Data-driven allotment: GRI 13.7 - Water and effluents

Introduction

This is one of a series of posts about one of my favourite topics - my allotment! And more importantly, how green it really is, using a selection of topics from GRI Standard 13: Agriculture, Aquaculture and Fishing Sectors.

I am a big believer in learning by doing, so when the Global Reporting Initiative released its standard for the agriculture, aquaculture and fishing sectors in 2022, I thought the best way of getting to grips with it was to try to apply it to my allotment. As I’m sure you can imagine if you’ve ever reported under the GRI standards, this was a challenging task. Firstly, quite a few of the topics simply aren’t applicable (or ‘not material’ as the Standard itself puts it), For example, I don’t keep livestock on the allotment so I don’t have anything to say about 13.11 - Animal health and welfare. Others I can address, but only indirectly, so whilst I don’t use pesticides, I can think about 13.6 - Pesticides use through the lens of ‘organic’ alternatives to conventional pesticides. You can find a complete list of the standard’s topics and which ones I have decided are immaterial or of limited applicability here.

Perhaps unsurprisingly, given my interest in horticulture and food systems, I am also interested in the ‘business’ side of the allotment - i.e. doing a very basic cost/benefit analysis. As I worked through the topics, it also became clear that this information was relevant to topics like 13.3 - Food security and 13.21 - Living income and living wage. Ultimately, I’ve ended up recording both environmental and financial data, some gathered myself and some from reliable sources like the Met Office and the Soil Association. The different types and sources of data are summarised here.

Whilst I have made every effort to be as thorough as I can be about this process, it’s important to remember that this is just an intellectual exercise for me, and I make no claim to the sort of scientific rigour that large food-producing businesses have to put into their reporting.

On to the interesting bit…

This month’s focus is GRI 13.7 Water and effluents.

Apart from Disclosure 3-3 - Management of material topics, there are five standard disclosures under GRI 13.7: Interactions with water as a shared resource, Management of water discharge-related aspects, Water withdrawal, Water discharge and Water consumption. Of these, 303-5 Water consumption is the only one of direct relevance to my allotment. Allotments are shared spaces, so you could perhaps define the water present on them as a ‘shared resource’, to the extent that there are some limited facilities to collect rainwater, though this would never be enough for the entire site. However, most of the water is provided via mains installed by the council that manages the site, who are therefore in control of the water withdrawal side. Water is ‘discharged’ to an extent on the site, and my particular site is close to a river branch, so run-off is a potential issue, but given that the watering is done by hand, it does not happen in volumes that could reasonably be considered material.

So that just leaves water consumption…

According to my records, I used a total of 1136 l of water over the main growing season (February-October). There are 12 main beds of 1 m x 3 m, plus approx. 7,5 m2 of growing space inside a small polytunnel and approx, 2 m2 where the previous occupant had planted some rhubarb plants, which I decided to keep. That makes a total of 45.5 m2 of growing space. This is only an approximation of course, but it means that I consumed roughly 25 litres of water per square metre of growing space.

This is complemented by the natural rainfall (except in the polytunnel), of which there was 642.17 mm in February-October according to Met Office daily records. This, incidentally, is significantly higher than the 1991-2020 average recorded by the nearest Met Office weather station at Writtle (Feb-Oct: 415.28 mm) and the average for the East Anglia region as a whole (Feb-Oct: 452.09 mm). It is worth noting here that the East Anglia region is usually regarded as one of the driest in the country, with an average annual rainfall over 1991-2020 of 626.91 mm compared to the national average for the same period of 1162.93 mm, and the situation is even more challenging in my area specifically, with the closest weather station recording an annual average rainfall of just 586.51 over 1991-2020. All of which means the way I use water on the allotment is clearly important. This data comes from the Met Office’s climate averages.

