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 desert Upper 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 Nevada Rain 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 19 th 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 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 construction Flood 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 – 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