The primary advantage
is that SPI is based on rainfall alone, so that drought
assessment is possible even if other hydro-meteorological measurements
are not available.
The SPI is also not adversely
affected by topography, it is defined over various timescales
(3, 6, 9, ..., 72 months) and this allows to use it as short, medium
and long-term drought index to describe drought conditions
over a range of meteorological,