Oct 2, 2008
Wheat harvest does not suffer as rainfall decreases
Most climate impact studies assume that reduced rainfall will result in lower crop yields in semi-arid areas. But wheat production in one such climate, Western Australia, has not declined since the 1970s despite a decrease in rainfall in the region. In fact, production actually increased by about 40 kg per hectare per year between 1980 and 2000. A team of scientists is now saying that this is because most of the reduction in precipitation has occurred in winter, a period when rainfall frequently exceeds crop demand.
The wheatbelt of Western Australia has a Mediterranean climate with most of the rainfall – which is the main factor limiting crop production – occurring in the winter months. But, since the 1970s, winter rainfall in the region has decreased by about 11%.
Using the Agricultural Production System Simulator (APSIM) for wheat, together with historic climate data, Fulco Ludwig of Wageningen University in the Netherlands and colleagues at CSIRO Plant Industry in Australia, calculated that despite this decrease in rainfall, crop yields have not suffered. Indeed, they actually increased from 1.1 tonnes per hectare in 1980 to 1.7 tonnes per hectare in 2000.
Ludwig and colleagues say the phenomenon may be related to a decrease in salt levels in the soil as rainfall decreases. Over the last century, large areas of land around the world have been cleared of natural vegetation to grow crops like wheat, which results in increased soil drainage when it rains. This causes groundwater levels to rise, bringing salt to the soil surface and making the land unsuitable for further cropping.
However, since deep drainage is intrinsically linked to the amount of rain falling on the land, a reduction in rainfall reduces the amount of water drained by soils. This reduces the spread of dryland salinity and could make some land more suitable for growing crops again. Indeed, researchers have already noted that dryland salinity has not spread as fast as expected over the last decade.
Decreased drainage also means that important nutrients like nitrogen are not leached from the soil, leaving it more fertile.
Ludwig's team obtained the results by studying nine sites in the central and northern areas of the Western Australian wheatbelt. The sites were chosen to cover both high and low rainfall regions as well as warmer and cooler parts of the territory. For all sites, the researchers had access to daily rainfall data from at least 1945. They downloaded additional weather data from the Silo website.
The scientists analysed the impact of recent climate change on different variables by comparing the periods 1945-1974 and 1975-2004. Three soil types typical of the region – sand, loamy sand and sand over clay "duplex" – were used in the simulations. These soils hold water differently due to their different textures.
The researchers employed APSIM-Nwheat, a crop simulation model that takes into account soil water aspects, crop residues, crop growth and development.
The team asks whether future changes in rainfall will follow the patterns observed so far and if the land can "continue to absorb further reductions in rainfall" – droughts may eventually have the upper hand when it comes to crop production. Crop technology, which could help increase or maintain current wheat yields, also needs to be taken into account.
The work was published in Climatic Change.
About the author
Belle Dumé is a contributing editor to environmentalresearchweb.