I love rain harvesting.

My rainwater barrel is super cool and a good metaphor for water conservation. But I find it interesting to actually run the math and see how little water they hold, and I wonder if they are taking the limelight away from more important organic gardening practices.

Rainwater Barrel Mathematics 101

If your roof is 1000 square feet (100 square meters) and you get 1 inch of rain on a spring Friday afternoon (don’t worry, it had to get it out of its system before the sunny, warm weekend ahead), you will have 625 gallons of water coming off that roof.

I don’t know about where you live, but where I live, 1 inch of rain is common.

Let’s say you are rain harvesting into a 60 gallon rainwater barrel that will get nice and full and you can use it when organic gardening to water your tomatoes another day. But 90% of that water is going elsewhere, maybe into your city’s sewer system. Your barrel can only take 1/10 inch of rain from that 1000 square foot roof.

So while I like the 60 gallon rainwater barrel, I think there are better solutions.

Rain Harvesting Into Cisterns

A 600 gallon rainwater cistern could handle almost a full inch and a 1500 gallon cistern could easily take 2 inches. Think of it as a giant rainwater barrel. I think every house should have one of these, the size dependant on the amount of rainfall in your area.

They can be a bit tricky to fit into the landscape and some people think they are an eyesore, but they can be cleverly hidden or even buried.

Rain Harvesting Into Ponds

A more attractive organic gardening solution for rain harvesting would be to build a small pond to catch the water. A pond the size of a king size bed and 2 feet deep would hold your inch of water and if constructed thoughtfully, would provide drinking water for insects and animals.

Rainwater barrel
This awesome pond holds way more water than a rainwater barrel

Every organic garden should have some open water in it for this purpose. Mosquitoes aren’t a problem if you keep the water moving and put some effective microorganisms in it, but that’s for another article.

While your rain harvesting into a pond, you might as well bring your greywater into it to be cleaned and used as well, rather than piping it to the sewer or septic system. Your sink, shower and laundry water can all be put to use in the garden. Make sure to use non-toxic soaps, shampoos and detergents.

Rain Harvesting Into Soil

What I think we should mostly be focusing on is the soil in our organic gardens. The soil is the best way to hold onto water and makes rain harvesting into a rainwater barrel look like child’s play (not that there’s anything wrong with child’s play).

Let’s look at how it works using 1 inch of rain as an example. If your roof is 1000 square feet (100 square metres) and you get 1 inch of rain, you will have 625 gallons of water coming off that roof.

A loamy sand, which is 70-85% sand (hence not very good at holding water), can hold 1.1 to 1.2 inches of “plant available water” (plant available water refers to the water that can actually be taken up by plant and microbes and also lost to evaporation; the soil can hold more water than that, but some of it is so tightly held by the soil that it can’t be used).

This loamy sand will already be fairly saturated from our 1 inch rain, though, so it won’t take much of the water from the roof.

A silt loam, which is 75-90% silt, is great at rain harvesting as it can hold the most “plant available water” at 2 to 2.5 inches. It can handle all of the 1 inch rain if you can spread the roof runoff over 1000 square feet. That might be possible with some smart engineering.

A clay soil, which has at least 55% clay, actually holds less plant available water than the silt loam because while it can hold more water, it is held so tightly that plants and microbes can’t get it. It holds 1.2 to 1.5 inches of water, again not much of our roof runoff.

So what to do?

Organic Matter: The Magic Ingredient

There is one thing that we can put in and on our soil that will hold the extra water, and that is organic matter. Organic matter is a rain harvesting genius. I don’t know exactly how much water organic matter can hold, but an article on the USDA website says it can hold 10 to 1000 times more water and nutrients than the same amount of soil minerals.

Even if organic matter holds only 4 times its weight in water, you can hold nearly an extra inch of water if you can increase the organic matter content of your soil by 1.5%, easily doable.

Rain Harvesting
Rain harvesting into the soil is the best option

A study in the Journal of Soil and Water Conservation found that regardless of the type of soil (sandy loam, clay, etc.), “as organic matter increased from 1 to 3%, the available water capacity approximately doubled. When organic matter content increased to 4%, it then accounted for more than 60% of total.”

That means we can now be rain harvesting 2 or 3 inches of rain from the roof, 20-30 times more than your rainwater barrel. This is what organic gardening is all about.

