Both runoff and discharge are important
in sustaining water resources around the world; Oki and Kanae (2006) have noted that river discharge
is a major part of renewable freshwater resources. As a planet, we are nowhere
near using 100% of all the water available – but the problem is one of temporal
and spatial issues in the availability of water resources. Such variability
includes variations in river discharge over the hydrological year, and
downstream water being so polluted that it cannot be reused (in this case, it
would be runoff that was the renewable resource, not discharge).
Variability is being exacerbated
by increasing variability in precipitation – climate change is leading to more
precipitation globally, but distributed unevenly with some areas wetter and
some drier (Arnell 1999). However, river runoff will likely decline globally,
as much of the land surface will see a reduction in precipitation and more
evaporation. Modelling using global climate models and projections of
population growth/water use allows tentative predictions of how climatic change
may affect the amount of people living under water stress – Arnell’s work
suggests the largest stresses will be seen in the Mediterranean, the Middle
East, and southern Africa. While this is valuable information, long term predictions (e.g. 2050 and
beyond) are difficult as the strength of the climate change signal becomes increasingly
less clear with time, and models are very sensitive to water use projections (which
are also variable). What is clear is that changes in the amount of runoff draining
into a country can have an effect on water resource stress.
However, we have to be careful
here. In models predicting future water resource stress, there are many
confounding factors – climate change, population growth and water demand.
Vorosmarty, et al. (2000) modelled three different scenarios in an attempt to
quantify the key factor influencing water resource stress. They did this by inferring
water demand from the ratio of water use against discharge. One scenario
involved only climate change, another involved only expanded water use (from
population growth) with no climate change, and the final involved both climate
change and expanded water use (Figure 1). Climate change alone proved to have
limited region specific impacts, whereas expanded water use alone increased the
demand per unit of river discharge for much of the world. Considering both factors
together shows that some interaction between population growth and climate
change causes some areas of stress per unit discharge decrease, but the general
pattern is still one of pandemic increase. The conclusion is that climate
change and resultant changes in discharge and runoff are something of a
secondary factor compared to population growth and expanded water use. Climate
change and water resource stress is directionless at a global scale – it will
both increase and decrease stress depending on the location.
 |
| The relative change in demand per unit of discharge for the three different scenarios. Red indicates an increase in demand, blue a decrease. From Vorosmarty, et al. (2000). |
So, an important message; while climate
change will have effects on runoff and discharge, many of these effects will be
mediated by humans. I personally see global scale models involving both climate
change and population trends into the future as limited, because of the huge
uncertainties and the large timescales involved. No doubt they are useful for
considering the overall direction of change, but there can be considerable regional
disagreement of the direction of change between models. This is a conclusion
often reached in these posts, but it is more often than not true – climate change
and the hydrological cycle are incredibly complicated and uncertain.
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