Observing actual evapotranspiration within a heterogeneous watershed: The case study of the Kamech site, Cap Bon Peninsula, Tunisia

There is a strong need for continuous long term observations of land surface fluxes, especially the latent heat flux, or actual evapotranspiration (ETa), a key component of both the surface energy balance and the hydrological cycle. The eddy covariance (EC) method is widely used to provide continuous measurements of land surface fluxes. However, missing data are inherent to EC measurements and several gap-filling methods have been proposed. Nevertheless, observing ETa by EC and testing gap-filling methods in heterogeneous and hilly watersheds have received little attention.

The main objective of this study was to obtain complete ETa chronicles from EC measurements collected within a small hilly watershed. This implied to adapt and test gap-filling techniques in the particular conditions of a hilly and heterogeneous watershed.

The experiment was conducted within the agricultural watershed Kamech, Northeast Tunisia. Kamech, one of the experimental sites of the OMERE environmental observatory, has a 2.45 km² area and is typified by spatial heterogeneities related to topography, pedology and crops. A 9.6-meter-high EC flux tower, located close to the center of the watershed, has been operating since 2010. Sensible and latent heat fluxes data collected from 2010 to 2013 were instrumentally corrected and quality controlled.         

The software REddyProc was used to fill gap of the chronicles of latent and sensible heat fluxes at hourly timescale. In order to account for the combine effects of wind direction, topography and heterogeneity, the gap filling was applied after discriminating between the two dominant wind directions, which slightly improved the estimation of the sensible and latent heat fluxes. Aggregating the hourly ETa estimates at daily and monthly timescales allows analyzing the temporal variability of ETa over the seasons and between years.