Integrating thermal stress indexes within Shuttleworth-Wallace model for evapotranspiration mapping over a complex surface

The main goal of this work was to evaluate the potential of the Shuttleworth-Wallace (SW) model for mapping actual crop evapotranspiration (ET) over complex surface located in the foothill of the Atlas Mountain (Morocco). This model needs many input variables to compute soil (r(s)(s)) and vegetation (r(s)(v)) resistances, which are often difficult to estimate at large scale particularly soil moisture. In this study, a new approach to spatialize r(s)(s) and r(s)(v) based on two thermal-based proxy variables was proposed. Land Surface Temperature (LST) and Normalized Difference Vegetation Index (NDVI) derived from Landsat data were combined with the endmember temperatures for soil (Ts-min and Ts-max) and vegetation (Tv(min) and Tv(max)), which were simulated by a surface energy balance model, to compute the soil (Ts) and the vegetation (Tv) temperatures. Based on these temperatures, two thermal proxies (SIss for soil and SIsv for vegetation) were calculated and related to r(s)(s) and r(s)(v), with an empirical exponential relationship [with a correlation coefficient (R) of about 0.6 and 0.5 for soil and vegetation, respectively]. The proposed approach was initially evaluated at a local scale, by comparing the results to observations by an eddy covariance system installed over an area planted with olive trees intercropped with wheat. In a second step, the new approach was applied over a large area which contains a mixed vegetation (tall and short) crossed by a river to derive r(s)(s) and r(s) (v), and thereafter to estimate ET. A Large aperture scintillometer (LAS) installed over a line transect of 1.4 km and spanning the total area was used to validate the obtained ET. The comparison confirmed the ability of the proposed approach to provide satisfactory ET maps with an RMSE, bias and R-2 equal to 0.08 mm/h, 0.06 mm/h and 0.80, respectively.

Automated summary

This study successfully evaluated the potential of the Shuttleworth-Wallace model for mapping crop evapotranspiration in complex surface areas near the Atlas Mountain in Morocco. By combining thermal proxy variables with soil and vegetation temperatures, a new approach was developed to estimate ET, providing satisfactory maps with RMSE, bias, and R-2 of 0.08 mm/h, 0.06 mm/h, and 0.80, respectively.