Abstract
Six reference evapotranspiration (ETo) methods including: Papadakis (1966), Turc (1961), Blaney and Criddle (1950), Blaney and Criddle modified by Shih et al. (1977), Penman modified by Frere and Popov (1979) and Stephens and Stewart (1963) modified by Jansen and Haise, were compared with the FAO-56 Penman-Monteith formula using rain-fed grass data within the period of 15 years (1967 to 1982) in Yaoundé, extracted from the records of climatological observations from meteorological stations published by the National Meteorological Center of Cameroon. The methods compared daily ETo using linear regression and statistical indices of a quantitative approach to model performance evaluation. The average FAO-56 PM ETo was 3.16 mm/day, but Penman modified by Frere and Popov (1979) overestimated ETo by 25% (12.72 mm/day) and Papadakis (1966) underestimated ETo by 8% (0.28 mm/day). In general, the Stephens and Stewart (1963) modified by Jansen and Haise method produced best statistics result (R2 = 0.96, RMSE = 0.072, MBE = -1.260, RMSEs = 0.980 and RMSEu = 0.693) and generated ETo results of 2.76 mm/day (2% underestimate), closest to that of FAO-56 PM method. The results of statistical comparisons delivered a confident statistical justification for the ranking of the compared methods based on performance indices.
Highlights
Water deficit is a key regulating element of plant growth and development under the panoply of environmental conditions around the world
Intending to reduce the alterations associated with tree canopy characteristics, Food and Agricultural Organisation (FAO) and working groups of the International Commission on Irrigation and drainage tested different formulas among empirical model equations extensively, and recommended the standardized Penman-Monteith reference evapotranspiration as the potential evapotranspiration (ETo) for short grass or a tall reference crop, whose characteristics have been well defined [4, 18, 24]
The average difference between model performances was measured by root mean square error (RMSE)
Summary
Water deficit is a key regulating element of plant growth and development under the panoply of environmental conditions around the world. To test the accurateness of these methods in a different environment employs the use of costly equipments (such as lysimeters) and profesionals or standard reference surface conditions (protocols). Many of these equations have limited global validity. Intending to reduce the alterations associated with tree canopy characteristics, FAO and working groups of the International Commission on Irrigation and drainage tested different formulas among empirical model equations extensively, and recommended the standardized Penman-Monteith reference evapotranspiration as the potential evapotranspiration (ETo) for short grass or a tall reference crop (alfalfa), whose characteristics have been well defined [4, 18, 24]
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