# Responses of Terrestrial Evapotranspiration to Extreme Drought: A Review
## 2 Methods to Estimate and Measure Evapotranspiration
### Estimation and Measurement of Evapotranspiration  
Numerous in situ techniques exist to measure the actual ET from point to region scales, including weighing lysimeters, eddy covariance, scintillometers, the Bowen ratio, sap flow, pan measurements, atmospheric water balances, and surface water balance techniques. Weighing lysimeters are the standard approach to measure ET without any assumptions and are often used to compare with other techniques [20]. Due to their costs and limited areal extent, however, there are very few long-term measurement records worldwide [29]. Eddy covariance provides a method for measuring ET over a diurnal cycle [30; 31] and hasbeen deployed as part of the FLUXNET network which covers over 500 sites worldwide [32] ([[https://fluxnet.org/sites/site-summary/](https://fluxnet.org/sites/site-summary/)]([https://fluxnet.org/sites/site-summary/](https://fluxnet.org/sites/site-summary/)), accessed on 22 November 2022). However, one also should note the issue with the eddy covariance system's energy balance closing errors caused by the spatial heterogeneity of atmospheric and surface conditions [33]. Nonetheless, the FLUXNET dataset provides valuable observations of terrestrial hydrological variables and can be useful for identifying the ET variability under extreme droughts on the point and regional scales. For example, several studies have evaluated ET dynamics using long-term FLUXNET datasets [34; 35]. However, their applicability may be limited for large areas as it is not feasible to achieve dense global coverage with such point instruments. Atmospheric water balances and surface water balances provide a robust estimate of ET at the basin to global scales with a low temporal (yearly) resolution based on precipitation and streamflow observations [29]. These approaches are simple and theoretically reliable, but one should also note the uncertainties introduced by the precipitation datasets and the lack of streamflow observations for several regions [36]. With the rapid development of satellite remote sensing technologies, varieties of models and approaches have been conducted as ET mapping methods over the past decades. Satellite remote sensing offers a reasonable estimation of land surface variables, such as albedo, vegetation index (VI), and surface temperature (Ts), but does not measure ET directly [29]. The satellite remote sensing techniques that are widely used to retrieve ET for long-term variability at different scales include the surface energy balance model (SEB, i.e., one-source model and two-source model) [37; 38], physical models (such as the Penman-Monteith model) [39; 40], and empirical models [41]. In the surface energy balance model, ET is determined by the residual term of the surface energy balance. The land surface temperature (LST) is a crucial RS parameter in SEB models; however, there are some limitations to applying LST data to estimate ET on a global scale [42; 43]. They have been widely used to estimate ET with routinely available meteorological data and relatively high resolution (i.e., Landsat and the advanced spaceborne thermal emission and reflection radiometer, or so-called ASTER) and modest resolution satellite data (such as the moderate resolution imaging spectroradiometer, or so-called MODIS). For example, recently, [PERSON] et al. [25] explored forest ET dynamics under extreme drought in Southwestern Amazonia using the mapping evapotranspiration with high resolution and internalized calibration (METRIC) model. A large number of remote sensing datasets have emerged and provide a useful method to obtain spatiotemporal continuous ET information, while studies also indicate that a 15-30% relative error remains between these RS-based ET products [44; 45; 46]. Different from the SEB model, the physical model and empirical model are flexible to adopt satellite observations and meteorology reanalyzing data that could be applied to estimate ET at a global scale, showing generally reasonable results [29].