Progress and Trends in Evapotranspiration Measurement and Estimation Techniques: Concepts, Challenges and Opportunities.

Evapotranspiration (ET) is a major variable in understanding environmental systems and climate change. This paper reviews the various ET measurement and estimation techniques that have been widely used over the years. The ET measurement techniques can be broadly grouped under direct & indirect methods. The two most common types of direct measurement methods, Water budget (Evaporation Pan, Water balance of basin, Lysimetry and soil moisture depletion) and water vapor transfer measurements (Bowen Ratio- Energy Budget, Eddy correlation) are discussed in this paper.  Indirect methods for measuring ET such as aerodynamic method or mass transfer method, Penman equation, Blaney Criddle method, Hargreaves method are briefly discussed.  Additionally, this paper also reviews remote sensing (RS) approach for estimating ET. The importance of Satellite Derived Feedback Mechanism, Biophysical processes with remotely sensed data were discussed owing to its advantage of high spatial resolution and coverage. A section of this paper is also dedicated for discussing , ET partitioning models such as Shuttlewoth-Wallace (S-W) models, ENWATBAL, Cupid-DPEVAP, SWEAT, TSEB, FAO dual Kc model, HYDRUS-1D . In conclusion the authors discuss the applicability, advantages and limitations of the various ET measurement and estimation techniques reviewed. Future challenges include development of models simulating the components of ET separately and advancement of methods for continuous measurement of E, T and/ or the ratio between the two. Scientists, researchers, policy-makers can benefit from this collective documentation on ET measurement and estimation techniques.