Modelling tools that improve understanding of how forest plantations affect streamflow in local catchments have been developed by The Australian Commonwealth Scientific and Industrial Research Organisation (CSIRO).
‘Many of the catchments that sustain significant areas of new plantations are biophysically complex, with varying rainfall, soil depths and textures, groundwater characteristics and possible planting locations. All of these can lead to variability in the potential impacts of any new plantation development on catchment yield,’ states The National Research Flagship’s Methods to Assess Water Allocation Impacts of Plantations: Final Report.
The CSIRO’s Water for a Healthy Flagship team – in conjunction with the National Water Commission, state authorities and the forestry industry – has developed techniques to account for these factors and to help meet Australia’s National Water Initiative, which aims to improve the use of water ten-fold by 2025.
The project consists of three components – an analysis of measured data, application of a forest cover flow change (FCFC) model for predicting the impact of plantation expansion on seasonal flows, and the development of a more complex model that incorporates dynamic tree growth.
The FCFC model has been used to analyse data from 19 catchments, ranging from 0.6 to 1,402km2 in various climatic conditions. Data are inputted on a wizard style interface to generate a time series and distribution of daily streamflow. These include rainfall, potential evapotranspiration, streamflow, proportion of forest cover and catchment area. An analysis of this data has concluded that the ‘plantation reduces not only mean annual streamflow but also affects [the] streamflow regime. In catchments with relatively low rainfall, large scale plantation expansion is likely to reduce low flow significantly, resulting in an increased number of zero flow days.’
Variability in tree growth and management is accounted for by linking an existing tree-growth model (3PG+) to a catchment hydrology model (2CSalt). This enables the team to predict the impact of landuse change and vegetation variation on streamflow.
This physiological growth tool is said to have several other advantages over the previously used 1D agricultural water balance model. Gilfedder says, ‘This approach allows us to model the impact of plantation management (planting, growth, thinning, harvesting) on catchment streamflow. This improves our existing approach, which had treated plantations as an unchanging fullygrown forest.’ The water balance also reflects the variations in climate, soil and species across the area and the effect they have.
He says, ‘From analysis it could be seen that the impacts of plantations on streamflow are not limited to small catchments (several hectares), but can still be seen in larger catchments’. However, he cautions that, ‘When applying models over large catchments, the variability of input data (such as rainfall, soil, geology, groundwater properties) across the region requires simplifications to be made’.
Nevertheless, Gilfedder says, ‘By applying the more complex model to a range of catchment areas across Australia, we have provided a proof-of-concept to demonstrate that the approach could be used. Such tools could be used to investigate and therefore trigger planning, management or regulatory measures to account for water use by plantations’.