Pan evaporation is a measurement that combines or integrates the effects of several climate elements: temperature, humidity, rain fall, drought dispersion, solar radiation, and wind. Evaporation is greatest on hot, windy, dry, sunny days; and is greatly reduced when clouds block the sun and 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]
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] 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.
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. [5] Also, research has been done about the installation practices of evaporation pans so that they can make more reliable and repeatable measurements. [6]
In the United States, the National Weather Service has standardized its measurements on the Class A evaporation pan, a cylinder with a diameter of 47.5 in (120.7 cm) that 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.
The sunken Colorado pan is square, 0.92 m (3 ft) on a side and 0.46 m (18 in.) deep and made of unpainted galvanized iron. As the name suggests, it 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. [7]
The Symons pan or tank is a standard instrument of the UK Met Office. It is a steel container 1.83 m (6 ft) on a side and 0.61 m (2 ft) deep, sunk into the ground with an above-ground rim of 7.6–10 centimetres (3.0–3.9 in) and is painted black internally. Its evaporation rate is lower than the Class A pan and conversion factors must be used. [8]
Over the last 50 or so years, pan evaporation has been carefully monitored. For decades, pan evaporation measurements were not analyzed critically for long term trends. But in the 1990s scientists reported that the rate of evaporation was falling. [9] According to data, the downward trend had been observed all over the world except in a few places where it has increased. [10] [11] [12] [13]
It is currently theorized that, all other things being equal, as the global climate warms evaporation would increase proportionately and as a result, the hydrological cycle in its most general sense is bound to accelerate. [14] The downward trend of pan evaporation has since also been linked to a phenomenon called global dimming. [15] [16] In 2005 Wild et al. and Pinker et al. found that the "dimming" trend had reversed since about 1990. [17]
Other theories suggest that measurements have not taken the local environment into account. Since the local moisture level has increased in the local terrain, less water evaporates from the pan. This leads to false measurements and must be compensated for in the data analysis. Models accounting for additional local terrain moisture match global estimates. [18] In a different view, an analysis of pan trends in records from 154 instruments shows no coherency and pattern of statistically significant trends, with 38% decreasing, 42% no change and 20% increasing. Changes in the local environment are implicated, in which increasing tree density near the pans elevating surface friction and slowing local wind runs, reducing pan evaporation. The evaporation paradox is a result of ongoing changes in the nearby environments. [19]
Pan evaporation is used to estimate the evaporation from lakes. [20] There is a correlation between lake evaporation and pan evaporation. [21] 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, and while light penetration in a pan is essentially uniform, light penetration in natural bodies of water will decrease as depth increases. Most textbooks suggest multiplying the pan evaporation by 0.75 to correct for this.[ citation needed ]
"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." [22]
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Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. A high concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humidity affects rate of evaporation of water. When the molecules of the liquid collide, they transfer energy to each other based on how they collide. When a molecule near the surface absorbs enough energy to overcome the vapor pressure, it will escape and enter the surrounding air as a gas. When evaporation occurs, the energy removed from the vaporized liquid will reduce the temperature of the liquid, resulting in evaporative cooling.
Hydrology is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and drainage basin sustainability. A practitioner of hydrology is called a hydrologist. Hydrologists are scientists studying earth or environmental science, civil or environmental engineering, and physical geography. Using various analytical methods and scientific techniques, they collect and analyze data to help solve water related problems such as environmental preservation, natural disasters, and water management.
A flood is an overflow of water that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are of significant concern in agriculture, civil engineering and public health. Human changes to the environment often increase the intensity and frequency of flooding. Examples for human changes are land use changes such as deforestation and removal of wetlands, changes in waterway course or flood controls such as with levees. Global environmental issues also influence causes of floods, namely climate change which causes an intensification of the water cycle and sea level rise. For example, climate change makes extreme weather events more frequent and stronger. This leads to more intense floods and increased flood risk.
Evapotranspiration (ET) refers to the combined processes which move water from the Earth's surface into the atmosphere. It covers both water evaporation and transpiration. Evapotranspiration is an important part of the local water cycle and climate, and measurement of it plays a key role in water resource management agricultural irrigation.
The sulfate or sulphate ion is a polyatomic anion with the empirical formula SO2−4. Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many are prepared from that acid.
The water cycle is a biogeochemical cycle that involves the continuous movement of water on, above and below the surface of the Earth. The mass of water on Earth remains fairly constant over time. However, the partitioning of the water into the major reservoirs of ice, fresh water, salt water and atmospheric water is variable and depends on climatic variables. The water moves from one reservoir to another, such as from river to ocean, or from the ocean to the atmosphere. The processes that drive these movements are evaporation, transpiration, condensation, precipitation, sublimation, infiltration, surface runoff, and subsurface flow. In doing so, the water goes through different forms: liquid, solid (ice) and vapor. The ocean plays a key role in the water cycle as it is the source of 86% of global evaporation.
