Abstract
Accurate estimates of evapotranspiration (ET) are essential for the conservation of ecosystems and sustainable management of water resources in arid and semiarid regions. Over the last two decades, several empirical remotely sensed ET models (ERSETMs) had been developed and extensively used for regional-scale ET estimation in arid and semiarid ecosystems. These ERSETMs were constructed by combining datasets from different sites and relating measured daily ET to corresponding meteorological data and vegetation indices at the site scale. Then, regional-scale ET on a pixel basis can be estimated, using the established ERSETMs. The estimation accuracy of these ERSETMs at the site scale plays a fundamental and crucial role in regional-scale ET estimation. Recent studies have revealed that ET estimates from some of these models have significant uncertainties at different spatiotemporal scales. However, little information is available on the performance of these ERSETMs at the site scale. In this study, we compared eight ERSETMs, using ET measurements from 2013 to 2018 for two typical eddy covariance sites (Tamarix site and Populus site) in an arid riparian ecosystem of Northwestern China, intending to provide a guide for the selection of these models. Results showed that the Nagler-2013 model and the Yuan-2016 model outperformed the other models. There were substantial differences in the ET estimation of the eight ERSETMs at daily, monthly, and seasonal scales. The mean ET of the growing season from 2013 to 2018 ranged from 465.93 to 519.65 mm for the Tamarix site and from 386.22 to 437.05 mm for the Populus site, respectively. The differences in model structures and characterization of both meteorological conditions and vegetation factors were the primary sources of different model performance. Our findings provide useful information for choosing models and obtaining accurate ET estimation in arid regions.
Highlights
Evapotranspiration (ET) refers to water transferring from the ground surfaces into the atmosphere, through soil evaporation (E) and plant transpiration (T) [1,2]
Several empirical remotely sensed ET models (ERSETMs) have been developed for ET estimation in arid and semiarid ecosystems. Most of these ERSETMs were constructed for riparian ecosystems under two simplified assumptions: (1) Plant transpiration predominates over ET with relatively weak soil evaporation in riparian ecosystems [10,11,12]; and (2) phreatophytes, which are the dominant vegetation types in riparian ecosystems, obtain water mainly from capillary fringe and groundwater, through root uptake, and have a reasonably steady supply of water [13,14]
Air temperature is an indirect reflection of solar radiation, water molecules could not move into the atmosphere if the wind is absent or if the atmosphere has already been saturated with humidity [59]
Summary
Evapotranspiration (ET) refers to water transferring from the ground surfaces into the atmosphere, through soil evaporation (E) and plant transpiration (T) [1,2]. Over the last two decades, the integration of the eddy covariance approach and the remote-sensing technique has been perceived as a feasible and effective way of estimating regional-scale ET [7,8,9]. Based on these techniques, several empirical remotely sensed ET models (ERSETMs) have been developed for ET estimation in arid and semiarid ecosystems.
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