One area can illustrate that the supply of & demand for Water Varies over Time & Space A case study...
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Transcript of One area can illustrate that the supply of & demand for Water Varies over Time & Space A case study...
One area can illustrate that the
supply of &
demand for
Water
Varies over
Time &
Space
A case study of the Colorado drainage basin
SW USA
Factors affecting SUPPLY are…
PHYSICAL
i) Geology
ii) Relief
iii) Climate
TECHNOLOGICAL
POLITICAL
PHYSICAL SUPPLY:
GEOLOGY
Permeable rocks –
Absorb water -
Give groundwater supplies
Aquifer sources are important in this arid area –
over 25% of supply
Loss of water by percolation through bed of reservoirs
e.g. Lake Powell
Over extraction from aquifers e.g. Phoenix
Water table becoming too deep
At Yuma – after flooding there was a localised rise in water table…
… Possible sewage contamination from septic tanks
North Phoenix - The purpose of CAP is to help Arizona conserve groundwater by importing a fresh, renewable supply of surface water from the Colorado River.
C.A.P. = ?
Central
Arizona
Project
PHYSICAL SUPPLY:
RELIEF
Many deep canyons
Suitable for dams & reservoir sites
Sierra Nevada – relief rainfall
Rain shadow to east in desertUpper catchment
(Rockies) much snowfall &
melt into the Colorado in Spring
Flows SW to Mexico
The Lower Basin depends almost entirely on the Upper Basin for its supplies
Boulder Dam – relief advantages?
PHYSICAL SUPPLY:
CLIMATE
Extremely arid
Low input of rainfall
Especially in the Lower Basin
Rain Shadow of Sierra Nevada Range
Much water loss by evaporation (95%)
high temperature and low humidity
Seasonal rainfall
Violent thunderstorms
Flash flood run-off
Not easily stored
High evaporation from reservoir surfaces
Much seasonal supply from snow melt
Relief rainfall
Sierra NevadaRain Shadow
Nevada and Arizona
PHYSICAL SUPPLY:
Other
Very little Vegetation
so transpiration loss is low in natural systems
Soils – friable and absorbent so
Groundwater supplies are adequate.
Rainwater drains quickly.
Soil eroded easily – puts sediment in river and silts up reservoirs quickly
Main supply is the Colorado river
Secondary source is groundwater / aquifers
Fieldwork in Arizona – a vegetation stop!
Supply factor
TECHNOLOGICAL
Colorado -
a River or
A Giant Plumbing System?
20 major dams and reservoirs
Denver and towns in Rockies supplied across the Great Divide (watershed) by the Big Thompson Project
33% of Southern California water supplied via Colorado river aqueduct
Irrigation for agriculture - 80% of use
California first to construct canals to take water to the Imperial Valley in late 19th C.
800,000 hectares of farmland is irrigated
Supply factor
TECHNOLOGICAL
Bureau of Reclamation set up in Depression (1930s) to plan and construct major engineering projects
With national funding
No overall Drainage Basin Management
Central Arizona Project - 540 km canal across desert from Parker dam to Phoenix and Tucson.
Lifts water 900 metres uphill
Problems of water quality and colour. Supplies 25% of Phoenix water
The main reason for technological interference was flood control.
The Colorado was very ‘flashy’ and dangerous
Supply factor
TECHNOLOGICAL
40 million people supplied with water & increasing
HEP – produces 120 million KW of electricity. Environmentalists oppose any further dam constructionFlood Control – 1983 –
disastrous floods below Parker and Davis dams – if the area is so controlled how did this happen?
… Surplus water drains from upper farmland with fertiliser and is reused downriver
High Technology allows wasteful use of water in lakes, swimming pools, golf courses, gardens and ordinary domestic use.
Problem of salinisation in water supplied to Lower Basin…
Supply factor –
POLITICAL
Colorado Compact 1922 –water allocation –
based on population figures of Basin states then.
Also shared with Mexico
No interstate transfers have been agreed
California dominates then and now.
Some states would like to renegotiate as things have changed!
Southern states tend to be non-interventionist – let business decide.
Growing environmental concern and lobby group.Different users have political clout to protect own interests e.g. Dole
N.A.W.A.P.A. scheme suggested but far fetched?
Demand factor
DEMOGRAPHIC
Compact agreed when area had low population – only California had a sizeable population. Now 35 million – more than total of all the other Basin states
‘Sunrise’ states with rapid population growth. Much in-migration in 1980s. Nevada 37% growth, Arizona 22% 1990 – 1998.
Mostly URBAN population – they expect and need water for swimming pools in such a hot environment. Tend to use water wastefully – e.g. golf courses and lakes but have ability to pay.
Native American population groups still exist in area and have some water rights. Will use the law now to protect their interests.
Demand factor
ECONOMIC
Agriculture uses 80% of the supply. Uses water wastefully e.g. furrow / flood irrigation. Grows specialist crops e.g Lettuce in Yuma. This demands regular irrigation – may not be SUSTAINABLE
Industrial – ‘Sunrise’ States – have a growing hi-tech industrial base. Many companies have landscaped grounds wit lakes and gardens watered by sprinklers. Business executives expect facilities like golf courses.
Domestic / Urban – a growing number of big cities like Phoenix. Demand water for homes and pools. Phoenix has built the 500 km Central Arizona Project to use Colorado water. Groundwater supplies already nearly exhausted
Rising price of water – litre for litre now costs more than petrol. Agriculture is least able to afford the rising prices. Some farmers have sold water rights to cities