CALCULATIONS: How big is an ideal surface to maintain the garden?

We calculated how big our surface should be to have enough water for the whole garden.
In the garden there is 36 m^2 of ground where people cultivate.

In the summer they need the rain the most so we calculated how much it rains in this period. 

This is 2,5 l / m^2 a day. (average)

We searched on internet how much water 1 m^2 needs to cultivate. We found 20 litres a week.

This leads to our calculations:

36 m^2 * 20l = 720 l /m^2 a week

Each weeks there falls 2,5*7 = 17,5 l of rain.

=> 720 l/ m^2   /  17,5 l = 41 m^2

This is a lot, because we want as much surface as possible we are thinking about a modular system with 9 m^2. In that way we can build multiple systems to put around the garden.