By Wilmer Noome
Abstract:
Evapotranspiration (ET) is one of the import parameters of the water balance, determining the amount of water vaporized from the soil and plants and it is important for different sectors (e.g. agriculture). Without satellite images calculating the evapotranspiration is only possible on small scale using lysimeters or pan-evaporation methods. Estimating evapotranspiration on a larger scale is possible by using remote sensing and satellite images.
The use of remote sensing images was merely focused on high resolution images over homogeneous areas, although some research looked at the possibilities of using lower resolution images. This research looked at the possibilities of using medium resolution images (MODIS images, 500 m) over heterogeneous areas with significant diversity in land-use (The Netherlands) using the S-SEBI method (Simplified Surface Energy Balance Index). The S-SEBI method needs a small number of additional meteorological parameters (incoming long and shortwave radiation), which makes it possible to use the method in areas without meteorological measurement sites. The validation of this method is done using several flux towers throughout the Netherlands and two Landsat images, these images were aggregated to the same resolution as the MODIS images.
The comparison of the MODIS result with the flux tower measurements show that the S-SEBI method estimate the ET and net radiation (Rn) comparable with the measured data (with an R2 ≈ 0.7 and R2 ≈ 0.9 respectively). For forested areas the S-SEBI method underestimates the Rn. The ET estimations show less similarity compared to the measured data, this is a result of the evaporative fraction (EF) estimations. The EF estimations and measurements show less similarity than the Rn data. Because the EF is used to calculate ET, this lower similarity is also visible in the ET estimations.
By comparing the MODIS results with Landsat results it was shown that the ET estimations are comparable, even when the Landsat pixel values under one MODIS pixel show some variance.
The use of remote sensing to estimate ET over large areas with medium resolution images as shown in this research is a promising way of estimating ET. The estimations in this research were not as accurate as the flux tower measurement, but with the development of the methods (e.g. the S-SEBI method) to estimate the ET and the use of sensor webs to give more additional (meteorological) data, this way of estimating ET will develop into more accurate estimations.