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

SummaryPan evaporation (Epan) 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 Epan 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 (Rs), air temperature (Ta), 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 Epan and quantify the contributions of climatic factors (Rs, net longwave radiation Rnl, Ta, U, and vapor pressure deficit D) to Epan 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 Rs was −4.5 W m−2 decade−1 (−2.6% per decade); the mean Ta increased by 0.27 °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 Epan were Rs and U; the causes of Epan 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 Rs.

► Epan over China showed a −3.1 mm/a2 (−1.8% per decade) change from 1961 to 2001.
► We proposed the PenPan-20 model for evaporation from the Chinese micro-pan.
► We quantified the attribution of changing Epan based on the PenPan-20 model.
► Main causes of changing Epan were declining wind speed in most parts of China.
► Main causes of changing Epan were reducing solar radiation in southwest China.