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Estimate Water use in Aquaponic Systems in 5 Easy Steps

Water is essential for growing pretty much anything.  Aquaponic systems and aquaculture systems are no different.  It is the be all and end all of food production.  If you don’t have enough water, your plants want grow and your fish will not be happy with you.  So how can you estimate how much water you will need for your aquaponic system?  First we need to understand how water is used in your food growing machine! Plants, Aquaponic Systems and Water Plant transpire, we know that.  Just like us when they are hot they sweat through groovy little pores called stoma.  Give some thought how you feel when it is hot, you sweat and when it is hot and humid you sweat more.  Its not all about temperature but more importantly the humidity and temperature combined. When the weather is cool, the humidity requirement of the plants is much lower, lets say 24c (about 50%) but as the temperature increases, so does the humidity requirement.  For example at 35c your plants will need the humidity to be well above 70%. If however, the temperature is high and the humidity low, the little pores (stoma) will close.  When the stoma are closed your plants are not moving those nutrients through them and their growth is reduced.  If this is severe, where temperatures are high and the air is dry, your plants will start to wilt, this is a sign of heat stress and where the plants pores are not open… and not transpiring. So the way plants take up nutrient, in its basic form is they sweat.  They transport the nutrients dissolved in water and vapor comes out of the stoma leaving behind the wonderful growing goodness, the nutrients they need.  In aquaponic systems this transpiration is where majority of the water is lost to the atmosphere and a great place to start calculating how much water you will need. Sunlight energy plays an important part in transpiration of plants so without going into leaf mass, vapor pressure differential and all the other things to get a dead set calculation, I want to share with you a few simple steps to estimating your  water use in your aquaponic system. You will need to find your local terrestrial solar exposure.  This will generally be in Mega Joules per square meter and will be found on most state or country weather site (for example Australia would be like this) Convert that number to Joules per square centimeter. Just add 2 zeros.  For example Alice Springs in January the solar exposure is 28mj/m2 so we just change that to 2800.  Simple. Work out how many square metres you have in your plant growing area. Define your transpiration rate.  This is the very flexible number somewhere between 0.001 and 0.003 litres. For mainly greens you would use the lower number and larger plants like tomatoes you would use the larger number (eg 0.003).  If you have a combination of both you can work out the square metres for each or just use an average of 0.002. Then apply those figures to this simple formula.  Transpiration rate x Solar exposure x 30.5 (days in a month) x square metres of planted space. Your done.  The result will be the estimated volume of water used over that period in litres. Example using Alice springs in January with 10 m2 of some leafy greens: 0.001 x 2800 x 30.5 x 10 = 854 liters of water for that month without accounting for evaporation and other “leaks” in the system. This very same calculation can be used to work out your irrigation cycles for NFT and towers.  Simply work it out for the days water needs (dont multiply by 30.5) and divide that by 24 or the hours of sunlight in a day. Using the same example 0.001 x 2800 x 10 (m2 of nft or verticals) = 85.4 liters for the day.  Divide that by 24 will give you 3.6 liters per hour over that 10 square meters.  You may only want the irrigation cycles happening during the day though, not the night.  We may switch this into a calculator later. Enjoy Paul…

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