Backyard Aquaponics
A place to discuss aquaponics

The wheelie bin is not a bad idea for cheap trial.

However, for an even cheaper trial we can do another thought experiment. I read through the cost benefit analysis on the main page (http://www.backyardaquaponics.com/Travi ... ystems.pdf) and whilst I think the dollar figures are well analysed, the water use is presented as "seven kilos of vegetables and/or herbs can be grown for every 22 litres of water".

To grow one kg of vegetables from this calculation you need 22/7 = 3.14 litres of water = 3140 mL
If we grow lettuce for example, then 1kg of lettuce contains about 950mL of water (see http://www.nal.usda.gov/fnic/foodcomp/c ... ut_edit.pl)
From our initial starting point of 3140 ml we have used 950 ml to "fill" the lettuce with the water that is removed when it is harvested. This is not water actually transpired during productive growth, just water to fill up the structure of the plant.
This leaves 3140-950 = 2190 mL of water for the plant to actually transpire.

My first thought experiment is to compare this 2190 mL to a vase of flowers. We have a vase at home of about 2 litres. If we have it full of flowers we might have to add another litre during the week to top it up so if we managed to keep them alive for a fortnight we would have used about 2 litres.

How much growth is likely to have occurred in this time? A 1kg weight gain would look massive in a vase - I am just guessing the initial starting weight of a bunch of flowers but perhaps 3-400g might be typical - so a 1kg gain would be very noticeable.

The second thought experiment follows from the first. There is a lot of research that shows that plants need to transpire around 300 litres of water to generate 1 kg of dry matter - this water loss occurs when the plant takes in CO2 from the air into the leaves and cannot prevent water being lost back to the atmosphere. This water loss is within the plant and is separate from leaching, runoff etc. To grow our 1kg of lettuce (which is only about 50 grams of actual plant material) we need at least 300 x .05 = 15 litres of transpiration. The 300 kg per kg does vary with species and conditions, but it does not really fall much below 300 and can go as high as 1000 kg per kg.

And you call yourself a "scientist"....

Cut flowers in a vase.... don't grow.... they cling to life and may transpire a certain amount of water...(through capillary action).... slowly dying, and probably transpiring less each day....

You might as well compare the amount of water needed to wash a Mack truck... with an apple orchard...

What ever happened to the "scientific method"...

Don't make assumptions...

Setup a double blind....

Gather data...

Analyse the data...

Draw conclusions from the data...


Please link to this data...

And while the open pores taking in CO2 might allow some transpiration... is this the the only process involved in "transpiration"???

And just where did you get the "we need at least 300 x .05 = 15 litres of transpiration. " from... where did you derive the ".05" factor from...

And lets be carful to keep our "apples" apples... and "oranges"... oranges...

Just takign the piss now.

Yeah - sounds more like a wild guess than a thought experiment.

Considering the cell structure is made of minerals etc and not just water

No, I was trying to place the figures into perspective. I should have clarified that I was not necessarily expecting the flowers to grow, but it is very easy for figures like kilo-litres and hectares to become meaningless so I wanted to refer back to something in a proportion that everyone can relate to. The several litres of water in a vase and a kilogram of growth are the figures that were presented in the paper that I linked to - do you not think that 3 litres and 1 kg are much easier to comprehend rather than 22kl and seven tonnes?

[quote="RupertofOZ"]And you call yourself a "scientist"....

What ever happened to the "scientific method"...

[quote]

Well I have not seen much scientific method in the links that claim aquaponics uses only 10% of the water that soil grown plants do - most of the links only show a conclusion without any explanation of the methods or research that was used.

The trials do not have to be double blind, this is only used in trials where the subjects can be influenced by the investigator having knowledge of the treatment such as a doctor biasing a patients response to a treatment. They do need to be replicated however.

Yes, the minerals and organic molecules are only 5% of the fresh weight of the plant, the other 95% is water.

This is where the figure of .05 came from in my calculation - you are not growing 1kg of plant material via photosynthesis, you are only converting sunlight into about 50 grams of sugars and the remainder of the cells is mostly water.

