160 The impactsof climate change
United States over a quarter of the groundwater withdrawn is not being
replenished, so every year the water has to be extracted from deeper
levels; and in Beijing in China the water table is falling by 2 m a year
as its groundwater is pumped out.
How variable are the stream and river flows? This question is particu-
larly relevant to arid and semi-arid areas. Detailed studies taking these
criteria into account have been carried out for a number of areas; one
example for the MINK (Missouri, Iowa, Nebraska and Kansas) region
of the United States is shown in the box.Study of the ‘MINK’ region in the United States
The United States Department of Energy has carried out a detailed
study^26 of the likely effects of climate change on a region (known as the
MINK region) in the centre of the United States comprising the states
of Missouri, Iowa, Nebraska and Kansas. Included within the region are
parts of four major river basins – the Missouri, the Arkansas, the Upper
and the Lower Mississippi. Water is already a scarce resource within
the MINK states; much of the area’s irrigation relies on non-renewable
groundwater supplies. These will diminish with time, so that even in the
absence of climate change less water will be available, especially for
irrigation.
To provide an analogue of the climate which might be expected with
increased carbon dioxide, the period of the1930s was chosen, when the
average temperature in the region was about 1◦C warmer than in the
period 1950–1980 (the ‘control’ period) and the average precipitation
about ten per cent lower than in the control period.
Water would become scarcer under the analogue climate compared
with the control.^27 The hotter and drier conditions would increase evapo-
ration and reduce runoff. Streamflow would drop by about thirty per cent
in the Missouri and the Upper Mississippi basins and by about ten per cent
in the Arkansas. Most streams would fall well short of supplying both
the desired instream flows and the current levels of consumption use.
Under the analogue climate, irrigated agriculture would be bound
to decline substantially because of the increased constraints on ground-
water use coupled with less water availability from other sources. This
would also result in a drive to increased efficiency, albeit at greater cost.
Maintaining the high priority currently given to navigation on the main
stem of the Missouri would become very costly.So far when mentioning changes in temperature or rainfall it is
changes in the average with which we have been concerned; for in-
stance, the simulations in Figure 7.7 are for average conditions. But,
as has been constantly emphasised, the severity of climate impacts