Pan evaporation

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 [cite web
url=http://www.engineering.usu.edu/uwrl/atlas/ch3/ch3overview.html
title=Class A Pan Evaporation] . Pan evaporation measurements enable farmers and ranchers to understand how much water their crops will need. [cite web
url=http://edis.ifas.ufl.edu/AE118
title=Irrigation Scheduling with Evaporation Pans]

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. [cite web
url=http://www.crh.noaa.gov/glossary.php?letter=e
title=NOAA Glossary:Evaporation Pan] The best known of the pans are the "Class A" evaporation pan and the "Sunken Colorado Pan" [cite web
url=http://www.fao.org/docrep/S2022E/s2022e07.htm
title=fao.org Chapter 3: Crop Water Needs] . In Europe, India and South Africa, a Symon's Pan (or sometimes Symon's Tank) is used. Often the evaporation pans are automated with water level sensors and a small weather station is located nearby. It has been noted that over the past 50 years, the amount of pan evaporation has gradually decreased. Researchers have found strong ties to Global dimming and Climate change.

Standard methods

A variety of evaporation pans are used throughout the world. There are formulas for converting from one type of pan to another and to measures representative of the environment [cite journal
quotes =
author = Bosman, HH
date = October 1990
year =
month =
title = Methods to Convert American Class A-Pan and Symon's Tank Evaporation to That of a Representative Environment
journal = Water SA
volume = 16
issue = 4
pages = pp. 227–236
issn =
pmid =
doi =
id =
url=http://md1.csa.com/partners/viewrecord.php?requester=gs&collection=ENV&recid=9104314&q=&uid=790778876&setcookie=yes
language =
format =
accessdate =
laysummary =
laysource =
laydate =
quote = ] . Also, research has been done about the installation practices of evaporation pans so that they can make more reliable and repeatable measurements [cite journal |url=http://md1.csa.com/partners/viewrecord.php?requester=gs&collection=ENV&recid=8807186&q=&uid=790778876&setcookie=yes
author=Bosman,H.H.
year=1987
title=The influence of installation practices on evaporation from Symon's tank and American Class A-pan evaporimeters
journal=Agricultural and Forest Meteorology
volume=41
pages=pp. 307–323
doi=10.1016/0168-1923(87)90086-4] .

Class A evaporation pan

In the United States, the National Weather Service has standardized its measurements on the Class A evaporation pan, a 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.

The Class A Evaporation Pan is of limited use on days with rainfall events of >30mm (203mm rain gauge) unless it is emptied more than once per 24hours. Analysis of the daily rainfall and evaporation readings in areas with regular heavy rainfall events shows that almost without fail, on days with rainfall in excess of 30mm (203mm Rain Gauge) the daily evaporation is spuriously higher than other days in the same month where conditions more receptive to evaporation prevailed.

The most common and obvious error is in daily rainfall events of >55mm (203mm rain gauge) where the Class A Evaporation pan will likely overflow.

The less obvious, and therefore more concerning, is the influence of heavy or intense rainfall causing spuriously high daily evaporation totals without obvious overflow.

Sunken Colorado pan

The sunken Colorado pan is square, 1 m (3 ft) on a side and 0.5 m (18 in.) deep and made of unpainted galvanized iron. As the name suggests is buried in the ground to within about 5 cm (2 in.) of its rim. Evaporation from a Sunken Colorado Pan can be compared with a Class A pan using conversion constants. The pan coefficient, on an annual basis, is about 0.8. [cite web
url=http://amsglossary.allenpress.com/glossary/search?id=colorado-sunken-pan1
title=AMS Glossary: Sunken Colorado Pan]

Decreasing Trend of Pan Evaporation

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. [cite journal | author=Roderick, Michael L. and Graham D. Farquhar | title=The cause of decreased pan evaporation over the past 50 years | journal=Science | volume = 298 | issue=5597 | year=2002 | pages=pp. 1410–1411] This trend has been observed all over the world except in a few places where it has increased. [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 The cause of decreased pan evaporation over the last 50 Years RODERICK & FARQUHAR / Science v.298, 1nov02 ] ] [ [http://www.agu.org/pubs/crossref/2004/2004JD004511.shtml A spatial analysis of pan evaporation trends in China, 1955–2000 ] ]

As the global climate warms, all other things being equal, evaporation will increase and as a result, the hydrological cycle will accelerate [ [http://intl.sciencemag.org/cgi/content/summary/298/5597/1345?ck=nck CLIMATE CHANGE: Is the Hydrological Cycle Accelerating? - Ohmura* and Wild 298 (5597): 1345 - Science ] ] . The downward trend of pan evaporation has been linked to a phenomenon called Global dimming. [ [http://www.bbc.co.uk/sn/tvradio/programmes/horizon/dimming_trans.shtml BBC - Science & Nature - Horizon ] ] [ [http://www.pbs.org/wgbh/nova/transcripts/3310_sun.html NOVA | Transcripts | Dimming the Sun | PBS ] ] In 2005 Wild et al. and Pinker et al. found that the "dimming" trend had reversed since about 1990 [cite web | url=http://www.realclimate.org/index.php?p=154 | title=Global Dimming may have a brighter future | accessdate=2006-06-12]

Lake Evaporation vs. Pan Evaporation

Pan evaporation is used to estimate the evaporation from lakes [cite news
url=http://www.brisbanetimes.com.au/news/queensland/officials-defend-dam-against-attacks/2007/04/18/1176696907459.html
title=Officials defend dam against attacks
date=April 18, 2007
publisher=Brisbane Times
author=Tony Moore] . There is a correlation between lake evaporation and pan evaporation [cite web
url=http://www-das.uwyo.edu/~geerts/cwx/notes/chap04/eoep.html
title=Ratio of lake to pan evaporation rates
author=E. Linacre
date=March 2002] . 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.

Relationship to Hydrological Cycle

"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." [cite journal
author=Brutsaert, Wilfried
title=Indications of increasing land surface evaporation during the second half of the 20th century
journal=Geophysical Research Letters
volume = 33
issue=20
year=2006
pages=pp. 1410–1411
doi=10.1029/2006GL027532]

Notes and references

ee also

*Atmometer (evaporimeter)
*Global dimming
*Global warming
*Hydrology