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A proposed robust approach for calculating the Standardized Evapotranspiration Deficit Index (SEDI) at the global scale
* 1 , 2 , 1 , 3 , 1 , 4 , 1 , 1 , 5 , 1
1  CSIC, Spain
2  Universiteit Ghent, Belgium
3  University of Gothenburg, Gothenburg, Sweden
5  University of Montana, Missoula, USA


Here we propose a new methodology for calculating the Standardized Evapotranspiration Deficit Index (SEDI) at the global scale using the difference between the actual evapotranspiration (ET) and the atmospheric evaporative demand (AED). ET was estimated by the Global Land Evaporation Amsterdam Model (GLEAM) v3a. The SEDI has been proposed recently to quantify drought severity based on the difference between actual evapotranspiration (ET) and the atmospheric evaporative demand (AED). Our findings demonstrate that, regardless of the AED dataset used for calculations, a log-logistic distribution is needed in order to fit the ED time series. As such, in many regions worldwide, the SEDI is insensitive to the AED method used for calculation. The SEDI showed significant correlations with the Standardized Precipitation Evapotranspiration Index (SPEI) across a wide range of regions, particularly for short SPEI time-scales. Overall, while this work provides a robust approach for calculating spatially and temporally comparable SEDI estimates, regardless of the climate region and land surface conditions, further studies remain needed to assess the performance and the applicability of the SEDI to quantify drought severity across varying crop and natural vegetation areas.

Comments on this paper
Two questions
Dear Authors:

1) Does the log-logistic distribution for ED arise from the same kind of distributions for ET and AED? If not, what are those for ET and AED?

2) What do your results tell about the Bouchet hypothesis (referred to at the Introduction), or more generally for the complementarity between Actual (ET) and Potential (AET) evapotranspiration, for both arid and humid environments?

Thank you very much for your contribution to the Conference.
Sergio Vicente-Serrano
Dear German,

thanks a lot for your comments.

We did not tested the suitability of log-logistic for ET and AED and only focused on the ED, which is the variable finally used to calculate SEDI. It is the same approach used for the SPEI, in which the series of P-AED are fitted to a log-logistic but other distributions can be more useful to fit P and AED.

And secondly, we did not test the bouchet hyphotesis based on the GLEAM ET, but this info could be useful for further validation.

Thanks again

Best regards