Predicting the impact of plantation forestry on water users at local and regional scales: An example for the Murrumbidgee River Basin, Australia

Abstract

To assess the potential impacts of land use change on river water users at both the local and regional scale requires models used in unregulated areas to be linked to river planning models for regulated river systems. This paper illustrates how a land use change model can be linked to the Integrated Quantity and Quality Model (IQQM) for the Murrumbidgee River system in southeastern Australia. Linking the two models allows the impact of potential plantation expansion to be assessed at various points throughout the river system and allows changes in streamflow in upland areas to be converted into impacts on allocations and diversion for downstream water users. To derive an envelope of responses, two plausible plantation expansion scenarios are considered. These are the planting of 30,000 ha in areas that will have the largest reduction in mean annual water yield, and the planting of 30,000 ha in areas that will have the smallest reduction in mean annual water yield. This paper shows that, at the regional scale, the impacts of these plantation scenarios are small on a mean annual basis, with reductions on allocations, diversions and end-of-system flows being 0.7%, 0.4% and 2.6%, respectively for the maximum impact scenario. However, when there is a large increase in the area of plantations in one sub-catchment, the local scale mean annual streamflow reductions can be significantly higher (up to 23% reduction for the modelled scenario), with larger percentage reductions seen in low flows than in the higher flows. Analysis of the annual impacts in the regulated system highlights the importance of looking at sequences of years and impacts in critical years with maximum annual reductions in allocations, diversions and end-of-system flows of 3.5%, 10% and 18% respectively for the maximum-impact water yield scenario. Linking the two models gives an insight into how management rules that control the regulated system can be triggered. This triggering of rules can result in small changes in upland area having large impacts downstream if key threshold values are affected. These impacts have the potential to be felt not only in the Murrumbidgee River system, but also outside in the Murray River system.