 Impacts on Stream Salinity and Flows
Planting trees on land previously cleared for agriculture or livestock can help to lower groundwater tables, reduce salinity problems both locally and downstream and increase land productivity. Both commercial and environmental plantings use water and will reduce stream flows. The CEF work has indicated that poorly located plantings may actually increase stream salinity by reducing dilution flows.
This work focussed on:
Regional-scale targeting Understanding the impact of revegetation on water quantity and quality at regional scale has been undertaken within the SWGB catchment as a case study, and for the whole of the Murray Darling Basin. Both commercial tree plantations and environmental plantings have been considered.  Salinity and water impacts
The BC2C (Biophysical Capacity to Change) model was specifically designed by the CRC e-Water to identify valleys where trees may or may not be used to reduce stream salinity and to provide a first estimate of the salinity reduction that can be achieved. Outputs from the BC2C modelling have demonstrated that targeting of plantations and environmental plantings can markedly influence their impact on salt interception compared to when plantings are not targeted.
Another model of groundwater flows and salinity (2CSalt) provides more detailed predictions once priority areas are identified. This model was developed by the CRC e-Water and was tested as part of the CEF project. 2CSalt provides more detail than BC2C in that it uses more detailed maps of terrain, soil and land use and predicts monthly changes in flow and salinity over time and along the river network.
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Erosion  A well-managed tree planting can strongly reduce soil erosion and river pollution by reducing sediment and nutrients. The SedNet (Sediment budgets for river Networks) model allows catchment managers to identify priority areas for managing these problems.
The tool is able to distinguish between different pathways for pollutant generation and transport to streams, such as surface erosion, gullying and river bank erosion. With this knowledge, the ability of new tree plantings to reduce erosion and pollutants can be estimated.
SedNet predictions for the SWGB are incorporated in the SPIF tool, enabling these impacts to be taken into account in planning and assessment of forest plantings.
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Farm-scale targeting  On any property there will be variation in soil depth, soil type, rainfall and climate. These variations are often related to position on the hill slope and can cause variation in tree growth, water use and groundwater recharge.
The CEF program developed the FLUSH (Framework for Land Use and Spatial Hydrology) in collaboration with the CRC e-Water. This model predicts the relative increase in forest productivity and environmental benefits that can be achieved through optimising planting design. It requires local information on the relationship between topography, soil properties and climate.
Integrating the FLUSH model, 3-PG+ and 2CSalt improves accuracy of predictions to assess:
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Growth rates in different positions in the landscape, and -
Impacts of revegetation on different locations in the landscape. Targeting environmental plantings on hilltops will improve biodiversity and reduce soil erosion, commercial plantations on mid-slope may provide a balance of growth and environmental benefits, however, establishment on shallow groundwater tables in valley bottoms can increase soil salinity discharge downstream depending on the local soil conditions.
...back to top Environmental plantings and on-farm remnant forests  Good data are available for rates of growth and water use of the traditional industrial plantation species, however there are almost no data for environmental plantings. This project has gathered data on growth rates and water use of environmental plantings to calibrate 3-PG+ and BC2C models.
The hydrological impact of the various types of environmental plantings and remnant forest in north-central Victoria was assessed, taking into account soil properties, water use, groundwater recharge, and surface runoff. There were large differences among sites in rates of water use. Reasons for this are still being investigated, but for example, it is likely that site factors such as soil type and site preparation (ripped or not) play an important part.
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Forest management impacts  Management of tree plantations for hydrologic impact depends on an integrated approach to balancing productivity and water recharge control. Consequently, the impacts of thinning Eucalyptus tricarpa (red ironbark) on growth and deep drainage were studied. At age 10 years, three thinning treatments were applied: 900 stems ha-1 (no thinning), thinning to 600 stems ha-1 and thinning to 300 stems ha-1. Detailed measurements of climate, soil properties, growth and water use were made during the subsequent three years.
Data were used to calibrate the 3-PG model for growth and water use of E. tricarpa plantations associated with different stem densities and thinning events. Results have shown that the timing and degree of thinning can have a large impact of drainage under the plantations, and by implication, on streamflows. Implications for landholders and catchment managers are improved predictions of water use and growth with different thinning regimes.
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