An overview of uncertainties in evapotranspiration estimation techniques

Accurate estimation of evapotranspiration (ET) is essential both at the regional and local scales for many management tasks. Numerous methods for estimating ET with various complexities and combinations exist which may be broadly classified as direct and indirect methods. Information on ET estimation uncertainties cannot be overemphasized and ignoring them can misguide decision-making in management of water resources. This study reviews the uncertainties in ET estimations and suggests ways to reduce them. Identified in this study are uncertainties associated with ET methods and input data, uncertainties due to spatial and temporal scales, and uncertainties based on region. Many studies have the ET method related uncertainties. The ground-based techniques generally used as a standard for comparing other methods have considerable uncertainty (10–30%) associated with the input components. The errors from the input reflect in the estimated ET output irrespective of the model used. Datasets from satellite products are based on in-situ network forcing as well as on model’s estimation and remote sensing (RS), and they are prone to errors as a result of differences in in-situ measurements, scale, sensor calibration and basics of model theory and parametrization. Generally, uncertainties associated with ET were found to vary temporally. Also, homogeneity and stability of potential evapotranspiration (PET) were worse in space than in time, indicating that the temporal distribution of PET was more uniform and stable compared to spatial distribution. Some ET RS products showed less uncertainty in coarse resolution and comparatively high uncertainty in fine resolution. This study identified five ways to minimize uncertainties in ET estimations. Minimizing uncertainty in ET estimation will definitely improve planning, management and use of water resources especially where accurate estimations are required.