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Uptake - Issue 1 - Update on trees and saline groundwater systems

Trees and saline groundwater systems – an update from WA

Stuart Crombie is a CRC Research Fellow based in the Forest Products Commission of Western Australia at Rivervale in Perth, WA. He is investigating uptake by trees of water from the saline-saturated groundwater systems which have risen since land clearing. These water tables are now discharging to the surface in large areas of valleys and other low-lying areas where they cause salinity.

The Strategic Tree Farming Project (STF) is an initiative of the Forest Products Commission that aims to establish nearly 20,000 ha of trees in the medium rainfall zone (MRZ, 600-450mm/yr) of south-west WA. These plantations are being established for commercial timber production and salinity control and to ‘kick start’ a commercial forestry industry with associated benefits in salinity control, farm income diversification, conservation of biodiversity and soils in this medium rainfall zone.

landscape of strategic tree farming project

 


Trees for the STF have been planted in various layouts (belts and small or large blocks) from the recharge zones higher in the landscape to the discharge zones in the valley floors. On the deep soils (20 – 30m) that are characteristic of south west WA, these trees often interact with complex layered groundwater systems.

  1.  Infiltrating rainfall becomes saline as it percolates through highly saline deeper soils so that deep groundwater that discharges in valleys is almost always saline. These deeper groundwater systems can develop considerable pressure as they are supplied by recharge higher on the slope and discharge is limited by overlying soil layers of low permeability (see photograph).  
  2. The discharge around the area in the photo is likely to be a combination of discharge from deep pressurised groundwater (20m+) in saprolite above base rock and from the shallow sandy clay topsoil overlying the B horizon at 0.5-1m depth. Whereas water in the deep system is highly saline water in the shallow system may have been relatively fresh until it mixed with the discharging deeper water.
Clive Hampton and Alex Winter measuring hydraulic pressure

Clive Hampton and Alex Winter from the STF project measuring hydraulic pressures in a deep groundwater system.  They are using a pressure gauge because the effective hydraulic head can exceed 10m above ground.



Tree water extraction from such a complex layered system will depend on many factors including recent rainfall, long-term climate, clearing history, and salt tolerance of the species planted. The relationship between the trees and the groundwater may also change with time if the trees reduce recharge, dry the soil profile and ultimately lower the water table.

We are investigating these interactions using transects of trees running from the lower discharge zone adjacent to the saline scald, uphill to the lower slope where there may be several metres of unsaturated soil above the groundwater. The main techniques being used are piezometers and neutron access tubes to assess soil water conditions. These will be combined with measurements of leaf water potential and development of leaf area and tree growth in the coming years to measure how the different tree species use the various bodies of saturated and unsaturated, saline and less saline groundwaters in different parts of the landscape and at different times of the year.