps at the base of mountains are often discharging wetlands formed by groundwater breaking through to the surface of the
ground at the base of steep slopes. Vernal ponds often occur in these areas, but are an exception in that groundwater flow to and from vernal ponds is practically nil in the northeast. Mountain top swamps are often recharging wetlands with groundwater out-flows to a water table much lower in elevation.
Inflow water reaches the wetland from precipitation, surface flow, subsurface flow and groundwater flow. Surface flow includes surface runoff, stream-flow and flood waters. Flood waters can carry nutrient laden sediments to forested floodplains where these sediments are deposited making the soil very fertile. Forested floodplains can be very productive.
Oufflow leaves the wetland by evaporation, transpiration, surface flow, subsurface flow or groundwater flow. Evaporation is water given off to the atmosphere as water vapor. Transpiration is the process whereby water taken up by plant roots is released as vapor to the atmosphere from the plant leaves. Surface flows out of wetlands can be small or large and are often the origin point of streams. Wetlands are often connected, to a degree, with surface and groundwater outflows of one wetland supplying the inflows to other wetlands lower in the watershed. The water supply to the lower wetland can be delayed until the upper wetland fills to a point where additional water runs off. As a result, some wetlands will not be as well supplied as others in dry periods.
As stated previously, the water storage capacity of the wetland is affected by the soil, the groundwater level and the surface contour. Wetlands generally occur in natural depressions in the landscape where geologic or soil layers restrict drainage. The surface contours act to collect precipitation and runoff water and feed it to the depressed area. Groundwater recharge can take place if the soil is not already saturated and the surface contours of the basin hold the water in place long enough for it to percolate into the soil. In many cases the shape of the basin is such that it can be rapidly filled by precipitation or flood waters and the water slowly released by a restricted surface outlet, by slowly permeable soil or by geologic conditions.
The water storage capacity of the wetland determines the volume and timing of water reaching the stream from precipitation. The precipitation reaches the stream in the form of groundwater or surface runoff to contribute to streamflow discharge. A hydrograph is a graph of the volume and timing of streamflow discharge measured at a certain point in the watershed. The hydrograph shows the lapse time, the amount of time between the onset of precipitation and the peak stormflow discharge. It also depicts the volume and distribution of the stormflow discharge over time. Wetlands tend to have longer response times and lower peak stormflows distributed over longer time periods. Urban and developed lands tend to have short response times and high volume, short duration stormflow discharges. The overall effect is that watersheds with wetlands tend to store and distribute stormflows over longer time periods resulting in lower levels of streamflow and reduced probability of flooding.
The Natural Valley Storage Project, a 1976 study by the U.S. Army Corps of Engineers (COE) concluded that retaining 8,500 acres of wetlands in the Charles River Basin near Boston, Massachusetts could prevent flood damages estimated at $6,000,000 for a single hurricane event. Projected into perpetuity the value of such protection is enormous. Based on this study, the COE opted to purchase the wetlands for $7,300,000 in lieu of building a $30,000,000 flood control structure (Thibodeau and Ostro 1981).Loss of floodplain forested wetlands and confinement by levees have reduced the floodwater storage capacity of the Mississippi by 80 percent increasing dramatically the potential for flood damage (Gosselink et al., 1981) The 1993 flood proved this prediction to be true and resulted in immeasurable damage. Yet governments are allowing and even assisting the rebuilding of some of these same levees fostering potentially more damage in the future instead of promoting land use change to restore the wetlands and reduce the potential for future damage.
After Thibadeau and Ostro 1981
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