Thornthwaite’s Potential Evapotranspiration Rate Assessment and Implication on Rainfall Induced Floods in a Coastal Expanse of Nigeria

David O. Edokpa; Precious N. Ede; Susan I. Ajiere; Bridget E. Diagi

Abstract

This study examined the potential evapotranspiration (PET) rate effects on rainfall-induced floods within Port Harcourt, a coastal city in Nigeria. Mean monthly rainfall and air temperature data for 39 years (1979–2018) was sourced from the Nigerian Meteorological Agency and used to evaluate the potential evapotranspiration (PET) of the area using the Thornthwaite’s method. Results revealed that the mean rainfall and PET values for Port Harcourt during the periods under review were 192.2 mm and 112.2 mm with a range of 145.3–238.8 mm and 94.6–131.8 mm,respectively. The total mean annual values for rainfall and PET for the years analyzed for the study area were from 1743.1–2866.0 mm and 1135.6–1581.3 mm respectively. The years 2006 and 2017 had the highest and lowest total mean PET, i.e., 1581.3 mm and 1135.6 mm, respectively while 2007 and 2011 had the highest and lowest rainfall amount, i.e., 2866.0 mm and 1743.1 mm, respectively. The mean monthly maximum temperature of 31.3°C for the area is enough to cause considerable evaporation of water from surfaces. The study showed that 58 % of PET was generated from the total mean annual rainfall amount for the period under review. This indicates that PET for the area is substantial and contributes significantly to the very humid environment. The values of the correlation coefficient (R) and the coefficient of determination (R2 ) for the relationship between air temperature and PET are 0.92 and 0.85 respectively indicating a strong relationship. The total mean annual PET for the domain, which is less than the mean annual rainfall value shows that excess rainwater will enhance enormous flood buildup in the domain. Also, the build-up of more moisture in the area will lead to increased rainfall amount with impact on flooding in the area. It is important to note that Port Harcourt city must be adequately drained to avoid increasing overwhelming flood events being witnessed presently.