TankSmart is a free tank water estimation program used for the calculation of water tank capacities. It uses estimated water inflows including rainwater, mains water and other sources, versus water usage and loss from various user definable sources to determine the sizes of tanks needed.
The output of this process is a series of graphs and other information which shows the ongoing water balances over a period of time. A second aspect of this is that it allows the complication of water restrictions to be factored in. TankSmart is 'written' in MSExcel and comes with a comprehensive manual to describe how it is to be used.
TankSmart is freely downloadable from an Australian website. Since I am not allowed to post a link in this forum if you search for a Australian professional electrical engineer called Kerry Nufer on Google, and follow the 'download' page link on his default web page, you can download the application and manual free of charge.