somewhat relevant articles (may be helpful

1. Climate trends and behaviour of drought indices based on precipitation and evapotranspiration in Portugal (doi:10.5194/nhess-12-1481-2012)

2.Comparison of suitable drought indices for climate change impacts assessment over Australia towards resource management (DOI: 10.1002/joc.1649)

3. Climatologist Drought Analyses and Projection Using SPI and PDSI: Case Study of the Arkansas Red River Basin (http://dx.doi.org/10.1061/(ASCE)HE.1943-5584.0000619)

Read More: http://ascelibrary.org/doi/abs/10.1061/(ASCE)HE.1943-5584.0000619

Regard

Muhammad Waseem

There is some evidence on variance but mixed reports about trends For RDI ina region http://www.ewra.net/ew/pdf/EW_2013_41_02.pdf or SDI and RDI in https://www.google.it/search?q=climate+change+SPI+rdi&ie=utf-8&oe=utf-8&gws_rd=cr&ei=iZ0jVY_dHI6y7QaJ4oCIDw. Their value is relative PET and variability increses acoording to mean precipitation. Possibly rivers run off would be more appropriate as compuond but effective index.

Comparison of seven meteorological indices for drought monitoring in Iran (DOI: 10.1002/joc.1264)

Analyzing spatial patterns of meteorological drought  using Standardized precipitation Index   DOI: 10.1002/met.33

Hi

before everything, you need to calculate this indices and after that you can monitor their amounts with change of precipitation and temperature. furthermore you need to model this changes for future with different model, such as LARS-WG, ASD, and other statistical downscaling model.

for calculating indices, you can use RDIT (Rained-based Drought Index Tool). This is a software for calculates 8-indices and draw graphs and assess severity and duration.

please visit:

http://www.agrimetsoft.com/

regards

Thank you

Article Estimation of meteorological drought indices based on AgMERR...

Hi

Please read this. This article has a software for calculating 8 drought Indices. SPI CZI, MCZI, ZSI, RAI, PN,DI, EDI