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Pan evaporation paradox and evaporative demand from the past to the future over China: a review

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In a warming climate, there was a long‐term expectation that the atmospheric evaporative demand would increase, which was challenged by an unexpected discovery two decades ago of decreases in measured pan evaporation, now widely termed as the ‘pan evaporation paradox.’ In this review, we summarize recent reports on the pan evaporation paradox around the world over the past half century, possible causes, implications for the water cycle, and its possible change in the future. We then present a case study of China based on the latest meteorological datasets and the state‐of‐the‐art General Circulation Models (GCMs). We confirm that pan evaporation (Epan) decreases in most parts of China at an average of about −2.60 mm/y2, and this decrease disappears around 1993. We introduce and develop a detrending approach in sensitivity analysis and find that changes in solar radiation, wind speed, and relative humidity overcompensate the positive contribution of increasing air temperature in Epan and lead to the well‐known pan evaporation paradox. Positive changes in Epan using 12 state‐of‐the‐art GCMs for 2021–2050 and for 2071–2100 under RCP4.5 and RCP8.5 scenarios, respectively, when compared with their corresponding multiyear mean over 1971–2000 as base line, shows that the evaporation paradox would not appear in the future. We highlight that it is vital and promising to move forward to interpreting how the atmospheric evaporative demand would change in the future. WIREs Water 2017, 4:e1207. doi: 10.1002/wat2.1207

Evaporation pans used worldwide; (a) the D20 and (b) E601B pans used in China and (c) the Class A pan used in Australia (with bird guard) and United States (without bird guard), etc. (d) The pan evaporation paradox and its implication in global water cycle and (e) the evaporative demand from the past to the future over China, the blue '−' means negative change and attribution while red '+' means positive effect .
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Changes of mean annual pan evaporation in 12 GCMs and their ensemble mean (EM) over China for 2021–2050 (black box and dots) and 2071–2100 (red box and dots, the HADGEM GCM ends in 2099) under RCP4.5 and RCP8.5 scenarios, respectively, in comparison with the control period of 1971–2000.
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Comparison of mean annual observed pan evaporation and calculated E pan of 12 GCMs as well as their ensemble mean (EM) (anomalies for 1961–2001) over China; the value in the brackets is the overall mean in the unit of mm/y of each GCM.
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Sensitivity analysis of pan evaporation for 1960–1993 (left column) and 1994–2014 (right column): (a1) the contribution of Sd for 1960–1993 and (b1) for 1994–2014, (a2) the contribution of u2 for 1960–1993 and (b2) for 1994–2014, (a3) the contribution of Tem for 1960–1993 and (b3) for 1994–2014, (a4) the contribution of rhum for 1960–1993 and (b4) for 1994–2014.
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Pan evaporation changes as well as annual time series of the sunshine duration (Sd), the wind speed (u2), the relative humidity (rhum), and the air temperature (Tem) in China for 1960–2014.
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The distribution of pan evaporation trends of China for two periods with the positive trend covered with a mask of red and negative with blue, respectively: (a) the trend for the period 1960–1993 of −24.56~+12.00 mm/y2 and (b) trend for 1994–2014 with range of −28.08~+34.39 mm/y2.
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Correlation of observed pan evaporation (Obs E pan) and calculated evaporation using the PenPan model (Cal E pan) (327230 pairs of monthly observed pan and calculated E pan plotted, and the color bar is the density of value within an interval of 61.05 mm/month).
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The newly adjusted parameters of as and bs used to estimate the net radiation from the sunshine duration (the actual value shall be the value in each counter line multiplied by 0.01).
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Distribution of radiation and meteorological stations over China.
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Science of Water > Stocks and Flows of Water
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