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