Quote: Water usage in agriculture has four main components:
1. Losses in transporting water to the plant
2. Water percolating past the rootzone and draining to the subsoil
3. Water evaporating from the soil surface
4. Water transpired by the plant
First before going into the numbers, keep in mind.
When many people make these claims (the first ones to make the big claims, not those of us simply repeating) they often exaggerate. I hate that people tend to do this, I would prefer that people make very reasonable claims so that it would be reasonable for real systems to live up to those claims. But anyway.
I expect the claims would be basing the "traditional agriculture" as the most water wasteful huge sprinklers running every other day during the hottest part of the day rain or shine wind or no. The losses from using overhead sprinklers are huge. The water savings from simply switching to micro sprinklers or drip irrigation is amazing so if you switch to a loss less method of delivering the water to the plants, you get even better. Remember that when people compare stuff to being better on water than dirt gardening they are generally comparing to big sprinklers.
Anyway, Aquaponics is gonna save huge on number 1.
Aquaponics is also going to save on 2 since they water can't percolate away, it is a recirculating system. You mention needing to have some leaching to avoid salt buildup, well in aquaponics we are not using chemical salts to fertilize the plants so we don't have to rinse or leach them away. There are only rare occasions that one might need to use large amounts of salt in an established system and even then large water changes are usually not needed unless something goes really wrong.
Number 3, evaporation from the soil surface, well since the surface of the gravel doesn't get wet it isn't as big an evaporation source as one might think. The evaporation from the fish tank where there is water splashing or bubbling would be a source of evaporation but it isn't as extreme as what evaporates from the soil surface of a whole garden or field after sprinkler irrigation.
Of course number 4 is gonna be the biggest source of water loss to an Aquaponics system of course the humidified air in the micro-climate created by the AP system and the transpiring plants will also help to reduce additional water loss from evaporation since humid air doesn't suck up more water as easily as dry air. Comparing water stressed plants not transpiring as much to well watered plants in an AP system kinda skips the whole production side since most water stressed plants don't produce as well.
So, I can't really answer about the exact amount of water savings from using AP since it will really depend alot on exactly what kind of agriculture and method you are gonna compare to and where it is.
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