Long-Term Trends of Pan Evaporation and an Analysis of Its Causes in Finland

Abstract:

The recent global warming affects the evapotranspiration which is an important factor of hydrologic cycle and water resources management. Many of the previous studies have reported the decreasing trends of pan evaporation in the area of the continental climate of the middle latitude. However, few studies in the region in a high latitude area such as Finland haven't been carried out so far. The purpose of this study is to investigate the long term variations of pan evaporation in Finland located in high latitude using a trend analysis. The causes of the trends of pan evaporation were discussed from two points of view: a complementary relationship and Penman's equation (1948). The results were summarized as follows: (1) The variations of pan evaporation showed decreasing trends at 5 stations and increasing ones at 2 stations. (2) The mechanistic causes for the decreasing trends at 5 stations were mainly the increases of precipitation and the aerodynamic term in Penman's equation. (3) The mechanistic causes for the increasing trends at 2 stations couldn't be revealed.

H. L. Penman. 1948. Natural evaporation from open water, bare soil and grass, Proceedings of the Royal Society of London, Series A, Mathematical and Physical Science, 193, 1032, 129-145.