Organic matter can be brought in via compost or mulch. Incorporating 2 inches of good compost into the top 12” of a new garden bed will easily increase the organic matter content by 2-3% (although you might as well go for 6 inches of compost while you’re doing it).

Mulch goes on top of the soil, but it too holds a lot of water.

Now all that’s left is to direct the runoff to your gardens with gutters, and perhaps slight grading of the ground and swales and berms.

Rain Harvesting Summary

The rainwater barrels are a nice idea, but they don’t go very far in rain harvesting from the roof. Cisterns and ponds can hold more and should be used, but the winner in this event is the soil and especially organic matter.

Most soils are low in organic matter and if that’s the case for yours, bringing in lots of compost and mulch will transform your soil into the ultimate storage tank.


  1. Melissa on November 6, 2010 at 11:13 am

    I never considered that soil should be the main water storage solution. Yet another respond to get my compost back in action. Thanks for the info.

  2. Jenny on November 9, 2010 at 10:31 am

    Very interesting. Organic matter seems to be a theme in your articles. Guess I’d better get out and turn my compost pile.

    • Cosmichrist Óforos on October 17, 2014 at 5:29 am

      Better yet: compost in place. No turning, just stacking.

      • Evan on June 20, 2021 at 10:38 pm

        Good way to grow mosquitos big enough to fuck turkeys

  3. Vemvaan on February 9, 2011 at 3:34 am

    Oh I am loving this course – thank you so much! what a clever idea – I just have a very small garden, but have often thought that a water feature would be great – now if I could use it to catch water from the roof that would make it even better! One other thing – I have a big tree which drops lots of leaves and as they are supposed to pull nitrogen from the soil I have been gathering them and mulching into my compost before putting back – is this necessary? at the moment I have a good layer of mulch under the leaves so assume they would not be affecting the soil is that right?

    • Phil on February 10, 2011 at 10:07 pm

      Hi, good question about the leaves. You don’t have to worry about thempulling nitrogen from the soil. In fact, they are the best food for thesoil, just what nature intended. The absolute best thing you can do is leavethem in the garden.Phil

    • Riana on October 9, 2021 at 6:43 am

      That is awsome I have started with swales and berms as we have a steap runoff in our urban garden and all the water lands in my neighbors garden. I have added another 5000 litre tank just to manage the runoff and directed the overflow to to the swales. I am also going to do a rain garden in a area on the lawn where there is still lots of water coming in. Planting the rain is the term I just love.

    • Elban on September 2, 2022 at 7:10 pm

      Leaves left on top of the soil do not sequester nitrogen. If you burried the leaves under ground then they would tie up some nitrogen while they are decomposting.

  4. Franck on March 6, 2011 at 2:21 pm

    Phil,I leave in CA. So we basically get rain from Dec to February if any. So it make sense to have a huge cistern 2500 gallon to get as much as possible during that time, but seeing the price of cistern, even small one, it is a deal breaker. Do you know of any cheap one around or an easy way to build your own? because otherwise, I’ll probably stick with my 4 barrels :(ThanksFranck

    • Phil on March 7, 2011 at 3:52 pm

      Hi Franck,Yes, they can get expensive, which is why it’s nice if you can find usedcisterns. A pond can often be made less expensively, but as you can see,cost is another reason why building up the organic matter in the soil isusually the best option.Phil

  5. Heidi on March 14, 2011 at 10:00 pm

    Could you include a photo or two of the type of plan you are describing where gray water is filtered through ponds in the yard? Maybe an elevation design and a photo of the system as it looks. This will become more important as summer water shortages intensify.Also, where do you get a cistern from? Is there something out there that could be re-used or salvaged to make a resevoir? How do you keep it from going septic? Finally, is there a way to figure in evaporation from said roof that is being rained on if a person lives in an arid northern climate like me? (We get 22 cm of rainfall a year including snow.)

  6. Phil on March 21, 2011 at 1:31 pm

    Your best bet to learn about gray water would be to find a book. The permaculture movement has many interesting approaches. There are many ways to go about putting in a system. The problem is that it’s against the law in most places, although people around the world are fighting these laws and sometimes winning.I don’t have any schematics in digital form. Cisterns can be expensive, which is why I focus on building the soil as being the ultimate solution. You keep it from going septic by using beneficial plants and microbes, as well as aeration and a cool product called Penergetic for water. There are evapotranspiration rates for many areas. Farmwest.com used to have them for BC.

