A new study in the USA has shown that intensive irrigation in rural areas lowers local air temperatures.

Using data collected over an 85-year period, scientists from the Lawrence Livermore National Laboratory in California, USA, compared temperature data with irrigation in California between 1915 and 2000, during which time the area of irrigated land in the state’s Central Valley doubled. The team found that maximum daytime temperatures dropped by between 0.9°C and 1.6°C following irrigation.

By extrapolating their findings back to 1887, the year irrigation first started in the region, the researchers also found that the intensively irrigated parts of the Central Valley are 1.8°C to 3.2°C cooler than they would have been if left untouched. ‘It was quite surprising how well we could distinguish a cooling trend that incrementally increases with the amount of irrigation,’ said Bonfils.

The cooling occurs because the sun’s energy evaporates the water on irrigated land rather than heating the air, but it isn’t set to last, according to the study, because the growth of irrigated areas worldwide – which currently contribute to around 40 per cent of global food production – is slowing.

October 2007