Hydrology

Hydrology = study of patterns of water movement

-extremely important for the maintenance of a wetland's structure and function

Hydrologic conditions affect many abiotic factors such as soil aeration, nutrient availability, and salinity

which then affect flora and fauna which then alter hydrology

 

The hydroperiod or hydrologic signature of a wetland is the balance between inflows and outflows of water (the water budget), the soil contours, and the subsurface conditions.

By measuring these you can determine the turnover time or residence time which is the amount of time it takes for water to cycle through the system

Generally, if turnover time is high, productivity is high. Decomposition is slowest when there is anaerobic standing water so productivity would tend to be low then.

*Hydrology is probably the most important determinant of the establishment and maintenance of specific types of wetlands and wetland processes*

Remember that wetlands represent the aquatic edge of terrestrial (emergent) plants and the terrestrial edge of aquatic (submersed) plants and animals. This means that small changes in hydrology can have big effects.

Vegetation, in turn, can modify hydrology through peat build up, sediment trapping, nutrient retention, water shading and transpiration (plants carrying water upward)

A beaver can completely destroy or create wetlands and alligators are know for their role in the Everglades in creating "gator holes" whihc serve as oases for aquatic animals during the dry season. Everything is inter-related.

Hydroperiod

-seasonal pattern of water level in a wetland

-could be subtidal or regularly or irregularly flooded or exposed if it's tidal; permanently flooded, seasonally flooded, saturated, intermittently flooded, etc. if it is non-tidal

inflows: precipitation, surface inflow, groundwater inflow, tides

outflows: evapotranspiration, surface outflow, groundwater outflow, tides

If you could measure all these, you could arrive at a water budget and assess residence or retention time (t) = average volume/total inflow rate

 

To define some of these terms:

Precipitation

Wetlands occur mostly in regions where precipitation is in excess of evapotranspiration and surface run-off. In a water budget, you must consider net precipitation or the precipitation minus "interception" (what gets intercepted before it hits the ground). For example, the average percent precipitation intercepted in forests varies between 8 and 35%.

Evapotranspiration

This is water vaporizing directly from water or soil plus moisture which gets passed up through vascular plants.

A neat way to measure it:

ET = Sy(24h+/- s)

ET = evapotransporation

Sy = specific yield for aquifer (unitless)

= 1 for standing water

= <1 for groundwater wetlands

h = hourly rise in water level from midnight to 4 am in mm/hr

s = net fall (+) or rise (-) of water surface in one day

Tides

Stress-submergence, saline soil, anaerobic soil

Subsidy-removes excess salts, provides oxygen, provides nutrients

Diurnal (one high one low) vs. semidiurnal (2 high two low)

Spring vs. Neap

 

In summary then,

1. Hydrology leads to unique vegetation but can limit or enhance species richness

-hydrology selects water tolerant vegetation; waterlogged conditions limit the

number and types of plants that can survive, therefore a wetland that is less

wet will tend to have higher species diversity

2. Primary productivity and other ecosystem functions in wetlands are often enhanced by flowing conditions and a pulsing hydroperiod and are often depressed by stagnant conditions.

-In general, the openness of a wetland to hydrological fluxes is probably the most important determinant of potential primary production, i.e. flow through is better than stagnant so hydrology influences wetland productivity by being the main pathway through which nutrients are transported

3. Accumulation of organic matter in wetlands is controlled by hydrology through its influence of primary productivity, decomposition, and export of particulate matter.

-Organics will accumulate more quickly the higher the productivity and the stiller

the water---to a point. If the water is so still that the soil is anaerobic, this is the

least favorable condition for decomposition. Organics are exported best by flow

through systems.

4. Nutrient cycling and nutrient availability are both significantly influenced by hydrologic conditions.

-When productivity and decomposition rates are high, nutrient cycling is rapid,

and vice versa.

 

Wetland Soil

hydric = "soil that is saturated, flooded, or ponded long enough during the growing season to develop anaerobic conditions in the upper part." (U.S. Soil Conservation Service) Wetland soils are of 2 types: (1) mineral soils or (2) organic soils.

(from Mitsch and Gosselink, 1993)

 

I. Organic Soils

Organic soils have lower bulk densities and higher water holding capacities than mineral soils.

Bulk density is an easy thing to measure:

bulk density = dry wgt. of soil material per unit volume

0.2-0.3 g/cm3 for organic soil (unless it's sphagnum moss than as low as .04)

1-2 g/cm3 in mineral soils because of the lack of porosity

For example, peat soils are 80% pore space (that's why you use peat moss in your garden to improve water retention)

 

Organic soil is composed primarily of the remains of plants in various stages of decomposition and accumulates in wetlands as a result of anaerobic conditons created by poorly drained conditions. Organic soils are classified as follows:

1. saprists (muck) = two thirds or more material is decomposed so less than

one third of plant fibers are indentifiable

2. fibrists (peat) = less than one third of the material is decomposed so more

than two thirds of plant fibers are identifiable

3. hemists (mucky peat or peaty muck) = something in between

4. folists = organic soils caused by excessive moisture that accumulates in

mountains (this is not considered hydric)

All these soils are dark in color.

 

II. Mineral Soils

Mineral soils are tougher to identify as hydric. One characteristic is gleying or gleization where iron reduction caused by mostly permanent flooding results in greenish or blue-gray color. Generally though, if a soil has low organic content and is very dark in color, it is likely a mineral hydric.

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