We have all heard the line “Water water everywhere, nor any drop to drink.” Well, soon we may have to rethink “everywhere”. There was a tolerably scary future hinted at by a recent letter in Nature (de Graaf, et al, 574: 90). It points out that groundwater is critically important for food production and currently pumping exceeds recharge from rainfall and rivers in many parts of the world. Further, when groundwater levels drop, discharges to streams declines or even stops completely, which reduces river flow, with potentially devastating effects on aquatic life. These authors claim that about 70% of pumped groundwater is used to sustain irrigation and hence food production. There are other problems, such as ground subsidence. If you take away matter from below you, then what you have between you and where you took it must eventually lower, but with land it does not have to do so evenly. Coastal flooding in some US cities is not really exacerbated by climate change to anywhere near the extent it is by the ground lowering due to groundwater removal.
If the streams to rivers are not recharged, there is a slow desiccation of the nearby land. The billions of tonnes of water locked in soils and bedrock and various aquifers is the biggest single source of fresh water on the planet. Life essentially depends on this resource yet we are unthinkingly depleting it. Most people know that people in dry lands will experience worse conditions due to rising temperatures. What most don’t realise is the inability to properly recharge the aquifers they depend on will lead to even worse problems, not the least through the requirement for more water as temperatures increase.
The paper also provided maps that outline the size of the crisis, but in my opinion also shows a problem inherent in such studies: there was a map that showed the head decline that might lead to a crisis, which really indicates how much groundwater there is. Included is two-thirds of the South Island of New Zealand. Now for parts of Canterbury that may well be the case, but it also includes the West Coast of the South Island. The geology there may well indicate there is not much groundwater, and in fairness I have never heard of anyone drilling wells there, but there is not exactly a water shortage there because the Alps get roughly ten meters of rain a year. The problem for farming in that region is not a shortage of water but rather too much. It is true that farming in Canterbury is probably over-drawing on aquifers, but that is in part because the farmers took to dairying in an area unsuitable for that activity. Prior to the dairy rush, the area was quite prosperous at farming, but without irrigation. Farmers tended to grow grain in the warm dry summers, and would also run sheep. Now wool is not really wanted, so the farmers switched.
Of course, just because I can find a problem in one place does not mean the paper does not raise a valid point. I suspect that when producing a world map like that the authors go to whatever resources they can find and may not check associated issues. So, what are the real problems? In a recent article in Physics World it was stated that within thirty years almost 80% of lands that irrigate through groundwater will reach their limits as wells run dry. It also states that for the other 20% of areas that rely on pumped groundwater, surface flow of streams and rivers has already fallen. This includes cities that depend on pumped water for a water supply. The effects are already being felt in the mid-west of the US.
This raises the question of what can we do about this problem? The most obvious answer is to use less. Most people domestically use far more than necessary, and those who rely on stored rainwater will show how to use less. At the city level, do we really need the number of home pools? On a lesser scale, how many houses really do not waste water? Irrigation needs to be managed in such a way as to lose less by evaporation (i.e. something other than sprinklers.) We need farming methods that are better suited to the local climate, except that also has the problem that we then lose production volumes, and it is far from clear we can afford that.
For coastal cities, desalination offers an answer, but it costs $US1 per 1 – 2 tonne so it is not cheap, and in terms of electrical energy, if we use reverse osmosis, roughly 5 kWh is required, which means we have to significantly increase electrical production. Reverse osmosis works by using pressure to force water through a membrane that will not permit salts to pass, so in principle it could be turned off during peak loads, which might make it useful for a base loading source like nuclear power, but this is not so useful for places distant from the coast. There is another problem with desalination. The seawater should be sterile and the membranes have to be regularly cleaned, which leads to the release of biocides, salts, chelating agents, etc into the sea, which in turn is not particularly good for the local environment.We could pipe water from somewhere else, except we may be running out of “somewhere elses”, and anyway, that place may well be what is feeding the groundwater aquifer. The unfortunate end-result is we may have to give up using so much. We have a problem, Houston!