Pan Evaporation Paradox Research Articles

Does evaporation paradox exist in China?

Abstract. One expected consequence of global warming is the increase in evaporation. However, lots of observations show that the rate of evaporation from open pans of water has been steadily decreasing all over the world in the past 50 years. The contrast between expectation and observation is called "evaporation paradox". Based on data from 317 weather stations in China from 1956 to 2005, the trends of pan evaporation and air temperature were obtained and evaporation paradox was analyzed. The conclusions include: (1) From 1956 to 2005, pan evaporation paradox existed in China as a whole while pan evaporation kept decreasing and air temperature became warmer and warmer, but it does not apply to Northeast and Southeast China; (2) From 1956 to 1985, pan evaporation paradox existed narrowly as a whole with unobvious climate warming trend, but it does not apply to Northeast China; (3) From 1986 to 2005, in the past 20 years, pan evaporation paradox did not exist for the whole period while pan evaporation kept increasing, although it existed in South China. Furthermore, the trend of other weather factors including sunshine duration, windspeed, humidity and vapor pressure deficit, and their relations with pan evaporation are discussed. As a result, it can be concluded that pan evaporation decreasing is caused by the decreasing in radiation and wind speed before 1985 and pan evaporation increasing is caused by the decreasing in vapor pressure deficit due to strong warming after 1986. With the Budyko curve, it can be concluded that the actual evaporation decreased in the former 30 years and increased in the latter 20 year for the whole China.

Wind Run Changes: The Dominant Factor Affecting Pan Evaporation Trends in Australia

AbstractThe Class A pan evaporation rates at many Australian observing stations have reportedly decreased between 1970 and 2002. That pan evaporation rates have decreased at the same time that temperatures have increased has become known as the “pan evaporation paradox.”Pan evaporation is primarily dependant on relative humidity, solar radiation, and wind. In this paper, trends in observed pan evaporation in Australia during the period 1975–2004 were attributed to changes in other climate variables using a Penman-style pan evaporation model. Trends in daily average wind speed (termed wind run) were found to be an important cause of trends in pan evaporation. This result is a significant step toward resolving the pan evaporation paradox for Australia.Data inspection and interstation comparison revealed that some of the significant wind run trends were discontinuous or spatially uncorrelated. These analyses raised the possibility that some of the changes in observed wind run, and by implication some of the significant changes in pan evaporation, may represent changes in the local environment surrounding the observing stations.Daily pressure gradients and NCEP–NCAR reanalysis wind surfaces were analyzed in an attempt to identify any climatological wind run trends associated with large-scale changes in atmospheric circulations. Unfortunately, the trends from the two data sources were not consistent, and the challenge remains to conclusively identify the cause or causes of the changes in observed station wind run in Australia.

Trends in pan evaporation and reference and actual evapotranspiration across the Tibetan Plateau

The Tibetan Plateau is one of the areas of the world where humans have had a relatively minor impact. The plateau thus provides ideal conditions for investigating evapotranspiration (In this paper, evapotranspiration terms are defined as follows: (1) “Actual evapotranspiration” includes evaporation from water and soil and transpiration from the vegetation of a specific region; (2) “potential evapotranspiration” includes the maximum quantity of water capable of being evaporated from the soil and transpired from the vegetation of a specific surface; (3) “reference evapotranspiration” includes the maximum quantity of water capable of being evaporated from the soil and transpired from a hypothetical reference grass with an assumed height of 0.12 m, a fixed surface resistance of 70 s m−1, and an albedo of 0.23; and (4) “pan evaporation” means evaporation from open circular pans with a diameter of 20 cm.) issues such as temporal trends of evapotranspiration, the pan evaporation paradox, and the complementary relationship hypothesis. We examined Penman‐Monteith reference evapotranspiration and pan evaporation from a 20‐cm pan by using a data set from 75 meteorological observatories across the plateau during the period 1966 to 2003. Actual regional evapotranspiration was estimated in 16 catchments across the plateau during the period 1966 to 2001. Reference evapotranspiration and pan evaporation significantly decreased at 47 and 38% of observatories, respectively, though air temperature at most of sites significantly increased (P < 0.05); wind speed and sunshine hours significantly decreased at 85 and 43% of observatories (P < 0.05). The annual reference evapotranspiration and pan evaporation averaged from all the observatories significantly decreased (P < 0.05) while actual annual evapotranspiration averaged from all the catchments increased (P < 0.1), indicating the existence of a pan evaporation paradox on the plateau. The analysis with a recovered stationary series method showed that decreasing trend in reference evapotranspiration was due to a decrease in wind speed and a decrease in net total radiation, and the increase in air temperature, however, showed little correlation with the declining trends in reference evapotranspiration and pan evaporation. Regional actual evapotranspiration and reference, Penman potential evapotranspiration, or pan evaporation exhibit complementary behavior, which, however, does not support Bouchet’s complementary hypothesis, perhaps because of the very low vapor pressure deficit. The current study suggests that the Bouchet’s complementary relationship needs to be reconsidered at high elevations.

A spatial analysis of pan evaporation trends in China, 1955–2000

Pan evaporation, an indicator of potential evaporation, has decreased during the last several decades in many parts of the world. This trend is contrary to the expectation that global warming will be accompanied by an increase in terrestrial evaporation, known as the pan evaporation paradox. In this paper we present an analysis of changes in the spatial patterns of pan evaporation in China based on data from 85 weather stations from 1955 to 2000. We found that pan evaporation decreased in China from 1955 to 2000. The decrease was statistically significant in all of China's eight climatic regions except northeast China. We also found that the decrease in solar irradiance was most likely the driving force for the reduced pan evaporation in China. However, unlike in other areas of the world, in China the decrease in solar irradiance was not always accompanied by an increase in cloud cover and precipitation. Therefore we speculate that aerosols may play a critical role in the decrease of solar irradiance in China. By subdividing China into eight climatic regions, we found that the rate of decrease in pan evaporation was highest in the northwest and lowest in the southwest. Although changes in solar irradiance are the main cause of decreasing pan evaporation, water conditions influence the sensitivity of pan evaporation to the change in solar irradiance in comparing the eight climatic regions. Thus the spatial trends of pan evaporation differ from those of solar irradiance among these regions.

Trends in pan evaporation and actual evapotranspiration across the conterminous U.S.: Paradoxical or complementary?

Pan evaporation (ETpan) has decreased at 64% of pans in the conterminous U.S. over the past half‐century. Comparing trends in ETpan and water budget‐derived actual evapotranspiration (ET*a), we observe the so‐called “Pan Evaporation Paradox,” which we confirm is no more than a manifestation of the complementarity between actual evapotranspiration (ETa) and potential evapotranspiration (ETp). Examining trends in the components of ETa—the radiative energy and regional advective budgets—we show that both components must be considered together to explain the relationship between ETpan and ET*a.