Dear Donald,

I advise you the Variable Infiltration Capacity (VIC) land surface hydrological model. You will find more details through the following links.

 http://www.hydro.washington.edu/Lettenmaier/Models/VIC/Overview/ModelOverview.shtml

https://books.google.dz/books?id=DakeBAAAQBAJ&pg=PA51&lpg=PA51&dq=hydrological+model+would+be+suited+for+drought+modeling&source=bl&ots=hrqOyYEexA&sig=k1bhMIAr8KZOrYgjSOsIIaH5h-w&hl=fr&sa=X&ved=0ahUKEwigpPD25o3TAhWGnBoKHdcDC3EQ6AEIQDAE#v=onepage&q=hydrological%20model%20would%20be%20suited%20for%20drought%20modeling&f=false

With my best regards

Prof. Bachir ACHOUR

Many Hydrologic model can be used to estimate water discharge that can be used to analysis the level of drought in catchment area. The model can be downloaded freely, such as: HEC-HMS (http://www.hec.usace.army.mil/software/hec-hms/),

SWAT (http://swat.tamu.edu/),

IFAS (http://www.icharm.pwri.go.jp/research/ifas/)..

Which model we use, depend on the available data in the study area. You can find in the link of web site.

Good luck...!!

Thank you all for your contributions.