Climatic factors influencing changing pan evaporation across China from 1961 to 2001

Pan evaporation (E-pan) is an indicator of atmospheric evaporative demand, and downward trends have been reported in many regions over the past several decades. It is important to understand why E-pan has changed and determine what the main causes are. This study analyzed daily climate data from 54 meteorological stations across China measured from 1961 to 2001. Climatic factors included Epan measured with the standard Chinese 20 cm diameter pan, global solar radiation (R-s), air temperature (T-a), relative humidity (RH), and wind speed (U). We modified the PenPan model for pan evaporation from the Class A pan, to estimate Chinese micro-pan evaporation E-pan and quantify the contributions of climatic factors (R-s, net longwave radiation R-n1, T-a, U, and vapor pressure deficit D) to E-pan using partial derivatives. For China as a whole, the arithmetic average of Epan from these 54 stations showed a significant decline of -3.1 mm a(-2) (-1.8% per decade). The observed change in R-s, was -4.5W m(-2) decade(-1) (-2.6% per decade); the mean T-a increased by 0.27 degrees C per decade; U decreased by -0.10 m s(-1) decade(-1) (-6% per decade); and D increased by 0.012 kPa per decade (2% per decade). For China as a whole, the dominant factors influencing changing E-pan were R-s and U; the causes of E-pan were very varied: regionally, the main causes for change were in most parts of China, it was the change in U, in southwest China it was the change in R-s. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.