Over February to October, my records indicate that my allotment produced just over 33.5 kg. That equates to approx. 34 l litres of crops per kilogram of produce. Is that good? This turns out to be a seriously complicated question to answer. A 2013 report on food waste by the Institute of Mechanical Engineers cites a then-recent study by the Water Footprint Network (note 37). Unfortunately it does not seem to name a particular publication and the link provided no longer works. The most recent relevant publication I can see on the Water Footprint Network’s list of publications is called The Water Footprint of Global Food Production and was published in 2020. However, its data on water-use compiles data on crop and livestock production from two other sources. The source for crop production is entitled The green, blue and grey water footprint of crops and derived crop products and dates to 2011, so it’s possible that this is the original source for the IME report. It certainly shares the same findings, which include that producing one kilogram of potatoes consumes a staggering 287 l of water, one kilogram of cabbage 237 l and one kilogram of tomato 214 l (note that includes processing, e.g. washing, of crops, which I don’t record). Of the crops on the IME list, these are the three that I grow. There are actually others in the original list, but they’re generally either presented in dry form (e.g. beans) so aren’t comparable or represent a relatively small proportion of what I produced this year (e.g. peas and carrots), so I have left them out for the sake of simplicity. However, I will integrate this data into my tracking spreadsheet for 2024.

So, with the above caveat, let’s assume that those were all I grew and that I grew them in strictly equal proportions. These three crops would theoretically require an average 246 l of water per kilogram of produce. My 34 l/kg seems paltry by comparison, given that producing 33.5 kg of potatoes, cabbage and tomatoes would theoretically consume 8733 l of water compared to my 1136 l. However, this is – and I apologise in advance for the pun – comparing apples and oranges, not least because of the scales involved. There’s no way I could, by hand, apply that amount of water, even if I wanted to. But then, my agricultural methods, insofar as I really have ‘methods’, are not comparable with the kind of industrial farming methods that this study primarily covers. If there exists a detailed study of how much water is used in small-scale or subsistence-level agriculture, I have not been able to find it. If you know of one, please do drop me a line! It’s also important to note that the water usage in this study is based on a model-estimation process using various assumptions, explained in detail in the original study.

All of this aside, the total figure of 1136 l did catch me slightly by surprise, and it seems like something I should be able to cut down on, So I have been thinking, and I’ve come up with three potential improvements.

Increased organic content to improve water retention

This is quite a fancy way of saying ‘mulch more’. Like most of my space, the basic unimproved soil on my allotment has quite a high clay content, probably not helped by how close the plot is to a river, the banks of which are completely unimproved. Though it’s never flooded in all the time I’ve been there, I imagine there have been deposits of clay over the years. After several years of cultivation before I arrived, I wouldn’t call it ‘heavy clay’ any more, but the ‘clay’ part would probably still be fair. I didn’t improve the soil in any way when I took over the plot - perhaps I should have.

Soil moisture is one of the things that I did try to measure over the year, but I suspect from the data that my instruments may have failed me. In a bid to avoid having to keep replacing the battery, I used an analogue instrument measuring on a scale of 1-10, which I think might have lacked the required level of sensitivity. And given the results, I’m not entirely convinced of its accuracy, but I will include what I did record for the sake of transparency:

Blue bars: rainfall (mm)
Orange line: soil moisture inside polytunnel (1-10)
Grey line: soil moisture outside (1-10)

A couple of things stick out here:

  • The soil moisture inside the polytunnel (orange line) went essentially unchanged throughout the year. This isn’t particularly surprising, since it conceivably wouldn’t be heavily affected by the actual rainfall, and the amount of water I added manually wouldn’t have changed the consistency significantly,

  • There was a peak in soil moisture in mid-April that doesn’t seem explicable by the actual rainfall, but then it went almost unchanged for the rest of the year, no matter how much rain fell (or didn’t - look at mid-May). This, in particular, leads me to think that the measurements were subject to either user error (perfectly possible!) or instrument failure. Either way, I think it might be worth investing in a digital monitor next year, despite the inconvenience of keeping it powered.

I can report anecdotally that the soil in its current state does not retain moisture particularly well, and its composition makes it prone to cracking in dry weather or getting waterlogged in heavy rain. Increasing the organic matter level should hopefully help with both of these issues. To keep the cost of mulching so much growing space low, and frankly because I am not inclined to cart vast amounts of manure or compost down to the site and spread it, I’ve chosen the green manure method. Specifically, I have sown any ground not in use by the end of October (i.e. most of it, excluding some of the hardy brassica beds) with field beans (Vicia faba). They will stay in place all through the winter before being cut down in the spring as and when I need the ground for actual crops. At time of writing, they are well established and, in addition to covering the ground to prevent soil erosion from rain over the winter, should hopefully be fixing nitrogen in the soil at the same time. It cost approximately £15.00 to buy enough to cover all of the plot that I measure and also quite a lot of my other ‘main’ plot, which I don’t measure as scrupulously - in other words, significantly less than the cost of buying enough manure to cover the same space.


