Trend Analysis of Reference Evapotranspiration; Case Study of Asaba and Uyo, South-South Nigeria

Abstract

Temporal trend analysis of monthly mean RET estimates using FAO-56 PM model was performed using the Non-Parametric Mann-Kendall (MK) and Sen’s test for Asaba and Uyo over the period of 41 and 35 years respectively. In Asaba, reference evapotranspiration (RET or ET_o) showed an overall increasing trend ranging from MK Z values of 0.80 to 2.93 except for June (-0.75) and September (-1.16) which declined insignificantly. For Uyo, RET showed an insignificant decreasing trend (P>0.1) with MK Z values ranging from -0.09 to -1.65 except for November (0.91) which insignificantly increased at P>0.1. For detailed evaluation of the trends, RET was separated into aerodynamic and radiation terms and their trend analysis were performed. In Asaba, the radiation term showed a general decreasing trend (ranging from MK Z values of -0.42 to -2.84) except for July (0.42) and August (0.46) which have an insignificant increasing trend. The aerodynamic term, increased in every month with MK Z values ranging from 0.64 to 2.17. The temporal study of the component terms in Uyo showed a more significant trend than the RET trend.  The radiation term trend increased at P-value of 0.01 (0.31 to 1.85) whereas the aerodynamic term significantly trends decreased (-0.86 to -3.04). It was also deduced that the general RET trend and the aerodynamic trend are of the same over the period of study. This implies that, aerodynamic parameters most especially the wind speed and the saturation vapour pressure have more effect on reference evapotranspiration in the two study areas.