Pan evaporation
From Wikipedia, the free encyclopedia
Pan evaporation is a measurement that combines or integrates the effects of several climate elements: temperature, humidity, solar radiation, and wind. Evaporation is greatest on hot, windy, dry days; and is greatly reduced when air is cool, calm, and humid.[1] Pan evaporation measurements enable farmers and ranchers to understand how much water their crops will need.[2]
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[edit] Evaporation pan
An Evaporation pan is used to hold water during observations for the determination of the quantity of evaporation at a given location. Such pans are of varying sizes and shapes, the most commonly used being circular or square.[3] The best known of the pans are the "Class A" evaporation pan and the "Sunken Colorado Pan"[4]
Since the results of measuring pan evaporation from one pan do not compare with results measured with others, the National Weather Service has standardized its measurements on the Class A pan.
A Class A evaporation pan is cylindrical with a diameter of 47.5 in (120.7 cm) has a depth of 10 in (25 cm). The pan rests on a carefully leveled, wooden base and is often enclosed by a chain link fence to prevent animals drinking from it. Evaporation is measured daily as the depth of water (in inches) evaporates from the pan. The measurement day begins with the pan filled to exactly two inches (5 cm) from the pan top. At the end of 24 hours, the amount of water to refill the pan to exactly two inches from its top is measured.
If precipitation occurs in the 24-hour period, it is taken into account in calculating the evaporation. Sometimes precipitation is greater than evaporation, and measured increments of water must be dipped from the pan. Evaporation cannot be measured in a Class A pan when the pan's water surface is frozen; therefore, the evaporation values shown on the map are for the evaporation season (May through October).
The Sunkend Colorado Pan is square, 36 in (92 cm) on a side and 16 in (41 cm) deep. And, as the name suggests is buried in the ground. Evaporation from a Sunken Colorado Pan can be compared with a Class A pan using conversion constants.
Often the evaporation pans are automated with water level sensors and a small weather station is located nearby.
[edit] Decreasing Trend of Pan Evaporation
- Further information: Global dimming and Global warming
Over the last 50 or so years, pan evaporation has been carefully monitored. For decades, nobody took much notice of the pan evaporation measurements. But in the 1990s scientists spotted something that at the time was considered very strange, the rate of evaporation was falling.[5] This trend has been observed all over the world except in a few places where it has increased.[6] [7] [8] [9]
As the global climate warms, all other things being equal, evaporation will increase and as a result, the hydrological cycle will accelerate [10]. The downward trend of pan evaporation has been linked to a phenomenon called Global dimming.[11] [12]In 2005 Wild et al. and Pinker et al. found that the "dimming" trend had reversed since about 1990 [13]
[edit] Lake Evaporation vs. Pan Evaporation
There appears to be a correlation between lake evaporation and pan evaporation[14] Evaporation from a natural body of water is usually at a lower rate because the body of water does not have metal sides that get hot with the sun. Most textbooks suggest multiplying the pan evaporation by 0.75 to correct for this.
[edit] Relationship to Hydrological Cycle
- Further information: Global dimming and Global warming
"It is generally agreed that the evaporation from pans has been decreasing for the past half century over many regions of the Earth. However, the significance of this negative trend, as regards terrestrial evaporation, is still somewhat controversial, and its implications for the global hydrologic cycle remain unclear. The controversy stems from the alternative views that these evaporative changes resulted, either from global radiative dimming, or from the complementary relationship between pan and terrestrial evaporation. Actually, these factors are not mutually exclusive but act concurrently." [15]
[edit] Notes and references
- ^ http://www.engineering.usu.edu/uwrl/atlas/ch3/ch3overview.html
- ^ http://edis.ifas.ufl.edu/AE118
- ^ http://www.crh.noaa.gov/glossary.php?letter=e
- ^ http://www.fao.org/docrep/S2022E/s2022e07.htm
- ^ Roderick, Michael L. and Graham D. Farquhar (2002). "The cause of decreased pan evaporation over the past 50 years". Science 298 (5597): pp. 1410-1411.
- ^ http://blue.atmos.colostate.edu/publications/pdf/roderick2004.pdf
- ^ http://www.dnr.sc.gov/climate/sco/pan_evap.html
- ^ http://www.mindfully.org/Air/2002/Decreased-Pan-Evaporation1nov02.htm
- ^ http://www.agu.org/pubs/crossref/2004/2004JD004511.shtml
- ^ http://intl.sciencemag.org/cgi/content/summary/298/5597/1345?ck=nck
- ^ http://www.bbc.co.uk/sn/tvradio/programmes/horizon/dimming_trans.shtml
- ^ http://www.pbs.org/wgbh/nova/transcripts/3310_sun.html
- ^ Global Dimming may have a brighter future. Retrieved on 2006-06-12.
- ^ http://www-das.uwyo.edu/~geerts/cwx/notes/chap04/eoep.html
- ^ Brutsaert, Wilfried (2006). "Indications of increasing land surface evaporation during the second half of the 20th century". Geophysical Research Letters 33 (20): pp. 1410-1411.
