Science, Technology and Engineering

Environmental Geoscience

Honours Projects

Modelling the impacts of land use changes on stream flow and salt load in the Hopkins catchment using the Catchment Analysis Tool (CAT)

John Sharples

Supervisor: Dr John Webb

This study looks at what impact likely land use changes will have on streams in the Hopkins catchment. The long-term effects that various increases in plantation forestry or cropping will have on stream flow and salt load have been examined. The land use change scenarios are based on increased plantation forestry in the north of the catchment and an increase of current areas used for cropping. The impact on each subcatchment within the Hopkins, as well as the whole catchment, has been examined. The Catchment Analysis Tool (CAT) modelling program was used to model these land use changes. The model simulated a 30 year period and was calibrated against historical stream gauge data for flow and salinity.

The plantation forestry scenarios all caused a decrease in both flow and total salt load in streams through out the catchment. A 1.9% reduction in flow and a 0.8% decrease in salt load were seen for every 100% increase in land used for plantation forestry. However, if plantations were located on alluvial, rather than hillslope, areas then the reduction in salt load was greater than the reduction in stream flow, effectively lowering stream salinity. The cropping scenarios all caused an increase in flow and total salt load in streams. Stream flow was increased by 7.5% and salt load by 30% for every 100% increase in area used for cropping. The increased flow was primarily from baseflow, and the increase in salt load was significantly higher than the increase in flow, resulting in an increase in stream salinity.

The magnitude of the effects caused by these land use changes was linearly related to the area of land use changed, provided an even amount of alluvial and hillslope land was changed. The plantation forestry scenarios showed that the amounts of hillslope and alluvial land planted could have a significant effect on the stream flow. This means that changes in the area and type of land use, as well as the spatial distribution of these changes, have an effect on stream flow and salt load.

Modelling of groundwater recharge on a small scale (paddock) using MIKE SHE

John Sharples

Supervisor: Dr John Webb

This study makes use of the MIKE SHE^TM modelling program to highlight the hydrological parameters affecting groundwater recharge on a small scale area near Willaura in Western Victoria. The observation area was a paddock, 400m by 800m, with a subdued topography rising only 7m from north to south. There is a natural drainage line which curves from the SW to the NW corners of the paddock. The drainage line is quite shallow; from the centre of the drainage line to the surrounding paddock there is only a 0.5m elevation difference. Twelve piezometers with loggers were installed across the paddock and the groundwater level was recorded from April 2006 to March 2007. Over this time period the paddock was effectively unvegetated due to the drought.

Despite only a minor topographical change across the paddock the pattern of groundwater recharge and salinity were quite varied. Groundwater recharge and salinity were both seen to correlate with the position of the slight drainage line. This is unexpected as it is generally thought that the main area of salinity do not coincide with recharge areas. In this paddock recharge was occurring under the bottom of the drainage line in, raising the water table to an extent where subsurface evaporation could concentrate the salt.

Using MIKE SHE^TM a model of the paddock was created to test the hypothesis that overland flow in the paddock could cause the pattern of increased recharge that was seen under the drainage line. The model showed that overland flow, directed by the sloped sides of the drainage line, was a plausible method leading to the observed pattern of recharge. The model also demonstrates that the recharge pattern is very sensitive to the infiltration rate (vertical hydraulic conductivity).