Drylands are among those regions most sensitive to climate and environmental changes and human-induced perturbations.
The most widely accepted definition of the term dryland is a ratio, called the Surface Wetness Index (SWI), of annual
precipitation to potential evapotranspiration (PET) being below 0.65. PET is commonly estimated using the Thornthwaite
(PET Th) and Penman–Monteith equations (PET PM). The present study compared spatiotemporal characteristics of global
drylands based on the SWI with PET Th and PET PM. Results showed vast differences between PET Th and PET PM;
however, the SWI derived from the two kinds of PET showed broadly similar characteristics in the interdecadal variability
of global and continental drylands, except in North America, with high correlation coefficients ranging from 0.58 to 0.89. It
was found that, during 1901–2014, global hyper-arid and semi-arid regions expanded, arid and dry sub-humid regions contracted,
and drylands underwent interdecadal fluctuation. This was because precipitation variations made major contributions,
whereas PET changes contributed to a much lesser degree. However, distinct differences in the interdecadal variability of
semi-arid and dry sub-humid regions were found. This indicated that the influence of PET changes was comparable to that of
precipitation variations in the global dry–wet transition zone. Additionally, the contribution of PET changes to the variations
in global and continental drylands gradually enhanced with global warming, and the Thornthwaite method was found to be
increasingly less applicable under climate change.

EB00000002306K

Available

• Sensitivity of Potential Evapotranspiration Estimation to the Thornthwaite and Penman–Monteith Methods in the Study of Global Drylands