In the WDI model, some inputs required to estimate the vertices
of the trapezoid are difficult to assess, for example, the aerodynamic resistance (highly affected by wind speed and plant structure), maximum and minimum canopy resistances (both closely
related to stomata closure) and soil heat flux (Mendez-Barroso
et al., 2008). Providing these data especially for natural vegetation
covers is difficult. Therefore, Sandholt et al. (2002) interpreted the
AVHRR LST vs. NDVI relationship in terms of soil moisture and proposed the Temperature Vegetation Dryness Index (TVDI) which is a
simplification of the WDI model. In the TVDI method, the study
area should be large enough to provide a wide range of NDVI and
surface soil moisture conditions. The TVDI is estimated using the
following equation: