software for hydrologic modeling of the watershed in arid and semi-arid zone.

To give a good and helpful answer is quite difficult. It depends on various prerequisits:
- time scale, time range, time steps for modelling
- spatial extension of model area: large or small scale
- wich data are availble? do I can feed the choosen software with convenient data?
- or reverse: wich data need the models?

ArcGIS with Arc Hydro
http://resources.arcgis.com/en/communities/hydro/01vn0000000s000000.htm

QGIS with Grass GIS is opensource software that has similar capabilities https://hub.qgis.org/wiki/17/Hydrology_and_Hydraulic_modelling

Dear Aiden, this may help you http://gwadi.org/sites/gwadi.org/files/wheater_1_L1.pdf