Harvest Time

Water harvesting, that is. We are often urged to try and minimise our use of mains water, with the added incentive that many of us are on a metered supply and stand to save money if we do. Save the world or save some money; it sounds good but I don’t have high hopes for either.

I have spent much of the past week making alterations to my rainfall storage arrangements and thinking there may be others out there considering rainfall harvesting or changes to their existing setup, I am going to describe the somewhat complicated installation I have created for myself.

In hotter countries than Britain, water harvesting is routine. Here, we rarely get long hot dry summers so setting up expensive and elaborate water storage makes little sense, just draw what is needed from the mains.

That at least has been the position but I am inclined to think that things are changing at an accelerating pace. In those same hot climates gardeners know what to expect and in the main will plant things that are capable of withstanding their conditions without excessive irrigation. Nevertheless, prolonged hot and dry spells are routine and unless one is willing to accept the considerable restrictions that such conditions impose on what can be grown, irrigation will be needed and water off the roof is free, even if the tank and pipework is not. We have done much the same here, planting for our conditions, which because they are less extreme means we can grow a very wide range of temperate plants without in general having to irrigate them.

Part of the problem is the level of uncertainty about what climate change will mean in a particular area. How will the annual rainfall pattern change, how much hotter will it be and so on.

The Met Office collects climate data from numerous weather stations around the country, the nearest of which to me here is at Cardinham. That’s only 8 miles away but at 200m altitude it is about 70m higher than here, which will mean it probably has significantly higher rainfall. The annual average rainfall 1981-2010 there was 1385.2mm.

I think it is safe to say that the average rainfall for here is around 1250mm pa. That means that  1250 litres fall on every square metre each year. My garden is around 500m², that’s an impressive 625,000 litres of water falling on the property each year. Digressing, at South West Water prices that’s worth £1156.25 or £3125.00 if you include the sewerage charge.

Taking a pro-rata proportion of the Cardinham figures for each month, I get the following, figures in mm:

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total
133.6 99.4 93.8 81 78.5 73.3 87.4 86.6 90.3 140.6 145.4 140 1249.9

I found an interesting article online which shows that annual rainfall for Cornwall has increased by 22% since 1940. That is then broken down to summer and winter, with winter rainfall up by 35% and summer rainfall by just 8%. For England in general there has been a much smaller annual increase with no difference between summer and winter.

For a long time I have collected rainwater off part of the house roof. We had an extension built and unfortunately it has a separate gutter and downpipe from the main roof. Nevertheless the rear half of it is an area of about 12.6. Via a rain diverter the rainfall is fed into a 1500 litre tank. I have recently set up a water butt which collects the water from the two glasshouses behind the house, a total area of 9.72. Here are the monthly yields for each, in litres.

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total
Roof 1683 1252 1182 1021 989 924 1101 1091 1138 1772 1832 1764 15749
Ghse 1299 966 912 787 763 712 850 842 878 1367 1413 1361 12150
Total 2982 2218 2094 1808 1752 1636 1951 1933 2016 3139 3245 3125 27899

In practice, the winter months can be disregarded because I’m watering very little and certainly using far less water than is coming in. In the summer months I’m not sure. Between April and September I may be using more than is coming in. If I start with all my storage full, I can afford to exceed what is coming in by as much as I have stored. Thus if I have 3000 litres storage capacity and in those six months receive 11096 litres as rainfall, I can use 14096 litres over the period.

It’s important to note that the stored water is only 21.3% of the total, which places a heavy reliance on the summer rainfall being fairly evenly spread if total water usage is anywhere close to the total available figure. It is also of course true that the less the summer rainfall, the greater the need for irrigation.

If I use the available water evenly over the period, I have 1849 litres a month available. If we get a six week dry spell, I use 2774 litres from my total storage of 3000 litres, assuming that it is full at the beginning of the period. If the dry spell comes well into the summer that is an unlikely scenario, if my stored water is down to 50% I would have 1500 litres and be wanting 2774.

Increasing storage capacity comes down to how much I am prepared to spend on storage and where I put it. I don’t want to give over garden space to a visually intrusive huge water tank. Putting in an underground tank would require machine access to the back of the house which we don’t have, would be massively disruptive and would be expensive.
Collecting more of the rainfall that falls on the roof is possible. I have a 100 litre water butt supplied by the gutter on the back of the main house roof and I pump it into my 1500 litre tank when there is space available. In a prolonged spell of rain I may empty it two or three times. I use a small pump that runs off an electric drill, far from ideal when it’s raining. I have a more suitable submersible pump on order. The main part of the roof is twice as wide as the extension, about 25, more than the area directly plumbed into my tanks.

I’m putting another similar water butt out the front to collect the rainfall from the front of the main roof. The main roof has an area of around 25. for each pitch, so 4mm of rainfall will fill a 100 litre tank, or two if I had one back and front. Over the summer six months it would yield me a further 12428 litres but given that this would be in 100 litre increments, it would need pumping out 124 times over six months, unless the tanks were already full.

