2.6 Remote sensing

Many ET models are available for use with remote sensing data. In addition, there are a variety of satellite data sources such as Moderate Resolution Imaging Spectrometer (MODIS), Landsat, Advanced Very High-Resolution Radiometer (AVHRR), Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), and many others. Additional information on satellite sources and available models can be found in [39]. Most remote sensing models are based on the energy balance where the reflectance from remote sensing is used with weather data from nearby weather stations and the four components of the energy balance are calculated. Typically, LE is calculated as the residual of the energy balance and converted to ET at an hourly and daily time step.

The biggest issue with using satellite data for creating ET maps is poor spatial and temporal resolution. Many energy balance-based models such as METRIC [40], SEBS [41], SEBAL [42], and others require thermal data to calculate surface temperature. These models are more limited on available data. ASTER, MODIS, and Landsat are the main data sources available with thermal sensors. ASTER has the highest spatial resolution at 15 m for visible wavelengths and 90 m for thermal wavelengths but has a return interval of 16 days [43]. Landsat data has 100 m resolution for thermal wavelengths and can provide data on an 8 day interval if Landsat 7 and Landsat 8 are both used [44]. MODIS provides daily data but has poor spatial resolution of 1000 m [45]. Although the models typically only provide hourly and daily ET estimates, methods are available to interpolate between satellite passes and for monthly and seasonal sums [46].

The aforementioned remote sensing models not only provide ETa maps but can also provide estimates of leaf area index, surface temperature, surface albedo, and many others. Although the spatial and temporal resolution of existing satellites limits applications to field-scale agricultural use, the rapid increase in unmanned aerial vehicle (UAV) technology shows vast potential to acquire remote sensing data with spatial resolution at a centimeter scale and as frequent as desired. Satellite-based ET maps typically have accuracy of 20–30% at best [47]; however, the accuracy of using UAV data for ET maps is not currently known.
