Irrigation You can use ...

Irrigation

You can use evapotranspiration method : The evapotranspiration (ET) method calculates the amount of water needed to maintain  landscaped area for a given location based on the water requirements of the type of plants, specific conditions of  the site, and precipitation received by the site.  ET represents the loss of water from the Earth’s surface through  the combined processes of evaporation (from soil and plant surfaces) and plant transpiration.   Or else, use irrigation audit method. You can find literature on both the methods.