What I have done this week is to install a row of six 210 litre water butts along the fence beside the house. I have connected them together and will fill them from the overflow of the butt that collects water from the two glasshouses. The seven water butts have a total capacity of 1460 litres and as far as I could determine, were the cheapest option available to me to get that volume of storage. Cheaper barrels and water butts are available but transport costs are sky high because of their bulk.


It also meant that I could install a wooden shelf along the top of them on which I will display all our potted fuchsias in summer; in other words, their visual impact is acceptable and the space they take up is being used effectively. The pipe coming in from the right is the overflow from the butt behind my new glasshouse.

I have kept the plumbing as simple as possible, linking them together via the threaded hole where the tap currently is with 20mm ldpe pipe and barbed fittings, thus not cutting any more holes. That way, if I want to change things later, I have six intact water butts to re-use or sell. The pipe and pipe fittings came from Easy-Irrigation.

The new glasshouse with a green water-butt behind it and to the right of that a black 1500 litre water tank, a second hand fruit juice container. I’ve squeezed the step-ladders in between.

My 1500L tank has in it a submersible pump, a Dab Divertron 1000 which I bought last year from Celtic Sustainables for £284.80. I see it has gone down in price and is now £270.00. It is claimed to pump 90 litres per minute and have a maximum head of 36 metres. By the time the output has gone 25 metres in a 20mm water pipe, round a couple of bends, through a tap, 25 metres through a half inch hose to a multi-nozzle spray lance I’m getting about 7.5 litres a minute. Closer to the tank I can get up to around 12 litres a minute. If it pumped into a 1 inch pipe, the size of its outlet, rather than a half inch hose, it would be considerably higher. To take a compromise figure of 10 litres a minute, from full to empty on my total 3000 litres storage will take me 5 hours and my hypothetical maximum available for summer of 14096 litres will keep me going for a full 24 hours, provided I take half hour for lunch.

It’s worth mentioning that we are on a meter for water, with the unit charge being £1.85 for water and £3.15 for sewerage, per cubic metre (1000 litres), plus standing charges of £37.34 and £20.20 respectively. In the six months to 7 Oct 2017 we used 36m³, to 16 Apr 2018 we used 27m³, to 12 Apr 2019 34.

The 14096 litres I may be collecting currently is thus worth £70.48 and each pitch of the main roof (12428 litres) is potentially worth £62.14.

13 thoughts on “Harvest Time

  1. Your set up looks like it will be quite efficient. I have a few rain barrels (aka ‘water butts’) but will be getting more since our summer dry spells are getting longer and tomatoes won’t grow on sunshine alone!


    1. Our rainfall has historically been fairly evenly spread through the year but I think that’s changing and that the pace of change is accelerating. It makes it difficult to know what you’re planning for.


  2. I have a 1500 litre tank buried under the patio (there was room next to the bodies!) But haven’t used it for a couple of years as the pump failed. When it was working it was great, I think it was a 3 bar pump, pressure activated so it came on when the tap was turned on. I should replace the pump. Fundamentally though, the economics of it arent that attractive, water is cheap (for now, anyway…). your point about the very time you need it is the driest period when there is no rain to replenish is a good one I think. I used to have several water butts but got rid when the tank was installed. Nearly my whole roof, front, side and back feeds the tank, the gutters are all connected, so it fills up quickly when it does rain. I should sort it out , I suppose.


  3. I see that you get a lot of rain at your home. I have an average of 750 mm of rain a year in recent years (recorded thanks to my rain gauge that I check every week for 10 years …. one day I will make a post about it too) The harvest of rainwater has become indispensable.


  4. That is some input in terms of facts and figures! I shall have do some adding up here. I use a hozelock submersible pump to pump from my roof filled water buts to a water tank at the other end of the garden. I think that holds 1000l. This year I had it faced in wood slats to hide the ugliness of it. I have 8 water butts dotted around the garden and would love to have them all lined up with a shelf over the top. There is usually a spell in late August when the hose has to come out but I’m pleased with what I can harvest.

    Liked by 1 person

    1. The figures reflect my inner nerd. I think the important underlying point is to maximise the amount you can collect, so as to replenish the storage efficiently, rather than have massive storage with a small input. That might work better in a different climate from ours, with highly seasonal rainfall.

      Liked by 2 people

  5. In our chaparral region, you would think that rain harvesting would make sense. However, because our rain happens within such a short time, the collected water does not last long. I mean, we get plenty of rain through winter, but then use all the stored water before summer. Without rain until autumn, the water does not get replenished. For those who live in town, where space is limited, big empty tanks are not exactly desirable.


    1. Rain harvesting is great in theory, in practice it falls down because of the volumes involved. Most nurseries here would harvest rainwater from their glasshouses or tunnels but it is stored in open reservoirs, a much cheaper option for large volumes than tanks.

      Liked by 1 person

      1. I’ve read and heard much the same here, though I don’t have a clue how much the law actually requires. Mainly to reduce run-off into already overloaded drainage systems in urban areas. Harvesting would work as well.

        Liked by 1 person

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