Thank you all for your replies, I will try and answer most of your replies in a summary
ivansng wrote:
Typically in the aquarium trade, a 10-20% water change per week (or fortnight) is required to ensure prolonged fish health, and possibly more when you are growing out fishes for maximum growth... so say 20% of water change per week for a 1000L tank... 200L per week dumped...
I agree that aquaculture dumping water or inorganic hydroponics consume a lot of water and it is great to see that aquaponics can recover and re-use this water. Thanks several people for the links - they show that aquaponics can be 50% more efficient than hydroponics but it did not mention soil based systems.
machineman wrote:
I beg to differ on that account. Plants that are water stressed (ie. lack of available water) have a tendency to shut down their Stoma and transpire less. AP planets do not have this need because of constant access to water.
I agree, plant water uptake and transpiration will be greater in aquaponics than in soil based system since the plants are never stressed. If the stomates never close then the plant will use more water and it will produce more growth. Ultimately the most meaningful measure of water use efficiency is
kg of water used to produce one kg of dry material. If plants grow twice as big in hydroponics but use twice as much water than do soil grown plants then the water use
efficiency of the plant is still the same. However, the water use
efficiency of the system will also depend on the non-productive losses of water that I mentioned in my first post. As an aside, plants typically transpire several hundred litres of water to produce one kg of dry material.
desertrat wrote:
have been curious to how much water I am actually using in my AP system and have started searching for a suitable water meter and in the process of digging up around all buried plumbing joins and double checking for leaks, although fairly confident of my plumbing skills
That would be great to know. I bought a mains water meter for about $300 for another project, perhaps you could get a secondhand one from ebay?
desertrat wrote:
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.
I agree that the evaporation from gravel will be quite low, but then evaporation from a mulched soil surface should be low also. I guess most readers on this forum are keen gardeners and if they weren't using aquaponics then they would have an efficient garden by using trickle or subsurface piping, mulching, watering from a timed tap etc. This allows us to compare like-for-like ie a well thought out soil garden or a well maintained aquaponics system.
TCLynx wrote:
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.
Plants grown in soil are often overwatered to intentionally leach some of the accumulated salts down into the subsoil. All natural waters contain some sodium but since plants do not use sodium it will accumulate in the soil and either become toxic to the plant or damage the soil structure. If the water is brackish then say 10-20% of the irrigation water might be lost to the subsoil in order to leach these salts. However, I must emphasis that this is only done when the irrigation water is saline enough to cause problems - if you used water of the same quality in an aquaponics system then sodium will gradually accumulate to the point where it becomes toxic and the water needs to be dumped. I want to start another thread in more detail about this later but for now I think that intentional leaching to displace salts could not account for the 10 fold difference between the systems, especially if we were to compare like-for-like by using the same quality water in both systems.
RupertofOZ wrote:
And 90% of that water applied.... just drains down beyond plant roots...
It really is a no-brainer... by pure logic...
I am not sure it is a no-brainer and I have to disagree that 90% of the water drains beyond the plant roots.
Plant transpiration can be estimated quite well as plant factor x actual pan evaporation. Pan evaporation is found quite easily from the met bureau - for example, the evaporation from an open pan in Perth in Feb 2010 was 257mm. The plant factor varies for different species. If we choose grass then about 0.7 is a typical plant factor - that is, a patch of grass will transpire about 70% as much water as is lost from an open pond. For Perth in February, this equates to 0.7 x 257 = 180mm of water is required by the grass to keep it growing at full productivity. If we use your example of 10mins of sprinklers and we run them twice a week (due to water restrictions) then in four weeks we have run them for 80 minutes. If we are generous and assume the sprinklers apply water at 1mm per minute then we have only applied 80mm of water to a plant that needs 180mm to grow to its full potential. Even if you ran the sprinklers for twice as long or four times per week then you are still falling short of what the grass can fully utilise. There is simply no water remaining to leach past the plant roots, and secondly, it shows that typical rates of water application do in fact limit plant growth. It would follow that having unlimited water available to the plant as in aquaponics will increase water consumption but as I pointed out earlier, if this is accompanied by increased production then it will not be a concern.
(Note: The lack of leachate can be confirmed by using a lysimeters (tanks buried in the subsoil to catch through flow) - lysimeters in Perth catch almost no leachate in irrigated ovals or golf courses over the summer months)
Going back to my original points, items 1-3 still seem comparable between soil based systems and aquaponics and item 4 is most likely to be larger in aquaponics than in the soil.
Regards
Backyard Aquaponics
