Abstract

The trends pan evaporation (PE) in 4 different climatic regions (Sahel, Midland, Guinea Savannah and Coastal/Tropical Rainforest) covering about 21 tropical stations in Nigeria (during the period 1970 to 2000) were investigated. The influence of meteorological variables, which have significant impact on the spatial and temporal trends of pan evaporation for three consecutive decades across the different regions in Nigeria was tested. Mann-kendal statistical test was carried out to investigate the trend. A decade to decade trend analysis was carried out for 3 consecutive decades over all the regions. A general decrease trend was observed in all the regions except Sahel region in the first decade under consideration.  The decade (1970 to1979) coincided with the period when there was a global solar dimming. However, the rest decades witness an increase trend in pan evaporation for all the climatic regions across Nigeria. The general trend analysis of PE for Nigeria shows that about 80% of stations involved in the study experience increase trend (development of arid condition) out of which, only 50% were significant. The rest 20% exhibited decrease trend (creation of humid condition) with none significant. Further trend analysis was carried out on the various seasons and this revealed that all the wet season (April to October) exhibits downward trend for all the regions while similar trend regime was established for dry season months (December to March) in Sahel and Guinea Savannah and upward trend for midland and the coastal or tropical rainforest. Evidence of pan evaporation paradox was also established for certain period in Nigeria.

Key words: Pan evaporation, trend, potential evaporation, meteorological variables.