We know river discharge is going
to change around the globe into the future, as a result of changes in the
hydrologic cycle forced by climate change. However a numerical change in
discharge means very little to anymore more than a poorly groomed, strangely
bearded (see figure), obsessive hydrologist. Potential changes in discharge can
affect humans in a number of ways, and one of the more obvious is changes in
flood risk. Flooding can often be a bad thing, but in some case it can be
important e.g. providing water for agriculture, recharging groundwater
supplies.
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| The mark of many a hydrologist. |
Modelling studies have indicated
that human-induced greenhouse gas increases have contributed to an increase in
the intensity of heavy rainfall across 65% of the Northern Hemisphere, and that
in the future many places will be subject to an intensification of heavy
rainfall. This is because as greenhouse gas levels rise (of which CO2)
is one, the planet will warm, which means clouds have to have more water in
them to be able to rain – so when it does rain, it rains a lot. These changes
promote increases in large flood events – more water = higher flood risk.
Throughout the 20th
century, the frequency of large food events (technically speaking, a flood that
would be expected to happen every 100years) have become more frequent, and
statistical modelling suggests that is trend will continue to occur as CO2
levels rise. Already historical changes in radiative forcing may have caused
the increase in large food events in the 20th century.
Recent work looks at the
changes in the magnitude of flood events with a 30 year return periods, and
clear patterns emerge (Figure 1). Increases in flooding are projected in central and
eastern Siberia, as well as southeast Asia, including India. Decreases in
flooding are projected in northern and eastern Europe, and parts of central
South America, around the Amazon basin. These patterns are similar to those
projected to river discharge – it is no surprise to see the two are
interlinked.
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| Figure 1 - Projected percentage changes in the magnitude of 30 year return flood events. From Dankers, et al. (2014). |
These changes in relatively
common floods (every 30 years) have to potential to impact humans, whether it is
destruction by floodwaters or agricultural drought from a lack of water
inundation. We will focus on specific impacts in another post, but it is clear
that humans are innately linked to river discharge and flooding.
I leave you with the song that inspired the name of this post.
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