Understanding and application of evapotranspiration is a prerequisite to the development and management of water resources, irrigation design, irrigation scheduling, agriculture, and agricultural crop productivity. Many macro-ecological and micro-meteorological analyses are based on the analyses of climatological data within which evapotranspiration estimates are of central importance. In this work, we present the results of the calibrated Hargreaves for the monthly reference evapotranspiration in a data sparse region and compared it with the original Hargreaves equation for the same site of study, and the FAO – 56PM. The results of the FAO – 56PM, and the Hargreaves and Hargreaves calibrated were; 5.19mm/d, 5.52mm/d and 4.25mm/d respectively. Summary statistics for analyses of mean monthly reference evapotranspiration estimated by the Hargreaves and Hargreaves calibrated against that estimated by the standard FAO – 56PM were; y = -0.0884x + 5.0739, y = -0.0788x + 4.0026 twice, and 4.0117, y = -0.0845x + 3.3977. Simple methods for estimating reference evapotranspiration requiring only temperature and day length of the year data are compared by reference to the result from the FAO – 56PM method. The calibrated equation is the Hargreaves that is tested for its degree of approximation of the Penman – Montieth method at the site of study for the period of 2000 – 2003. The calibrated value of 0.00188 correlated well with the accepted value of Eto in a humid – coastal region like Port Harcourt.
 

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