Improving the complementary methods to estimate evapotranspiration under diverse climatic and physical conditions

F M Anayah,J J Kaluarachchi

doi:10.5194/hess-18-2049-2014

Abstract

Abstract. Reliable estimation of evapotranspiration (ET) is important for the purpose of water resources planning and management. Complementary methods, including complementary relationship areal evapotranspiration (CRAE), advection aridity (AA) and Granger and Gray (GG), have been used to estimate ET because these methods are simple and practical in estimating regional ET using meteorological data only. However, prior studies have found limitations in these methods especially in contrasting climates. This study aims to develop a calibration-free universal method using the complementary relationships to compute regional ET in contrasting climatic and physical conditions with meteorological data only. The proposed methodology consists of a systematic sensitivity analysis using the existing complementary methods. This work used 34 global FLUXNET sites where eddy covariance (EC) fluxes of ET are available for validation. A total of 33 alternative model variations from the original complementary methods were proposed. Further analysis using statistical methods and simplified climatic class definitions produced one distinctly improved GG-model-based alternative. The proposed model produced a single-step ET formulation with results equal to or better than the recent studies using data-intensive, classical methods. Average root mean square error (RMSE), mean absolute bias (BIAS) and R2 (coefficient of determination) across 34 global sites were 20.57 mm month−1, 10.55 mm month−1 and 0.64, respectively. The proposed model showed a step forward toward predicting ET in large river basins with limited data and requiring no calibration.

Highlights

A reliable estimate of ET in river basins is important for the purpose of water resources planning and management
A careful screening of available meteorological, land use/land class and related hydrologic data in typical rural river basins suggest that ET is more challenging to calculate given the limited data
Data limitations in most rural river basins highlighted the importance of using alternative methods as opposed to the classical methods using land use/land cover data

Summary

Introduction

A reliable estimate of ET (evapotranspiration) in river basins is important for the purpose of water resources planning and management. ET represents a significant portion of rainfall in the water balance especially in semiarid regions where most rainfall is typically lost as ET (FAO, 1989). A careful screening of available meteorological, land use/land class and related hydrologic data in typical rural river basins suggest that ET is more challenging to calculate given the limited data. Data limitations in most rural river basins highlighted the importance of using alternative methods as opposed to the classical methods using land use/land cover data. Complementary methods initially proposed by Bouchet (1963) and others are alternative methods that can be used to calculate ET using meteorological data such as relative humidity, temperature and sunshine hours

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Conclusion