Landholders anxious about water supplies are concerned that plantations intercept and take up rainfall that would otherwise replenish local streams.

An international study published in Science found that plantations use a tremendous amount of water; and that overall, afforestation of pastures and cropland reduced streamflow by around 40 per cent.

Currently Australia has around 1.7 million hectares under plantations, 40pc of them planted since 1990, and a target of three million ha by 2020.

Researchers have found that plantations can reduce runoff and stream flow, particularly if the plantations cover a large part of a catchment.

However, most plantations cover only a small proportion of any catchment.

When plantations are established, the impact on stream flows is initially minor, and then increases until the tree canopy closes.

Water uptake by the growing trees can reduce stream flows for 10-20 years, possibly longer in drier environments.

During this time, flood peaks are likely to reduce, and the number of low or zero flow days increase.

Flow changes are likely to be more significant for streams that are not regulated by large dams.

Trees can also take up water from shallow groundwater.

The impact on groundwater storage depends on the area of plantation, depth of water table and plantation management.

Plantations established close to drainage lines intercept more water than those established further away.

Planting further away from streams could reduce impacts on flow, but may lead to slower tree growth.

Plantations established in strips across the contour may use more water than the same area of trees planted in blocks or perpendicular to the contour.

Management options to reduce impact on stream flow favour a “farm forestry” approach where the plantations are interspersed with open land and provide shade, shelter and land protection.

Scientists recommend using less thirsty species of trees planted in stages, so that there is a spread of age classes.

They also recommend that plantations be dispersed across the landscape and cover less than 20pc of the catchment.

Another recommendation is to establish plantations in lower rainfall areas where stream flow impacts are less than in high rainfall areas.

However, care needs to be taken here, as environmental degradation has been caused by single species plantations in arid areas.

The BRS paper suggests that water rights do need to consider plantation impacts, and that more research is needed to assess water use by different types of land cover.

The real challenge is to apply scientific knowledge, to develop plantations in parts of selected catchments, where they have the most commercial and environmental benefit with minimum impact on water flow.

You can read the international study by Jackson et al in the December 23 issue of Science at http://www.sciencemag.org/cgi/content/abstract/310/5756/1944.

In response to landholders’ concern, the Bureau of Rural Sciences has produced a short paper, Plantations and water, summarising the Australian research done in this area, available at http://affashop.gov.au/product.asp?prodid=13357.