Use of remote sensing for evapotranspiration monitoring over land surfaces

Abstract

Monitoring évapotranspiration (ET) at large scales is important for assessing climate and anthropogenic effects on natural and agricultural ecosystems. This paper describes techniques used in evaluating ET with remote sensing, which is the only technology that can efficiently and economically provide regional and global coverage. Some of the empirical/statistical techniques have been used operationally with satellite data for computing daily ET at regional scales. The more complex numerical simulation models require detailed input parameters that may limit their application to regions containing a large database of soils and vegetation properties. Current efforts are being directed towards simplifying the parameter requirements of these models. Essentially all energy balance models rely on an estimate of the available energy (net radiation less soil heat flux). Net radiation is not easily determined from space, although progress is being made. Simplified approaches for estimating soil heat flux appear promising for operational applications. In addition, most ET models utilize remote sensing data in the shortwave and thermal wavelengths to measure key boundary conditions. Differences between the radiometric surface temperature and aerodynamic temperature can be significant and progress in incorporating this effect is evident. Atmospheric effects on optical data are significant, and optical sensors cannot see through clouds. This has led some to use microwave observations as a surrogate for optical data to provide estimates of surface moisture and surface temperature; preliminary results are encouraging. The approaches that appear most promising use surface temperature and vegetation indices or a time rate of change in surface temperature coupled to an atmospheric boundary layer model. For many of these models, differences with ET observations can be as low as 20% from hourly to daily time scales, approaching the level of uncertainty in the measurement of ET and contradicting some recent pessimistic conclusions concerning the utility of remotely sensed radiometric surface temperature for determining the surface energy balance.

Regional estimation and validation of remotely sensed evapotranspiration in China

“…However, the use of radiometric surface temperature as a substitute for the aerodynamic temperature in the original Penman-Monteith model leads to substantial error, especially over partial vegetation cover, because the radiometric surface temperature is a composite temperature consisting of soil and vegetation temperatures. Yet such error can be reduced in several ways: through an introduction of an extra-resistance (Kustas and Norman, 1996;Kustas et al, 1989), by switching from singleresistance models to a two-source model, by using more complicated multilayer models (Choudhury and Monteith, 1988;Lhomme et al, 1994a,b;Norman et al, 1995), by developing an empirical formula to estimate the aerodynamic temperature (Huang et al, 1993) and by incorporating a vegetation index within the model (Moran et al, 1994(Moran et al, , 1996. A Surface Energy Balance System (SEBS) for the estimation of atmospheric turbulent fluxes and surface evaporation using satellite earth observation data in the visible, near infra-red and thermal infrared frequency range has been designed for composite terrain with heterogeneous surfaces at a larger scale (Su, 2002).…”

“…Detailed knowledge of ET is vital for monitoring regional and global climate change through the hydrological cycle, and its estimation has significant applications in agriculture especially in areas such as runoff prediction, soil moisture prediction, crop yield prediction, and land use planning (Bastiaanssen et al, 2007;Kustas and Norman, 1996). The conventional method that uses point measurements to estimate ET values is only representative of local areas and cannot be extended to large areas due to the dynamic nature and regional variation of ET (Courault et al, 2003).…”

Spatial evapotranspiration, rainfall and land use data in water accounting – Part 1: Review of the accuracy of the remote sensing data

“…Review papers on advanced algorithms for estimating spatial layers of ET have been published by Moran and Jackson (1991), Kustas and Norman (1996), Bastiaanssen (1998), Courault et al (2005), Glenn et al (2007), Gowda et al (2007), Kalma et al (2008), Verstraeten et al (2008), and Allen et al (2011). While these review papers provide a good understanding of the evolution of ET algorithm development, they rarely report the accuracies attainable, especially at a seasonal or longer time frame.…”

A review on evapotranspiration data assimilation based on hydrological models

“…Evapotranspiration (ET) is the main process controlling the water cycle and energy transport between the atmosphere, hydrosphere, and biosphere (Priestley and Taylor, 1972), and is an important subject of research on global and regional water and energy budgets (Rosenberg, 1983;Kustas and Norman, 1996;Vinukollu et al, 2011). Remote sensing and hydrological modeling are two key approaches to estimate evapotranspiration (Liu et al, 2007).…”