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 ( E_pan), rainfall ( R), radiation ( S_R), wind speed ( W_S), temperature ( T_a) and humidity ( R_H) for the period 1973-2008 were collected from the International Institute of Tropical Agriculture (IITA) in Ibadan, Nigeria. Mann-Kendall 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 E_pan, S_R and W_S significantly decrease ( P < 0.001) while R, T_a, and R_H showed insignificant increasing trends in the last four decades. E_pan and S_R decrease at the rate of 8.3 mm year⁻² and 37.8 MJ m⁻² year⁻¹, respectively. Similar to E_pan, S_R 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 E_pan and other explanatory variables is rather recent. Regression between E_pan and other variables indicates that about 30, 15 and 6% of its variance can be explained by S_R, W_S and VPD (vapour pressure deficit), respectively. The possible roles of dust-haze known as 'harmattan' winds and monsoon clouds in attenuating S_R and hence reducing E_pan are discussed. Copyright © 2011 Royal Meteorological Society [ABSTRACT FROM AUTHOR]