Making deserts bloom - centre pivot irrigation

Dr Jonathan Hare, The Creative Science Center, Sussex University.

from air

Circular crops fields in the Arizona desert (Note: left photo taken from reference 4 below)

In March 2011 I spent a happy two weeks visiting friends in Phoenix, Arizona USA. First we travelled north to the Grand Cannon and then later we went south near to the border with New Mexico. Much of Arizona is mountainous and high desert with extreme temperatures and often low rain fall [1]. Somehow farmers make this desert bloom; how do they do it?

The ancestors
One answer is in the clever use of water. Giant dams in the area provide water for irrigation. The Native Americans in the Phoenix area used the confluence of four rivers to channel water into irrigation ditches they designed and constructed. Much later the Mormon settlers rediscovered these ancient canals which allowed them to settle in the area.

cpi

CPI - Centre Pivot Irrigation
Out in the southern desert I saw how modern farmers use a system called Centre Pivot Irrigation (or central pivot irrigation or just circle irrigation) [3]. Here watering equipment rotates around a center point or pivot creating a circular irrigated patch where plants can grow.

I had seen these circular patches looking out on long flights and had often wondered what they were. After visiting the Chiracahua Mountains in the south of Arizona we drove on to Bisbee. On the way we stopped off by the road to take a look at, and photograph CPI in action. So on this trip I was able to see them close up.

CPI uses a long moving metal water tube fixed at one end to a water source. Very slowly the pipe-wheel contraption moves around the canter point rather like an enormous hour-hand on a giant clock (ca. one rotation a day or less [3]). This creates a large circle of water irrigated soil on which plants can grow under the intense and bright blue desert skies.

The moving pipe is ca. 100 - 500m long and is suspended a few meters above the ground using cantilevers and supports. Every 20 m or so they are supporting by wheels. Early equipment used water pressure to drive the wheels but electricity is now generally used.

cpi close up

Along the length of the tube there are thousands of small holes from which water continually sprays out onto the ground. Usually hanging tubes (called 'goosenecks') direct the water close to the ground to minimise water evaporation, wind dispersal and allow the equipment to work over rough or uneven ground. The tubes further out from the cantre have to cover more ground than those close to the centre so each tube flow needs to be adjusted so that the soil gets an even watering during a cycle. For more information about CPI see the links below.

References and Links
[1] Arizona wiki page
and Arizona history wiki page
[2] Lonely planet site
[3] wiki center pivot irrigation
[4] Farm in Arizona
[5] CPI in Saudi Arabia

JPH, these notes made April 2011

THE CREATIVE SCIENCE CENTRE

Dr Jonathan Hare, The University of Sussex
Brighton, East Sussex. BN1 9QJ.

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