Geography Reference
In-Depth Information
Box 4.1 Groundwater and subsidence in Tucson, USA
Tucson, Arizona lies in a semi-arid area surrounded by mountain ranges
and desert. The population and its water use, including that for farming,
grew rapidly at the start of the twentieth century and most water then
had to come from groundwater wells as surface water supplies had
been outgrown. Abstraction grew at a much faster rate than replenish-
ment from rainfall or snowmelt from the mountains nearby. This meant
water tables dropped in some places by 80 metres. The Santa Cruz
River around which the city is based used to flow all year round but now
only flows for short periods during the year as there is no baseflow into
the river. During the mid-twentieth century hundreds of fissures, gullies
and collapse features formed creating havoc for roads, pipelines and
other infrastructure. In some places, subsidence of the general surface
level occurred by around 8 metres. Water was therefore urgently
needed from other sources to prevent further subsidence and also to
ensure the city would have enough water to survive. A massive engin-
eering project was devised which diverted water from the Colorado
River for around 400 kilometres across the desert (and uphill) to towns
across central Arizona including Tucson. The link was made in 1992 but
the taste and odour of this new water, which had travelled across the
desert and undergone evaporation and concentration of dissolved
matter, was not to a lot of people's liking. Therefore, much of this new
water was used instead to replenish groundwater, being filtered on its
percolating journey before later being abstracted. Other activities such
as recycling waste water are now being used to encourage the slow
recharge of the aquifers in the Tucson area.
land flow dominates the runoff response then the
hydrograph
is
likely to have a short lag time and high peak flow. The hydrograph
(a graph of river discharge through time) will therefore be quite
steep (perhaps even spiky) in shape, rising quickly from low low
to the peak flow. Urbanisation increases flood risk as it reduces the
infiltration capacity of the surface through construction, leading to
rapid runoff to the river resulting in steeply rising hydrographs with
sharp peaks. If matrix throughflow dominates runoff response then
the river may rise and fall very slowly in response to precipitation
and the peak may be small. However, since throughflow contrib-
utes to saturation-excess overland flow then throughflow can still