    • Variciuc on May 9, 2011 at 8:32 pm

      Hey Phil, first off let me thank you for your website and all your wonderful knowledge, but quick question, what exactly is that Penergetic product? I’ve done a brief google and everything I’ve found basically distills down to “it activates aerobic microorganisms.” They have various types of penergetic as well, what differs?Thanks,Silv

      • Phil on May 9, 2011 at 9:58 pm

        Hi Silv, it’s a homeopathic product. It’s an energetic thing. Search forhomeopathy to learn a little more what it means. I don’t know a whole lotabout the product other than that I have seen mild benefits from using itand I have been impressed with some of the research. The differences in theproducts are in manufacturing process, so just be sure to buy the rightone(s).

        • Silv on May 9, 2011 at 11:07 pm

          Ah, I see. In other words, placebo.If homeopathy were truly legit, then the water we all drink out of our tap would kill us dead. I mean hey, it’s been in contact with innumerable toxins, viruses, bacterium, etc… The whole “leaving an energetic imprint in water molecules” theory just sounds absurd.Plants and Effective Microorganisms, yes. Homoepathic bunk, not for me.Now I didn’t write this to start a fight or offend anyone. Just giving my two cents, for what they’re worth. Feel free to point to any studies that you found significant and proves otherwise.Silv (B.S. in Biology)

          • Classichomeopathy on June 17, 2011 at 4:59 am

            Uh, now I know why it’s called a B.S. in biology.Try it please before you attack it, or disbelieve it, and certainly before you post about it.If you just look at the facts and actual research where T cells, and weed seed germination have proven the effectiveness of homeopathy, maybe you’d have a bit of education behind those letters.  Not to mention the animal studies, double and tripple blind studies which all show a definite and positive response. The only bunk is your assertion that we’d all be poisoned by extremely small amounts in our water or whatever we’re exposed to.  Look up Ardnt-Schultz law based on the wheat seed germination studies.  It shows the opposite, and stipulates how the extremely small amounts of even a poison stimulate life. I have a doctorate, and a few more letters none of which are BS.  Ron  D.H.M.,CNHP, D.I.Hom.  

          • Variciuc on June 18, 2011 at 5:43 am

            Whoa there, my educated friend. I only gave my -somewhat educated- opinion(what I actually meant by “2 cents”). No need to launch an attack.

          • Farmer N Potter on March 4, 2012 at 1:02 pm

            Wikipedia?  It’s hard to believe you have a Bachelor of Science degree if you are using wikipedia as a source.

          • SecularHumanist on September 17, 2012 at 5:15 pm

             This is called “smiling gardener” – please don’t fall so low as cheap shots.  Instead of attacking variciuc’s sources and his degree, why don’t you post some links of your own?  Be informative, not insulting.

          • Classichomeopathy on June 17, 2011 at 5:07 am

            Uh, now I know why it’s called a B.S. in biology.Try it please before you attack it, or disbelieve it, and certainly before you post about it.If you just look at the facts and actual research where T cells, and weed seed germination have proven the effectiveness of homeopathy, maybe you’d have a bit of education behind those letters. Not to mention the animal studies, double and tripple blind studies which all show a definite and positive response.The only bunk is your assertion that we’d all be poisoned by extremely small amounts in our water or whatever we’re exposed to. Look up Ardnt-Schultz law based on the wheat seed germination studies. It shows the opposite, and stipulates how the extremely small amounts of even a poison stimulate life.I have a doctorate, and a few more letters none of which are BS. Ron D.H.M.,CNHP, D.I.Hom.     

  7. Mdunn80 on November 5, 2011 at 12:34 pm

    I hear you and agree with your position on the importance of adding organic matter to our soil to improve just about everything with it including water holding capacity. But I love my rain barrels. I use them all the time. Like I said, I agree with soil management and adding organic matter to boost soil’s water retention abilities thus turning our soil into rain barrels as you call it. But rain barrels are great for those extended dry spells when plants may just need a drink to get through the next hot day when even the most organic soil and mulches are just working too hard. I also use a rain barrel to make compost tea. Fill the barrel 1/4 with compost and let the rain pour in. Let it sit and voila, compost tea. I also use my rain barrels to water my potted plants on my porch and in my home. I love my rain barrels. So for those of us who don’t have a pond or a cistern can a rain barrel be the next best thing?

