New study predicts the impact of forests on water

CSIRO researchers have developed a new approach to predict the impact of afforestation on local and regional water resources.

This new approach - developed in a study of the Murrumbidgee catchment in south-eastern Australia - has the potential to be applied to other national and international catchments, for estimating plantation impacts on water security.

It has demonstrated that 'the impact of plantations on water resources could be estimated by linking catchment water balance models with river planning models'.

The paper said: "This way it enables us to express the impacts in terms of changes in water allocation and diversion, which are directly relevant to water resources management".

The paper, entitled 'Predicting the impact of plantation forestry on water users at local and regional scales. An example for the Murrumbidgee River Basin, Australia', was done by CSIRO Land and Water researchers Alice Brown, Geoff Podger, Andrew Davidson (NSW DNR), Trevor Dowling and Lu Zhang.

The River Murray and its tributaries form Australia's largest and most developed river system. The changes faced by the region's people, as they adapt to the physical limits of water availability, are challenging.

Extending from south of Cooma in south eastern Australia, to Balranald in the west, the Murrumbidgee catchment covers 84,000 square kms and the river – the third longest in the Murray-Darling Basin (MDB) - flows for approximately 1,600 kms.

The study area is upstream of Wagga Wagga and covers about 38,000 square kms, with a mean annual rainfall of between 500 mm - 2500mm.

The National Water Initiative (NWI) recognises that large-scale plantation development is a land use change that must be considered due to its potential to intercept significant volumes of surface and/or groundwater.

Expansion of tree plantations has economic, social and environmental benefits, but the Study found that 'substantial changes in forest cover might also lead to decreased stream flow in the MDB river systems'.

The researchers examined two scenarios relating to the simulated planting of 30,000ha in the Uplands region of the Murrumbidgee catchment, considered the upper limit of likely plantation expansion. They show how a water balance model can be linked to the Integrated Quantity and Quality Model (IQQM), for the Murrumbidgee River system.

"Linking the two models allows the impact of potential plantation expansion to be assessed at various points throughout the River system," the paper said. "It allows changes in streamflow, in upland areas, to be converted into impacts on allocations and diversion for downstream water users".

The research indicates that 'on a mean annual basis, the whole-of-catchment scale impacts of the vegetation change on streamflows, allocations, and diversions are minimal. However, when local responses, critical years, and sequences of years are considered, some potentially high impacts are identified'.

The results also confirm that afforestation can potentially reduce the percentage of time that water can be extracted, in each of the water access classes. "This reduction will impact on the volume of water that may be extracted under the unregulated licence conditions, which may have significant impacts on water users," the Study said.

The research was carried out through the Water for a Healthy Country Flagship. The Flagship is working with governments, industries and communities to achieve a tenfold increase in the social, economic and environmental benefits from water by 2025.

Research in the Murray Basin is delivering knowledge about the Murray water system plus tools to predict how it responds to change. Research findings aim to help governments, irrigation companies and catchment management agencies achieve their water management goals.

Contact: Alice Brown, CSIRO Land and Water

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