what are you looking for exactly? depending on the topic of interest, there are different 'definitions' of drought. I think Niemeyer 2008 is a nice paper to read if you want an overview of this variability.

During the last years, the Standardized Precipitation-Evapotranspiration Index has been shown one of the best indicators of drought derived from meteorological measurements. Vicente-Serrano has written sevarl papers about that (see also attachment).

If you are interested in using soil measurements to study drought effects, you should have a look at Granier et al 2007, and to my own letter about the data needed for this (Vicca et al 2012).

I may be able to help you more if you make the question a bit more specific...

Please go though this , this is really good for developing preliminary understanding about drought

http://drought.unl.edu/DroughtBasics.aspx

Hi, check this out: Acta Geophysica vol. 61, no. 2, Apr. 2013, pp. 404-421

I suggest this recent "perspective" paper:

http://www.nature.com/nclimate/journal/v4/n1/full/nclimate2067.html

My Dear Janab Jalal Shiri; Here are few links which may be quite useful and helpful. Regards, Dr Dubey

http://onlinelibrary.wiley.com/doi/10.1029/WR016i002p00297/full

http://www.sciencedirect.com/science/article/pii/002216949502713Y

http://www.tandfonline.com/doi/pdf/10.1080/02508068508686328

http://onlinelibrary.wiley.com/doi/10.1029/2004JD005182/full