which led to the development of SWSI (Shafer and Dezman
1982), probably the most popular of this group. Later, RDI
(Weghorst 1996) improved SWSI by incorporating temperature
and hence calculated a variable water demand as input.
RSDI (Stahl 2001) bases its model on homogeneous
drought-stricken regions that comprise several neighbouring
low-flow gauging stations. RSDI first calculates the deficiency
in streamflow compared with historic values and then
uses cluster analysis to delineate the drought-stricken regions.
Two later indices consider a water balance model: GRI (Mendicino
et al. 2008) and Water Balance Derived Drought Index
(Vasiliades et al. 2011). The former focuses on groundwater
resources and uses geo-lithological conditions information in
a distributed water balance model, while the latter uses a
model that artificially simulates runoff for ungauged and
low-data watersheds