Simple arithmetic tells us that a few tweaks to farming could make a huge difference to Britain’s ability to cope with floods – and with the droughts we could be facing in a few months’ time. The catchment area that picks up the rain is likely to be at least a thousand times larger than the area on which all that water is finally dumped. So one inch of rain on the surrounding hills becomes 1000 inches – more than 80 feet – in the river, or in the high street if the river can’t cope.

Catastrophes are rare only because rain doesn’t fall all at once, and by the time the last drop has fallen the water that fell first has already been safely drained away. So the total amount is obviously important – six inches of rain over the catchment is harder to cope with than one inch. But timing matters just as much: how quickly, or slowly, the water flows from the hills to the drains to the river and so to the sea. If the water that falls on the hills takes a day to drain away, as opposed to one hour – or a week rather than a day – then the rivers would probably cope.

Much of the catchment that funnels the water into the rivers is likely to be farmland – and farmers can do a very great deal to stem the flow, and to buy time. In fact they, rather than civil engineers (or the inexhaustible sand-bags that David Cameron has offered, or the emergency services and the army whom he has cause to praise so fulsomely) could almost certainly make the greatest contribution of all. They can do this in four main ways:

1. Planting

Deep-rooting perennial plants in general hold more water for longer than annuals, so here is a good reason to follow the lead of Britain’s Pasture-Fed Livestock Association and raise animals on home-grown pasture rather than home-grown cereal or imported cereal and soya. PFLA members are showing that this can be done – cattle and sheep on 100% pasture, and pigs and poultry on more than is generally supposed. The best water-catchers of all, though, are trees. Once (and in some countries, still) it was standard practice to plant trees at the tops of hills to stem the flow of water from the top and to prevent erosion. Trees stop at least some of the rain from reaching the ground at all: they catch it on their leaves and some of it then evaporates away. The rain that does reach the ground is in part transpired away. In any case, they certainly slow the run-off. Trees (and hedgerows) are even more helpful when planted in rows or in narrow copses along the contours. Ideally they should then be integrated into the farming system as a whole to become a true exercise in agroforestry. This all seems obvious. But while much of the world including the EU has been promoting agroforestry, Defra has not.

2. Cultivation

Ground-cover and contours help a lot. If fields are not left bare, and if the land is ploughed at all, it should be around the contours of the hills, rather than up and down. I seem to remember learning that in O-level geography, but in much of Britain, this is not the norm.

3. Topography

Chinese and South East Asian farmers hold millions of tonnes of monsoon rain on hillsides and mountain sides by terracing – paddy fields in the sky. We don’t grow rice in Britain but the principle can still be applied. Swales (barriers) can be made along the contours with straw-bales or logs or stones covered in earth. It may be possible to create ponds, sometimes permanent ponds, on the upward side – good for wildlife, and a reserve against future drought.

4. Soil structure

Last but certainly not least, the soil can be made more spongy, able to hold far more water, just by increasing the organic content – which of course has many other advantages too.

So should we blame the farmers for the floods? Occasionally, perhaps, but in general, absolutely not. Like all working people they are hard-pressed to make a living and often cannot afford the kinds of changes that are needed. It costs a lot to introduce significant agroforestry, and to convert to organic. If farms are to do what needs doing, society as a whole must bear at least some of the cost.

Here indeed is yet another instance where governments are required to govern: to do what obviously needs doing on behalf of the society they were elected to serve, and on behalf of humanity and the biosphere as a whole. But the general strategy of all British governments of all parties since about 1980 has been to leave the country’s affairs to the free market, which means that power is ceded to the corporates, especially the transnationals, who play the market most adroitly; and to the banks, who lend the money (or, to be accurate, have been given the right to deem that the money has been lent, and to demand pay-back with interest).

Government strategy in agriculture, as in all things, has been to offer the corporates and banks the opportunity to fill their boots. Corporates have little or nothing to gain from agroforestry or organic farming, but big engineering companies can make a killing by building dams and ramparts.

To control floods we need a different approach to farming – and a different approach to governance. Sir John Beddington in his Foresight report of 2011 on The Future of Food and Farming said “we cannot continue with business as usual”. More to the point, we cannot continue with governance as usual. Truly the world’s affairs need re-thinking from first principles.

Photograph: The approximate photographer