Underwater direct-to-root irrigation - ollas

This is something I have been thinking about for quite a while, ever since I first heard of them. In fact, it’s one of those ideas that seem so obvious once you’ve heard it that you can’t believe you didn’t come up with it yourself. An olla is essentially an unglazed terracotta vase with a very long spout that you bury in the ground with only the very top sticking out. The fact that the terracotta is unglazed is important, as that means it remains water-permeable. As a result, when you fill it up with water, the moisture is slowly wicked out into the soil around it by the power of osmosis, faster during dry periods as the soil dries out and more slowly when the soil is moist.

Ollas have several advantages over conventional watering practices. Firstly, the water that goes into them is released into the soil at a much lower level, protecting it against waste through surface run-off and, to an extent, against the evaporation that happens at shallower soil depths, especially in hot spells. This should mean greater water efficiency, as comparatively less water will need to be added. Plant roots should also theoretically be drawn towards them, so you don’t have to water the ground around the crop plants, hopefully inhibiting the germination of any weed seeds in the soil. They also act as a slow-release mechanism, which is a huge advantage to me as someone who is not able to water every single day. Some plants, like tomatoes, require consistent watering when cropping to avoid growing at uneven rates and suffering from phenomena like tomato cracking or splitting.

There are downsides, however. For one thing, I’ve not really been able to find any reliable information about what their effective range is. I suspect a complication here is that the soil texture will strongly impact how far the water can permeate – this might be something worth measuring. Secondly, they’re a relatively labour-intensive practice. They have to be buried in the soil in the first place, then dug up in the winter as frost may damage them. This is not only time-consuming, but also counter to my generally ‘low-dig’ approach, so it’s definitely a limiting factor on how many I can really use. I also imagine they’re of limited use for root crops like potatoes and carrots. I can imagine that rapidly growing potato tubers could damage them, and even if they didn’t, any potato that came into contact with one would run the risk of essentially sitting in water - a recipe for rot and blight. And it’s difficult enough in my heavy, stony soil to get carrots to grow straight, so putting in a water source that might attract the roots probably wouldn’t help. Finally, they’re a relatively large investment for a hobby - at about £15 for a 2 l olla and an uncertain range of effect, it’s difficult to know how many to buy. For the first year of use, I intend to buy enough to support plants that need consistent watering (e.g. tomatoes) and ‘cluster’ crops, i.e. climbing plants grown up bamboo support structures with a relatively small soil footprint, like cucumbers and climbing beans.

Once I’ve seen how well they work, I can consider expanding their use, though I will need to take the other limiting factors into account.

Watering patterns

This is a bit of a nebulous one, but should hopefully be aided in part by the use of ollas for certain plants. The Royal Horticultural Society (RHS) also recommends watering in the morning in order to make the best use of water and avoid waste. I did tend over the past year to water in the evening when it was coolest, so this is a good tip. It should also mean a nice little walk to the day, which I’m sure will be good for my wellbeing too!

Generally though I will try to be more conscientious about my watering. For example, I will look to focus my water use on plants that are still getting established rather than more mature plants, which should have deeper root systems to access water in the ground. I did notice that I got very poor germination from my carrot seeds last year, and I suspect that this was because I just didn’t keep them moist enough whilst they were getting established. Focusing more on new plants should hopefully help me increase the efficiency of my watering.

How exactly I could measure this is unclear - though I love data, I’m not sure I would have the time or motivation to consistently record exactly how much water I used on which bed on which date. That’s a level of granularity that even I can’t commit to! I suppose the most relevant measure will bed the amount of water consumed per kilogram of produce, which should hopefully be reduced by this and all of the previously mentioned measures.

So will these make a difference?

Time will tell! Patience is essential, but I’ll certainly report back as when I have any relevant updates.

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CPD Roundup: November 2023