In Florida, sometimes our lakes dry up, our rivers run low, our swamps become mud bogs, our marshes become sandy.... Rainfall has much to do with our fluctuating water: in some years the rainfall is plentiful and the water is high; in other years rainfall is relatively scarce and water is not replenished at the surface or into the aquifers, and water is low. However, rainfall amounts and aquifer levels are not the only reasons why a lake may cycle between a full lake this year and a grassy depression a few years later.Water moves from our lakes, rivers, marshes and swamps in a variety of ways. For examples, water may seep into the ground, it may be pumped out, or water may flow away. Close examination of almost all Florida lakes will reveal a natural outfall slough or constructed canal, ditch, or other conveyance regulated by a weir, dam, levee, flood control structure, etc.
Or water may be evaporated from the open surface, or it may be transpired through aquatic and wetland plants. Evaporation may account for the loss of many inches, thousands of tons, of water from a water body in a single year. Evapotranspiration by plants may account for much more water loss than from evaporation.
EVAPORATION
Evaporation is the process by which water vaporizes and escapes from the water surface, rising into the atmosphere. Thus clouds may form. In many open water lakes and wetlands, evaporation is the major outflow component of the hydrologic cycle. Evaporation rates are measured using a number of formulae: the Priestly-Taylor Equation, Penman Equation, DeBruin-Keijman Equation, Papadakis Equation and others. Penman's Equation is the favored one.
Evaporation rates are affected by latitude of the water body (solar energy input), air and water temperatures, air pressure, wind velocity over the water surface and turbulence in the water. In low rain years, evaporation loss may exceed the amount of gain from rainfall.
Evaporation rates are also affected by the plant coverage on a lake's surface. In general, evaporation decreases as emersed plant (e.g., cattail) and floating plant (e.g., water hyacinth) coverage increases.
However, in the case of floating-leaved plants (e.g., water lily), most species actually reduce the evaporation loss of water in a lake, when plant leaf coverage is high. Research shows that evaporation losses in lakes and ponds covered with floating-leaved plants are lower than in lakes with no plant coverage. Floating-leaved plants that reduce evaporative water loss include Salvinia, Azolla, Nymphaea, Nelumbo, Lemna, Wolffia, and Spirodela.
Some Reported Evaporation Rates
Open water evaporation rates reported from around the world range from 30 inches/year to 103 inches/year.
An open water lake in Orlando loses 58 inches to evaporation in a typical year. Daily losses range from 0.04 inches/day in January to 0.26 inches/day in May.
In a north Florida lake, evaporation was calculated to be 33 inches for the six warmest months of the year.
Total water loss in an aquatic system, among the other factors mentioned above, has to do with a combination of open water evaporation and plant evapotranspiration.
The more floating-leaved plants present, the less evaporation, because there is less exposed water for evaporation. At the same time, emersed and floating plants, such as cattail and water hyacinth, because of their structure and leaf area, transpire more water than would evaporate in the same area. Therefore, lakes filled with emersed and floating plants will lose more water to the atmosphere than will open water lakes having few plants. Lakes covered with water hyacinth will lose water much more quickly than will open water lakes.
On the other hand, floating-leaved plants, such as duckweeds and lotus, that have flat, often overlapping leaves, reduces evaporation, because there is less exposed water for evaporation, but also, because their structure and habit is different from emersed and floating plants, do not transpire as much water as would evaporate in the same area. Therefore, lakes with many floating-leaved plants will lose less water than will open water lakes. Lakes covered with duckweed will hold water for a longer time than will open water lakes.
EVAPOTRANSPIRATION
Evapotranspiration is the process by which water is taken up by plants, used and transpired (emitted as water vapor) to the atmosphere. In most situations, aquatic macrophyte evapotranspiration greatly increases water loss from a water body. (Exceptions include swamps with much dead plant material accumulated over years; dense floating-leaved plant coverage having leaves in a single horizontal layer diminishes water loss.) Research shows that most kinds of aquatic plants enhance water loss in a water body. In subtropical areas, such as Florida, water hyacinth has high evapotranspiration rates throughout the year.
Water loss through evapotranspiration by aquatic plants must be taken into account when storing water or when planning for the construction of large water storage systems such as reservoirs.
Evapotranspiration rates of plants depend on those plant characteristics that expose more photosynthesizing cells to the air, such as height (especially for cattail), and leaf area index (for water hyacinth and water lettuce). Water hyacinth leaf and petiole surface area is greater than cattail leaf and stem surface area.
Environmental factors also greatly affect evapotranspiration rates on a water body. Primary environmental factors are latitude (solar energy input), air and water temperatures, wind speed, relative humidity, plant growth phase, and nutrient availability. Evapotranspiration takes place mainly from 10 am to 6 pm.
Some Reported Evapotranspiration Rates
In studies, water hyacinth evapotranspiration water loss has been calculated to be from 1.3 to 2.7 times greater than evaporation water loss, depending on lake location and conditions. In some studies, water hyacinth transpired up to .5 inch/day, with most loss occuring between August and October.
In one study, a single acre of water hyacinth transpired 21,000 gallons (87 tons) of fresh water per day. Water hyacinth increased water loss over evaporation losses by 30 inches per year.
An acre of open water under the same conditions would evaporate about 9,000 gallons (37 tons) of fresh water per day.
An acre of water lettuce transpired 9,000 gallons per day; salvinia transpired 8,500 gallons per day; azolla transpired 7,600 gallons per day.
In other experiments, other aquatic plants also transpired much more water than would be lost by evaporation of open water.
Some Reported Ratios
Evapotranspiration Losses Over Evaporation Losses
(Numbers less than 1.0 mean that those plants actually reduce water loss.)
Eichhornia crassipes (water hyacinth) transpires 1.26, 1.62, and 2.7 times the amount of water as would evaporate over open water
Typha latifolia (cattail) 1.75, 1.8, 2.5, 2.0
Acorus calamus 2.0
Scirpus validus (bulrush) 1.9
Panicum rigidulum (panic grass) 1.58
Juncus effusus (rush) 1.52
Carex lurida 1.33
Alternanthera philoxeroides (alligatorweed) 1.26
Pontederia cordata (pickerelweed) 1.2
Justicia americana 1.17
Nymphaea odorata (water lily) 1.0
Lemna minor (small duckweed) 0.9
Wolffia columbiana (water meal) 0.89
Spirodela polyrhiza (giant duckweed) 0.85
http://plants.ifas.ufl.edu/guide/evaptran.html
CAIP-WEBSITE@ufl.edu
3 comments:
Nice article...everyone should know this.
Please provide some examples of lakes or ponds covered by water lillies or duckweeds. Please inform the evaporation losses in ponds covered by water lillies or duckweeds.
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