Trends and variability in pan evaporation and other climatic variables at Ibadan, Nigeria, 1973-2008

Understanding changes in evaporation rates is expected to be of great importance for water resource planning and management. This paper examines pan evaporation data as an example of the detection and attribution of trends in climate variables. Records of pan evaporation (Epan), rainfall (R), radiation (SR), wind speed (WS), temperature (Ta) and humidity (RH) for the period 1973-2008 were collected from the International Institute of Tropical Agriculture (IITA) in Ibadan, Nigeria. MannKendall trend and Sen's slope tests were performed on the respective meteorological variables and a variability index (VI) was also computed for these. The results showed that annually Epan, SR and WS significantly decrease (P < 0.001) while R, Ta, and RH showed insignificant increasing trends in the last four decades. Epan and SR decrease at the rate of 8.3 mm year-2 and 37.8 MJ m-2 year-1, respectively. Similar to Epan, SR decreased significantly in all the months (P < 0.01) and the reduction ranged from 5.1% per decade in March to 9.3% per decade in August. The result of VI showed that the decrease in Epan and other explanatory variables is rather recent. Regression between Epan and other variables indicates that about 30, 15 and 6% of its variance can be explained by SR, WS and VPD (vapour pressure deficit), respectively. The possible roles of dust-haze known as harmattan winds and monsoon clouds in attenuating SR and hence reducing Epan are discussed. Copyright (C) 2011 Royal Meteorological Society