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. Precipitation occurs when a portion of the atmosphere becomes saturated with water vapor, so that the water condenses and "precipitates" or falls. Thus, fog and mist are not precipitation; their water vapor does not condense sufficiently to precipitate, so fog and mist do not fall. Two processes, possibly acting together, can lead to air becoming saturated with water vapor: cooling the air or adding water vapor to the air. Precipitation forms as smaller droplets coalesce via collision with other rain drops or ice crystals within a cloud. Short, intense periods of rain in scattered locations are called showers.
Global dimming is a decline in the amount of sunlight reaching the Earth's surface. It is caused by atmospheric particulate matter, predominantly sulfate aerosols, which are components of air pollution. Global dimming was observed soon after the first systematic measurements of solar irradiance began in the 1950s. This weakening of visible sunlight proceeded at the rate of 4–5% per decade until the 1980s. During these years, air pollution increased due to post-war industrialization. Solar activity did not vary more than the usual during this period.
In hydrology, discharge is the volumetric flow rate of a stream. It equals the product of average flow velocity and the cross-sectional area. It includes any suspended solids, dissolved chemicals like CaCO
3(aq), or biologic material in addition to the water itself. Terms may vary between disciplines. For example, a fluvial hydrologist studying natural river systems may define discharge as streamflow, whereas an engineer operating a reservoir system may equate it with outflow, contrasted with inflow.
Ecohydrology is an interdisciplinary scientific field studying the interactions between water and ecological systems. It is considered a sub discipline of hydrology, with an ecological focus. These interactions may take place within water bodies, such as rivers and lakes, or on land, in forests, deserts, and other terrestrial ecosystems. Areas of research in ecohydrology include transpiration and plant water use, adaption of organisms to their water environment, influence of vegetation and benthic plants on stream flow and function, and feedbacks between ecological processes, the soil carbon sponge and the hydrological cycle.
Streamflow, or channel runoff, is the flow of water in streams and other channels, and is a major element of the water cycle. It is one runoff component, the movement of water from the land to waterbodies, the other component being surface runoff. Water flowing in channels comes from surface runoff from adjacent hillslopes, from groundwater flow out of the ground, and from water discharged from pipes. The discharge of water flowing in a channel is measured using stream gauges or can be estimated by the Manning equation. The record of flow over time is called a hydrograph. Flooding occurs when the volume of water exceeds the capacity of the channel.
The Global Energy and Water Exchanges Project is an international research project and a core project of the World Climate Research Programme (WCRP).
Rain is a atmospheric phenomenon of water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water for hydroelectric power plants, crop irrigation, and suitable conditions for many types of ecosystems.
A Hook gauge evaporimeter is a precision instrument used to measure changes in water levels due to evaporation. It is used to precisely measure the level of a free water surface as an evaporation pan or a tank. The main users are meteorologists and water engineers, especially in hot, arid countries where water conservation is of vital importance.
The following outline is provided as an overview of and topical guide to hydrology:
Droughts are a relatively common feature of the weather in the United Kingdom, with one around every 5–10 years on average. These droughts are usually during the summer, when a blocking high causes hot, dry weather for an extended period. However this means that droughts can vary in their characteristics. All types of drought cause issues across all sectors, with impacts extending to the ecosystem, agriculture and the economy of the whole country in severe cases of drought. The south east of the country usually suffers most, as it has the highest population and the lowest average precipitation per year, which is even lower in a drought. Even in these areas in severe droughts, the definition, impacts, effects and management are all minimal in comparison to drought prone areas such as Australia and parts of the United States. In recent years however, the summers of 2007, 2008, 2009, August 2010 and 2012 were wetter than normal, 2007 being wettest on record. Droughts have continued to occur in recent times, with spring 2011, July 2013, summer 2018, spring 2020, spring and summer 2022 and May and June 2023 all featuring excessively dry periods for part or all of the UK, and will likely become more severe due to climate change over the 21st century.
Global terrestrial stilling is the decrease of wind speed observed near the Earth's surface over the last three decades, originally termed "stilling". This slowdown of near-surface terrestrial winds has mainly affected mid-latitude regions of both hemispheres, with a global average reduction of −0.140 m s−1 dec−1 or between 5 and 15% over the past 50 years. With high-latitude showing increases in both hemispheres. In contrast to the observed weakening of winds over continental surfaces, winds have tended to strengthen over ocean regions. In the last few years, a break in this terrestrial decrease of wind speed has been detected suggesting a recovery at global scales since 2013.
Climate change in Colorado encompasses the effects of climate change, attributed to man-made increases in atmospheric carbon dioxide, in the U.S. state of Colorado.
The biotic pump is a theoretical concept that shows how forests create and control winds coming up from the ocean and in doing so bring water to the forests further inland.
The effects of climate change on the water cycle are profound and have been described as an intensification or a strengthening of the water cycle. This effect has been observed since at least 1980. One example is when heavy rain events become even stronger. The effects of climate change on the water cycle have important negative effects on the availability of freshwater resources, as well as other water reservoirs such as oceans, ice sheets, the atmosphere and soil moisture. The water cycle is essential to life on Earth and plays a large role in the global climate system and ocean circulation. The warming of our planet is expected to be accompanied by changes in the water cycle for various reasons. For example, a warmer atmosphere can contain more water vapor which has effects on evaporation and rainfall.