Though we talk of dry weight of plant material produced
So what is the volume of transpiration, I was under the impression that droplets of water could be seen on a plant at the end of the leaves.
That this water could help maintain a micro climate and be recycled within such.
So long as it had an ground cover, midlevel and a canopy?

Please link to this data...
[/quote]

If you want to get a comprehensive overview of this then you will probably need a text book rather than the internet. I had a quick look for you but it is hard to find an easy to read summary.

Here is something below that might be useful background from the Ag Dept of WA. They state that beans produce about 10kg per ha per mm which is about 1kg per 1000L of water. http://www.agric.wa.gov.au/PC_92182.html?s=1001


When a crop uses water it passes through the plant and evaporates from the leaves. Very little is retained by the plant itself. Water also evaporates directly from the soil surface. Only water passing through the plant (the process is called transpiration) contributes to crop growth. Minimising the amount of water lost by soil evaporation is one means to improve water use efficiency because more of the water is then available for use by the plant. Rapid canopy development, as in field peas and faba beans, helps by maximising soil surface shading (hence reducing soil evaporation). The slow canopy development of chickpea contributes to its poor water use efficiency because it allows more soil evaporation. As well, water does not evaporate from dry soil surfaces, so water use efficiency is sometimes higher in dry seasons and dry locations than in wet ones.

Transpiration is an inevitable consequence of the way plants grow. They absorb carbon dioxide (CO2), from which the bulk of plant biomass is built, through pores in their leaves called stomata. The insides of the leaves are wet (and must be, since most of the necessary chemical reactions only occur in solution) so at the same time water vapour escapes to the dry external atmosphere through the stomata. If the stomata close to restrict transpiration, CO2 assimilation, and hence crop growth, is reduced. If the stomata open up to allow faster CO2 assimilation, the crop grows faster but, more water is lost.

The efficiency with which water passes through the plant is used also varies. The hotter and drier the air, the faster transpiration will be for a given stomatal opening. When the surrounding air is at 100 percent relative humidity, there will be no transpiration irrespective of stomatal opening, but CO2 will continue to be assimilated. Crops therefore use transpired water more efficiently during winter and early spring, and in the early morning, when the air is cool and humid, than in late spring and summer, or in the late afternoon. This is a further reason why early maturing pulses which complete most of their life cycle during cool, humid weather, like field peas and faba beans, have higher water use efficiency than crops like chickpeas which continue to grow as the weather becomes hotter and drier.

Research generally uses dry weight to make a real comparison since fresh weight is not really "produced" by growth and it does not contain any useful energy. That is why lettuce is such a good diet food

The water lost by transpiration moves quickly to the atmosphere and is lost irreversibly.

The water you can sometimes see at the end of the leaf is guttation which is a separate process.http://en.wikipedia.org/wiki/Guttation

Well a whole of what you said is different to what I understand.
Also, the mid range and canopy I was talking about were not the leaves of a bean or pea.
I was talking about other species that grow above a ground cover and a midrange plant, then helping to 'recycle' the transpired water
for the benefit of the 'other' species and the closed loop system.
Is also why I plant, not in groups but in a spread of ground mid and canopy.
Transpiration I was to understand will stop if the temperature become too high and why plants droop.
We don't( well I don't) grow chickpea.
Guttation is also the use and release of water by a plant is it not?
BYAP is so different(mix of species) than broad acre or even market gardening(plots of same species),
so much so that I don't think the same equations can be drawn.

The mechanics of transpiration are reasonably similar between plants, apart from three biochemical groupings (C3,C4, CAM). They all have to inevitably lose water in order to gain CO2 and this is invariably several hundred litres of water per kg of dry matter produced.

Agreed totally.... and I thought that had been accepted previously...

So if we assume that transpiration rate is a constant.... then I don't see how you could conclude anything other than.... aquaponics is the most efficient usage of water for plant growth...

The other factors of water loss... are drainage waste in soil, and water/nutrient replacement....(both soil and hydroponics).... neither of which is applicable to aquaponics, as it is a constantly recirculated/nutrient replenished system....

