Backyard Aquaponics
A place to discuss aquaponics

I expect that a plant can transpire a whole lot more than the amount of gravel it's planted in could possible evaporate. (A tomato plant can easily take up over a gallon of liquid in a day, most of that liquid is being transpired out to evaporate and cool the plant.) Plants are very good at holding water out where it can evaporate, they have lots of open airy surface area being held up for the breeze and sun.

10% is a good figure to work on, I have been keeping records and my average water usage for the past 6 months (our summer period) is 250 litres/week.

I have:
1x 1,800 litre fish tank
2x 350 litre sump tank
1x 600 litre isolation tank (all in a green house)
Okay, so that works out to 8%

The GBs and fish tanks do not have direct sunlight on them as there is 50% shadecloth blocking out the sun...the water level in the GBs is kept below gravel level so is not exposed either...so most of the water loss is from transpiration