# Responses of Terrestrial Evapotranspiration to Extreme Drought: A Review
## 2 Methods to Estimate and Measure Evapotranspiration
### Comparisons of the Methods  
There is no agreement on which approach or product is the best because all of them have certain strengths and limitations and all have proved to be of acceptable quality for drought impact research in different situations. Ground-based observation measurements can obtain relatively accurate estimates of ET at the point scale, while their applicability may be limited over large areas [59]. The water balance approach is simple and theoretically reliable, allowing for basin areas of long-term accurate ET estimates with the limitation of a relatively low temporal resolution (yearly). Therefore, most models and products evaluate the performance of ET estimation by comparison with the flux site observations and water budget-derived ET. Satellite remote sensing techniques have been considered the most feasible way to provide spatial coverage, cost-effectiveness, and a reasonably accurate method for estimating ET at the regional to global scales. The SEB model is one of the earliest RS models to estimate ET, while the accuracy largely relies on the reliability and quality of the thermal remote sensing data. In addition, the SEB model is only reliable in clear-sky conditions because of the utilization of thermal RS data. Empirical models are easy to apply with the minimum climate inputs, but they usually lack a solid physical basis and depend significantly on calibration. LSMs and data assimilation methods provide a useful way to obtain long-term ET data at a regional to global scale and can be used to forecast ET dynamics in the future. Nevertheless, the quality of input data and model structure continue to have an impact on the accuracy of ET simulation.  
Numerous studies have been conducted to compare the performance of different global ET products. [PERSON] et al. [46] compared the seasonal variations and multiyear mean spatial patterns of the LandFlux-EVAL datasets based on observational data, including satellite-based products, LSM simulations, and reanalysis products. The results revealed that the investigated datasets demonstrated similar spatial patterns despite the differing observation constraints, while the IPCC AR4 simulations displayed an underestimated ET within several continents. [PERSON] et al. [60] supplemented [PERSON]'s research, which focused on the latent and sensible flux, and the results demonstrated that RS-based products exhibit a higher correlation between the latent heat flux and the net radiation compared with the off-line LSM models and reanalysis products. Aiming to develop high-quality and consistent global ET datasets, [PERSON] et al. [61] derived monthly global ET benchmark synthesis products by merging 40 distinct datasets, including diagnostic datasets, LSMs products, and reanalysis products. The synthesis products also provide distinct statistics for muti-data, and the annual mean ET shows similar interannual variations in all categories, while the merge products based on diagnostics and re-analyses exhibit the largest interquartile ranges. Above all, to reduce the uncertainties in quantifying ET and to increase the accuracy of input data, a substantial effort should be made to establish improved multi-sensor RS measurements and observation networks, especially in areas with poor spatial coverage. An overview of current global ET datasets, includingobservation-based products, satellite RS-based products, LSMs, and reanalysis products, is provided in Table 1.