Last week, I learned that water comes in three colours: green, blue and black. I’m not talking about the amazing turquoise seas of the Caribbean nor the steely slate of the North Sea. All fresh water is classified by colour. Green water is the water that falls from the sky and flows across fields as run-off. Blue water is water in streams and rivers or ponds and lakes. Black water is found in underground reservoirs.
The different categories are important because they help us think about how water can be used. A lot of green water is wasted through evaporation or run-off, so farmers can use much more of it than they do now. That’s why rainwater harvesting is such a great way to grow more crops or bring clean water to a village.
Most of the water we use is blue water. We take water from rivers and lakes to irrigate farms, run factories and keep our taps flowing. There will be a lot more competition in the coming decades. Cities in developing countries are getting thirstier as they grow bigger and richer. At the same time, the environmental reasons why we should leave more water in rivers are becoming clearer. Fish numbers are down and some species can no longer get to their breeding grounds because rivers are running dry.
Black water is the one that has people really worried though. Globally we are mining water: taking water out of underground reservoirs that took eons to collect there.
Why so much fuss about water? We need to double the amount of food we grow by 2050 to make sure everyone has enough to eat. That’s going to take a lot more water. Unfortunately, climate change will actually reduce the amount of green and blue water in many of the world’s poorest places. We should be much more careful about how we mine the black stuff - oil that causes climate change and water that would help us adapt.