    • Phil on November 7, 2011 at 1:20 pm

      For sure, there’s nothing wrong with rain barrels at all. In the previous article to this one, I just showed that they don’t make a dent in the rain that actually falls on most houses, and so cisterns, ponds and soil do a much better job. But barrels are a great start and certainly useful.

  8. Wmhouston on November 25, 2011 at 4:24 pm

    Phil,I’m curious to see what you think about collecting the water which comes from air conditioning condensate drains. I channel mine towards one of my flower beds, but is it safe to use for edible plants?Bill

    • Phil on November 25, 2011 at 9:46 pm

      Great question. I would tend to be on the safe side and test it for heavy metals like lead. I wouldn’t worry about any biology issues, but the heavy metals can accumulate in the soil. Same goes for rain harvesting.

      • Heather on April 13, 2012 at 4:47 pm

        Do you recommend any certain, reliable water test? I’ve wondered about that as it relates to testing tap water, and now this makes me more curious if there is a suitable home test to use for these various applications or what the best method is.

        • Phil on April 14, 2012 at 3:36 pm

          Where I live, I can send water into my municipality for free and they test it. That’s only for tap water, though. To test other sources, I have to pay a lab. If I recall correctly, it’s $100-$200 for me. I’m not really familiar with the water testing process though.

  9. Mrpappyg on February 29, 2012 at 5:50 am

    Hi Again Phil. I know a newbie, LOL, I also have 55 gal drums on every downspout on my 5 bedroom Colonial, it’s a pretty big House, and over the last four Years of organic Gardening they have sustained my garden’s well, I do also have a 2400 Gal Koi Pond that I built and in times of severe drought I would definately use, but I also constructed 4 French Drain’s in all four Garden’s and after the Deluge of last year in New Jersey if I hadn’t I would have lost everything.                                                                                          Thanks again Phil.

    • Phil on March 1, 2012 at 6:00 pm

      Ya, they definitely can be helpful. I mostly just wanted to show people that we need to learn to harvest all of the water from every rain, not just a tiny bit.

  10. Mcs818 on March 4, 2012 at 1:03 pm

    Thanks, Phil.  This is mostly reinforcement for me with some new info added in for wonderful fodder for new research for me. 

  11. Ruminantia on March 19, 2012 at 3:09 pm

    I am anxious to hear about using EM in ponds (and rain barrels?) to control mosquitos as this has been the only drawback I have found.   I do have two rainbarrels which I use to water container plants.  Have tried mosquito fish, but the pet store variety gold fish have better survivability in the barrels.  I am interested in hearing how the EM controls the mosquito larvae.  I am working on improving the life of my soil also…nice to know there is yet another reason for doing so. 

    • Phil on March 20, 2012 at 1:40 pm

      I don’t have that research with me (I will in 6 weeks), but EM applications have decreased mosquitoes, I think by discouraging the laying of eggs. But I think in some of the research they also fermented certain plants with pest control properties to make kind of a homemade, safe insecticide. I have to look into that again. Keeping the surface water moving is also very important.

  12. Nanegoat67 on April 7, 2012 at 8:20 pm

    Another way to keep those pesky mosquito larvae out of your water is to add a goldfish or two.  You don’t have to have expensive koi in your pond, just a few ten cent “feeder” fish will do.

    • Phil on April 8, 2012 at 3:09 pm

      Thanks, that’s a good idea, too.

      • Merryj on September 14, 2012 at 1:45 pm

        I wonder if tilapia would do that job, too?  Then, when the population gets too high for your little pond, you can eat the extras 🙂

    • David_R59 on October 9, 2014 at 2:21 am

      To keep mosquitos from reproducing in your rain barrels simply put a few drops of dish washing liquid in it every seven days.This changes the surface tension of the water so that insects cannot stand on the surface to lay eggs. If a mosquito does somehow manage to lay an egg and it hatches then the nymph cannot attach itself to the surface in order to emerge as an adult.You do it every seven days because the lifecycle of a mosquito in the water is 7-10 days.Remember, if you barrels are open, to float a piece of 2X6X1ft wood in it so that birds can get a drink if they need it and they won’t fall in and drown.

