Decreasing relative humidity dominates a reversal of decreasing pan evaporation in mainland China after 1989

Pan evaporation (Epan) reflects the evaporation potential of surface water and is a key indicator of atmospheric evaporative demand. Previous studies have found a substantial decrease in Epan across China, dominated by declining wind speed and solar radiation before the late 2000 s. However, how Epan responds to the recovery of wind speed and solar radiation since the late 2000 s remains unclear. Here, we investigate the spatial and temporal patterns of Epan and analyze its controlling factors, based on the daily meteorological observations during 1965-2018 at 2,018 stations over China. We find that the annual Epan in the northwest and south of China is higher than that in the northeast and central parts of China. The national mean Epan decreases significantly during 1965-1989 at a rate of-6.57 mm yr(-2), then reverses at a rate of 4.58 mm yr(-2). Distinct seasonal discrepancies and geographical heterogeneities in Epan trends are observed. Declining wind speed mainly contributes to the decrease in Epan before 1989, while relative humidity is shown as the dominant factor that drives the growth of Epan after the turning point. The recovery of wind speed still has significant impact on Epan variation in parts of China, while Epan is only mainly affected by solar radiation in parts of central China in autumn. Our findings highlight the importance of understanding Epan variations and driving factors, and call for precautions and adaptation actions to combat and prevent flash droughts over China, especially in the south of China.