Abstract
The performances of five empirical models, namely: Hargreaves-Samani, Makkink1 (1957), Makkink2 (1984), Priestley-Taylor and FAO 56-PM in estimating reference evapotranspiration (REF-ET) were separately compared with Epan data and FAO 56-PM, respectively. Based on statistical analysis, Hargreaves-Samani method compared best with daily and monthly Epan data, while Makkik2 (1984) ranked first with FAO 56-PM. In terms of regression analysis, Priestley-Taylor performed best with daily FAO 56-PM method while Hargreaves-Samani ranked first with daily Epan data. Hargreaves-Samani also correlated best with mean monthly Epan data. The quantitative evaluation of cumulative daily and monthly reference-evapotranspiration (RET-ET) values showed that Makkink (1984) produced the least overestimation and percent relative error against FAO 56-PM while Hargreaves-Samani performed best with Epan data with the least overestimation and percent relative error. In terms of cumulative monthly ETo totals for the farming season (Dec-April) over the study period, Hargreaves-Samani ranked best with Epan data with the least overestimation and percent relative error while Priestley–Taylor ranked best with FAO 56-PM producing the least overestimation. Overall, Hargreaves-Samani with its original coefficient was adjudged best, capable of approximating FAO 56-PM and Epan data in the Lower Niger River Basin, followed by Makkink (1984) and Priestley-Taylor. Penman-Monteith estimates were used to develop monthly correction factors for adjusting Empirical models for their potential use in Lower Niger Basin. A comparative study such as this has not been undertaken in the Lower Niger River Basin. The models recommended in this study are economical, lesser-data demanding and can be applied to predicting REF-ET in remote agricultural areas. Key words: Reference-evapotranspiration (RET-ET), empirical models, radiation-based methods, temperature-based methods, FAO 56 –PM, Lower Niger River Basin.
Highlights
The accurate knowledge of evapotranspiration and consumptive use of water is an index of successful food production programme
Irrigation water demand is usually determined through evapotranspiration estimation procedures, namely; (i) direct field measurement methods such as Lysimeter apparatus and US weather Bureau Standard Class A pan and (ii) empirical relationships and mathematical model based on weather data to determine Reference Evapotranspiration (REF-ET) (Jensen et al, 1990; Allen et al, 1998)
The following conclusions were drawn from the results of the study: i) Based on the statistical analyses, regression analysis, accumulated REF-ET values (2000 to 2010); monthly REF-ET estimates for the farming season (Dec to April)
Summary
The accurate knowledge of evapotranspiration and consumptive use of water is an index of successful food production programme. The availability of water and efficiency of its economic use are dominant factors controlling or limiting food production and a better understanding of water requirements can, result in large benefits (Hargreaves and Samani, 1981). Irrigation water demand is usually determined through evapotranspiration estimation procedures, namely; (i) direct field measurement methods such as Lysimeter apparatus and US weather Bureau Standard Class A pan and (ii) empirical relationships and mathematical model based on weather data to determine Reference Evapotranspiration (REF-ET) (Jensen et al, 1990; Allen et al, 1998). In view of the human resources and costs implications of using direct measurement methods, empirical and mathematical models based on weather data have become an attractive alternative
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