Formulation of the problem ...

Formulation of the problem needs further input such as :

• Describing inflow such as rain fall intensity and distribution,
• Quantifying out flow such :
• crop planning,
• evaporation and transpiration needs which in turn depends on temperature and sunshine hours.
• type and frequency of irrigation,
• storage evaporation and conveyance losses etc
• topography of the land 

Storage is a function of the inflow and out flow to satisfy the irrigation requirement which has to be worked out based on cropping plan for the area for  individual cases.

CROPWAT is a program by  FAO site for the purpose