Cheap water fuels an
8 October 1999
Cheap water fuels an
agricultural paradise
California is famous for its cheap water
and prodigious agricultural output.
Lincs farmer Nicholas Watts went to
see the irrigators in action
WHILE in Britain we talk about adding inches of water to our crops, in California they talk about feet of water. Average rainfall in the main agricultural areas is between 75 and 200mm (3-8in) a year, with a lot more evaporation than we get here.
Anyone who has built a winter storage reservoir will know how expensive water is. Californian farmers are rather more fortunate in that the American government has over the years kept them supplied by paying for big irrigation schemes. In places water is pumped and channelled up to 100 miles, enabling them to grow over 200 different crops.
The biggest irrigation scheme of all was to take surplus winter water out of the Sacramento-San Joquin delta, water that would otherwise run out to sea under the Golden Gate bridge at San Francisco. This water is lifted into the massive San Luis reservoir near Los Banos by way of the Mendota Canal. It supplies the eastern side of the Central Valley, which is the largest area under cultivation in California. The western side is supplied by another system of reservoirs and canals that takes water from the Sierra Nevada mountains.
All these schemes are supplemented by underground water which farmers have had to bore for. How deep they have had to bore depends on where they are situated, but without exception all these bores have had to be deepened over the last 20 years.
One area that has not been included in these irrigation schemes is the Salinas Valley, which runs along the coast. Though the valley receives up to 500mm (20in) of rain annually, it still relies on bore water. These bores are having to be driven deeper and deeper into the aquifer, with a risk that salt water will contaminate the bores.
This valley is so productive, only about 1% of it is cereals. Just to rent an acre of ground for cropping costs $2000 (£1250)/yr and the main crops are lettuce, broccoli, cauliflower, onion, grapes and melons with other minor crops also grown.
Another big scheme financed by the American government is the Imperial Valley project near the Mexican border to take water from the Colorado river. Water is channelled west from the Colorado river north of Yuma to the flat lands of the Imperial valley, most of it below sea level. Any surplus water runs into the Saltern Sea which is 83m (274ft) below sea level.
Salt accumulates
All naturally-occurring water contains salt. If water is continually added to land without any run-off, salt will gradually accumulate in the soil until crops cannot be grown. Many fields in the Imperial Valley are underdrained and as there is plenty of water in the Colorado River, fields are able to be flushed every so often to avoid any build-up of salt.
In Central Valley water is more expensive and not in surplus. Here, land doesnt get a regular flush-out and salt is gradually building up in the soil. However, much of this land wasnt used for agriculture until after the San Luis reservoir was built in 1969 and farmers arent expecting to feel any salt build-up effects for at least a generation.
Nearly every crop is grown on ridges, similar to potato rows. These are formed in the autumn and left to weather. In the spring they are lightly cultivated with a Lilleston rolling cultivator. The cultivator destroys any weeds that have germinated and freshens the bed up. If a second crop is grown during the summer, ridges are again formed after discing several times. No signs of any power harrows!
The crop is then drilled on top of the ridges in a single or double row, usually with a Stanhay Webb precision drill, and a pre-emergence herbicide applied to the top of the row in the same operation.
Three main types of irrigation are used – flood, trickle and sprinklers. I saw very few pivot irrigators, no modern linear systems and none of the rain guns that are so common in the UK.
Sprinkler systems that went out of favour in the UK in the 1970s are used everywhere to start crops growing. They are normally left running for 6 hours, sometimes moved to irrigate the land adjoining but moved back again within a day or two. With most of the land being fairly well-bodied, it is important to keep the surface moist with fine droplets.
Flood irrigated
These sprinklers are usually operated at around 3.5 bar (53psi) pressure with one 3mm nozzle working on each standpipe.
Once the crop has been established, the sprinklers are collected up and taken off to establish another crop. This will have used anywhere between 150 and 300mm (6-12in) of water an acre. Flood irrigation then takes over on nearly every crop except potatoes, and this is the reason for growing all crops on ridges.
Before the crop is flood irrigated it will be cultivated and probably again after each subsequent flooding. Light cultivations on flood irrigated land freshen the soil up and helps to stimulate growth and improve yields.
Flood irrigation is labour-intensive but requires little capital. Usually 100-150mm (4-6in) is applied at each application. Its not the most efficient way of irrigating but if the land is level it does get to every row even on windy days. Trials are being done with trickle irrigation in some areas but while water is still relatively cheap most farmers are content to continue in the same way, using 3-4ft/acre for most crops.