  13. Guillaume on April 11, 2012 at 9:59 am

    Thanks for your article on harvesting water Phil. I have 7 water butts (UK for water barrels) taking water from almost all my roofs. The overflow from each barrel goes down into hardcore cisterns under each of the flower beds. The hardcore is covered with ‘weed-stop membrane’ to stop the soil seeping in. Using the hardcore was a very cheap way of making underground cisterns; I didn’t even have to pay to have the hardcore taken away. With about 2 feet of good soil above the membrane, the flowers should do well.In my garden of about 1/3 acre I have dug-in about 20 tons of well-rotted horse manure that comes from my previous home where I kept horses. The originally sandy soil is now a nice loam that I can dig immediately after a rainstorm with no problems. What with green manure in the form of peas, beans, and Hungarian grazing rye plus all the weeds, garden compost, and leaves collected from a nearby footpath going through 7 compost bins, I reckon I have just about fed the soil with organic matter. (Can you have too much organic matter in the soil? I have not seen a mention of this as a problem)In addition, I have a 30 foot well that normally provides an ample 10 foot of water; but this year the ground water level has dropped so much that the pump switches itself off as a safety measure. The 7 water butts will store ample water until the well gets enough water soaked into it again.I didn’t think I could do better than an underground cistern in the form of the well (It collects water from miles around), but this year has sprung a surprise on me. Guillaume

    • Phil on April 12, 2012 at 12:45 am

      Wow Guillaume, sounds like an excellent system you have there. To answer your manure question, yes, you can definitely add too much. It can throw off your soil nutrient ratios, especially bringing in too much potassium and phosphorus, sodium, even nitrogen. That’s more an issue with fresh manure than composted, but even compost can cause some issues. Plus it can get compacted and go anaerobic if you add too much, but if you incorporated it into your sand, that shouldn’t be a problem. So it’s probably mostly about nutrient ratios for you. You may want to test your soil. I teach soil nutrient testing and fertilizing in month 2 of the Academy. I’d love to see some photos from you this year if you get around to it!

  14. Trish Eilers on July 13, 2012 at 8:17 pm

    Thanks for putting all this info together! It could be overwhelming, but you explain it very well, where I’m not to that point yet…lol.I’m a newbe at this, so please bare with me and my “newbe” questions :0)When you say plant cover…do you mean cover the soil and plants with mulch or just the soil and have the plants poke thru to absorb the sunlight??? I would think you would mean the latter but want to make sure I’m understanding correctly.

    • Phil on July 15, 2012 at 3:23 pm

      Hi Trish. What I was saying was to try to have it so that plants are covering all the soil in your garden. We don’t want a lot of bare patches. That’s where groundcovers and cover crops come in, to make sure the soil is always covered. And then we want mulch covering the whole soil, too, underneath the plants.

  15. 1505 on March 28, 2013 at 1:49 am

    Looking to store lots of water for a fair cost look at rainwater harvesting at http://www.aquascapeinc.com/

  16. Cait on April 24, 2013 at 3:31 pm

    I am so glad you talked about this because I just read a similar article in Mother Earth News (a past issue) and wondered what you and others would think. Thank you! We have a couple rainbarrels and are only renting, but I’m interested to try something more like this if we can.

  17. april on June 10, 2013 at 4:49 pm

    Hey Phil,I apologize,if this is a repeat question. I have 2 rain barrels, they are currently full, how can I keep them from growing, mold, stinking, and incubating mosquitoe?Thanks

    • Phil on June 12, 2013 at 4:34 pm

      One thing would be to have various screens/filters to stop debris from getting in there. Also, buying some ‘effective microorganisms’ and mixing it in there at a 1:10,000 ratio could help control smells and maybe even mosquitoes. Mostly, it should be covered to keep insects out.