Ergo.... aquaponics is the most efficent water user of the three methods....

Thanks for confirming what we've always known... and has been oft stated by others...

The question then comes back to... does it only use 10% of the water used in soil production....

Many studies of hydroponics have suggested that the hydroponic method of growing uses about 60-90% less water than equivalent soil methods... granted this is dependant on water minimisation techniques employed in soil methods... but assume best practise...

And without the need to replace spent nutrient solution... hence water....

Aquaponics is at least 50% more efficient than hydroponics....

Ergo ... if hydroponics only uses about 10-40% in comparison to soil methods... and aquaponics only uses 50% in comparison to hydroponics...

Then aquaponics only uses 5-20% in comparison to soil methods...

As you rightly point out... transpiration is a "constant"...

The real question to determine differences... would surely be to examine the other factors... "evaporation", water loss through drainage...

My contention would be that per sq mtr... aquaponic systems... (given the density of plantings)... would be no where near the evaporative losses of hydroponics... which are near nil anyway... and significantly less than soil systems...

And aquaponics does have the water losses of soil drainage... or nutrient replacement of hydro....

I really just can't understand how you can't see that...

Ok... let me try and relate this to a real example...

Here's a hydroponics table full of green oak lettuce that I grew at home... 200 plants... let's assume each is a kilo...



The table measures (in total) ... 6mtr x 2mtr.... a total footprint of 12 sq mtr....

(In fact there are 10 channels, each of which are 6mtr x 100mm).... so the "true" total surface area of plant production is 6mtr x 0.1 x 10... or 6 sq mtr)... but let's assume 12 sq mtr...

I'm assuming, and I suspect correctly.... that the figure you quote of 300L/kg lost through transpiration... represents the total loss to fully grow a plant to 1kg... in a soil based system...

So... for the 200kg of lettuce on that table.... transpiration losses should represent 200 x 300L... a total water usage over the plants growth of... 60000L

That table of 200kg of lettuce took about 4 weeks, in summer, to grow from seedlings....

If we assume some evaporative losses, say 10%... from my experience, while the channels certainly heat the water when they are exposed during seedling stage, very little... then we might have another 6000L of water loss through evaporation of the life of the 200kg of lettuce....

Let's then turn to the water replacement required... due to nutrient replenishment...

The nutrient tanks are approx 100L... and I would have replaced the nutrient solution maybe twice per week...over 4 weeks... thats an additional.. say 1000L..

So that's theoretically a total water requirement of 60000 + 6000 + 1000L = 67000L of water for 200kg of lettuce...

I can tell you absolutely... that it didn't require anything like that amount of water... I would have been required to refill the 100L bin 670 times within the month of growing... or 22 times/day....

So the 300L/kg figure for transpiration losses.. while it might be valid for soil based operations... just doesn't appear correct IMO...

I suspect that the figure probably represents the TOTAL water water requirment for 1kg of growth in a soil system... not just for transpiration... but also evaporation and drainage losses...


To replace the supposed transpiration losss for the equivalent number of plants, and sq mtr area... in a soil garden... accepting your figure as transpiration losses alone....

Assuming sprinklers can use as much as 1,000 litres of water per hour ...

Would require that you water your garden 67 times per month... about twice a day, for an hour each time... just to cover transpiration losses..

And that might seem to be about right.... and would tend to support your figure...

But it doesn't, supposedly, account for losses of evaporation or drainage...

And the reality is tht the hydro operation pictured didn't use anywhere near that amount... from my estimation more likely about 10% of that water requirement...

And that represents a portion of straight water losses through physically dumping expended nutrients...

I don't have to... and don't do that in an aquaponics system...the only constant is "transpiration"...

And I really don't believe the figure you quoted represents "transpiration" alone...

My 1000L fish tank... with associated (approx) 6 sq mtr ... doesn't require 670000L of water replacement/month...

It actually probably only requires about 3000L/month... or 100L/day... in hot weather... maybe twice that...