Remote Sensing of Evaporative Fraction in Big Cypress National Preserve: A Comparison of Methods

Abstract

The evaporative fraction (EF) of available radiation energy (A) is useful for determining evapotranspiration in the landscape. This paper compares three methods for the remote sensing of EF. These methods utilize surface temperature (T) calculated with LANDSAT band 6. The first method is a version of the triangle method utilizing relationship between T and NDVI. The second method fits a linear relationship between EF and T using eddy flux tower measurements. The third method is based on the residual method assuming a constant surface roughness so that sensible heat (H) can be linearly related to T. The 3 methods were applied to 2 study areas: 1) Big Cypress National Preserve and 2) Waldo, Fl and vicinity. Method 1 consistently over estimates EF compared to method 2 and 3. Method 2 and 3 have very little difference on average, but the variance of the difference is significant. These differences form spatial patterns where method 2 over/under estimates EF compared to method 3 depending on surface type. Determining whether method 2 outperforms method 3 will require validation data which can be difficult since the availability of eddy flux data is limited and needed for calibration.