Groundwater has some really important economic benefits and it's extraction has exploded in the last fifty years. But the extent of groundwater use has lowered the water table in some places, increasing the cost of extraction and causing subsidence (which also adds to sea level), and reducing water quality (Konikow and Kennedy, 2005). It's all a question of scale really - there are billions of us who need to drink, and there are millions of square kilometres of crops that need watering, and there are all the industries that need water too. And in some areas there just isn't enough rainfall, so we pump water out from the ground, and when we've finished with the water we don't put it back underground, more often than not it runs out to sea and starts to add to sea level (Konikow, 2011). But how much of a problem is this in terms of sea level?
Thing is, it is really difficult to model groundwater depletion and recharge. It's even difficult to know how much there is in the first place. In some places it has been accumulating for thousands of years, or accumulated when there was a wetter climate, so actually it can be considered a non-renewable resource - rather than fossil fuels, we're talking fossil water (NOAA).
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| It's older than you think (Hydrology) |
Pokhrel et al (2012) modelled global land based water storage and how it was used with a particular emphasis on human activities such as reservoirs, and agriculture. Reservoirs are interesting because they actually lead to fall in sea level because so much water is stored on land. Their model suggested that overall, changes in groundwater had been contributing a whopping 0.77mm a year between 1961 and 2003. This accounts for 42% of sea level rise. Wada et al (2012) also modelled the contributions of groundwater to sea level rise. They used three general circulation models, with 50 km grid spacings, which suggested that groundwater contributed 0.035 mm a year to sea level in 1900 and that this rose to 0.57 mm a year by the year 2000. Which ever way you look at it, this is alot lower than the calculations by Pokhrel at al, but both examples do show that groundwater is hugely important in terms of sea level.
Wada et al (2012) also went on to model groundwater contributions for the future. Their model included socio-economic projections for each country or region, so they could try and include how water use in various areas would change with future developments. They found that by 2050 groundwater would be adding 0.82 mm a year to the global sea level rise.
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| Wada et al (2012) projection of groundwater contribution to global sea level until 2100 using two general circulation models and three emission scenarios. The B1 emssion scenario is the only one which starts to suggest that groundwater contributions to sea level could start to decrease in ~2050. This scenario involves increased awareness about the environment from the population, as opposed to the A1 in which economic development continues unabated, or the A2 scenario in which there is no unified response from the global population (IPCC). |
So I think there are two issues here, first that the current rate of groundwater use is unsustainable and may ultimately mean that wells run dry or that any water that does come out of them is undrinkable. And secondly that groundwater withdrawal is contributing to sea level rise. I think the important point for both issues is one of sustainability - we are already into the mindset of sustainable forests, but is it possible to extend this to sustainable water supplies? Looking at the projections for sea level rise as a result of groundwater alone it is obvious that we are going to have to do something really drastic to combat groundwater use, or to stop it from getting into the oceans. The B1 scenario , the only one to show an eventual decrease in groundwater use, is an extreme case, and one that frankly I don't think we will able to replicate.
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