Yesterday morning early I was trudging back and forth to the water trough, worrying about just how dry the soil has become over the past couple of weeks. We had a decent spell of rain before that, but my impression is that we’re distinctly down over the average. Here in Bath the average annual rainfall figure is given as 761mm. I did a bit of investigating and soon realized that I had no idea what that figure really meant, and so it came as a surprise to me that the figures given in mm aren’t ‘per square metre’ – in fact they’re not related to area at all. If you want to find out how many litres of water fall on your patch of ground in an average year, it seems that 10mm of rain is equivalent to 10 litres of rain per square metre – that’s to say, in average year our 250 square metres of allotment receives very roughly 20,000 litres of rainwater.
So far so boring, you may think, but many allotments are completely dependent on a water supply that can be turned off if there should be a drought. Most of us are familiar with the need to have a waterbutt attached to the shed and/or the greenhouse but when you look at the data, a single waterbutt is hardly going to save the day – and that’s where I came in. Yesterday, let’s say I used 20 cans of water (it’s peak watering season), and so that’s 200 litres of water – or pretty much one whole water butt full – that’s to say if the Council or the water companies decided to pull the plug, on our allotment we’d have four good waterings left. So, at a pinch, we could probably survive for a month, but what about a three month drought? It’s a fairly simple calculation – we’d need 12 water butts – 3000 litres of stored water. However, gathering that amount of water in order to store it would be a huge challenge – we have about 7 square metres of roof altogether, so we would need somewhere in the region of 500mm of rain allowing for some inefficiency. Not only that, water butts take up a lot of space – we’d need to go for 1000L caged containers. Any way you look at it, making provision for a drought is both complex and expensive.
The only way to mitigate our water use is to increase the water capacity of the soil by increasing the amount of organic matter and making use of mulches to reduce surface evaporation. It’s clear from our experiments with mixed plantings, that covering the ground completely (courgettes, for instance under sweetcorn) lashings of leafmould and so-on really increase the moisture holding capacity. The Potwell Inn allotment needs most watering on the areas where the soil is exposed – it’s obvious really. Another approach – one which I’m sure the seed merchants will soon be all over – is to develop drought resistant varieties, but we could also start to develop extremely locally adapted plants through seed saving.
The take home lesson from all this, so far as I can see, is that in the teeth of global heating, we allotmenteers are going to have to adapt very quickly. It’s easy to feel virtuous when we grow our own organic food, but we need to be modest about our potential. Collective action across a whole allotment site could be worth investigating – we’ve got three underground streams running through the site – it would take a lot of work and investment but at the moment it’s just running into the river.
When I sat down to write this I felt a bit glum, but now I’ve done the maths and given it some thought, sustainability is a real possibility on the allotment. It seems almost a crime to be pouring purified drinking water on our plants when alternatives are possible. It’s easy to think that the only harvest that matters on an allotment is the stuff we can eat, but we need to harvest every scrap of green waste we can get our hands on, and every available fallen leaf so we can make quality moisture retaining composts and mulches; and then every possible harvestable drop of rain to reduce our tapwater consumption.
To repurpose an old saying – “We have seen the enemy – it is us!”