    • David_R59 on October 9, 2014 at 2:24 am

      To keep mosquitos from reproducing in your rain barrels simply put a few drops of dish washing liquid in it every seven days.This changes the surface tension of the water so that insects cannot stand on the surface to lay eggs. If a mosquito does somehow manage to lay an egg and it hatches then the nymph cannot attach itself to the surface in order to emerge as an adult.You do it every seven days because the lifecycle of a mosquito in the water is 7-10 days.Remember, if you barrels are open, to float a piece of 2X6X1ft wood in it so that birds can get a drink if they need it and they won’t fall in and drown. see more 0

  18. thoomfoote on September 2, 2013 at 4:58 pm

    Yes cisterns are nice and big and building up the organic matter is a goal. However, do not downplay barrels. They can be located to catch small amounts like from a chicken coop or well house and used at that location. IPC food grade totes, in the 275 gal size make great catchments and overflow can be directed into ponds or barrels or rain gardens. In addition, they are cheap ($125 each). Cisterns cost MONEY which as a retiree, I do not have a lot of. I go for diverse catchment approaches that are low cost. I do not have $5k for a 5k gal cistern. Build small ponds with a tractor or by shovel and put numerous barrels with overflows around your land. Put gutters EVERYWHERE you can.

    • Phil on September 5, 2013 at 10:03 pm

      All great advice – thanks for that. And certainly a 275 gallon size is much better than a 60 gallon, too.

  19. Nancy on June 20, 2014 at 5:10 pm

    I live in a small town on the Front Range of Colorado (right at the foothills of the Rocky Mountains and, as weird as this sounds, it’s actually illegal to even have a rain barrel or use one’s greywater. The state apparently owns all water that falls from the sky (!) and the water rights issues here are intense.

    • Michelle on December 28, 2018 at 8:37 am

      Surely digging grit and gravel into your soil for drainage is acceptable? As one commenter above suggested, French drains with hardcore/large stones covered by gravel and then a weed suppressant membrane can’t be seen once you replace the soil and add plants. You’re not taking the rain as such, just slowing it down which helps to prevent flash flooding. Hard work in the short term but a good investment of time and effort. Gravel is fairly cheap too.

  20. Bob V on December 8, 2014 at 5:00 pm

    Hi, I spend a lot of time trying to add micro-organisms to my soil and therefore i worry about using chlorinated water from the hose for my garden. I live on a lake and it is pretty healthy with fish and fowl and some gators. I have a sprinkler system that pumps from the lake for the lawn. Would it be OK to use this water in the garden too ? any worries ?

    • Phil on December 12, 2014 at 3:04 pm

      Good question. Lakes are often polluted, the question is how much. I would tend to lean towards lake water over chlorinated water, but I suppose it really depends on what is in that lake vs what is still in that tap water.

      • Bob V on December 12, 2014 at 3:41 pm

        yea, the lake seems to be healthy and not too many houses around the lake to contaminate the water with lawn fertilizers. The fish are really healthy and the gators seem to be too. We have a lot of water fowl too like ducks. Like I said, we’ve been sprinkling the lawn from the lake for years and it grows well too. I pointed my sprinkler heads towards the garden and it’s already on a timer so it’s a easy way to periodically water the vegetables. Don;t want to kill that fungi and bacteria

  21. Maria on January 12, 2015 at 2:23 pm

    I am following your lessons, But is it possible for you to give a lesson on harvesting your own indigenous Mycorrhiza

    • Phil on January 12, 2015 at 6:03 pm

      I actually don’t know much about it. I’ve looked into propagating it, but it’s very involved. Other than taking a few shovelfuls from a natural area to use in my garden, I’ve never gotten into the technical details of harvesting it properly. If I ever do, I’ll definitely write about it.

  22. rtj1211 on April 18, 2015 at 7:10 am

    I came to these conclusions too a couple of years ago. However, if you don’t have a greenhouse (I don’t) and your climate isn’t reliable for growing tomatoes in the soil (mine isn’t), you do need something to store water for your tomatoes grown in pots. We have 77 gallons worth of water collection butts and this April, it’s been so dry that I’ve not only been watering tomatoes with it (not much as they are still in smaller pots), but also the potatoes, carrots and parsnips I sowed in early April which haven’t had a drop of rain since and won’t for at least another 5 days. I’ve also had to water the peas and beetroot once as they were wilting due to the drought.However, my over-wintered cabbages which benefitted from being planted the end of last September into freshly laid compost on top of a bed undug for the whole of the 2014 season, are still doing just fine without rain. Every no-dig bed I plant into has plenty of moisture below the surface even though the very top of this clay soil is now as dry as a bone.So my take is that you could do with some butts/cistern capacity for plants you grow in pots, balconnieres etc etc but diverting rainwater into your garden is the way to go for everything planted the natural way.

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