Determining hydroperiod thresholds for vegetation

A field-based project was undertaken to develop and test a mechanism which would allow for correlation of the health of vegetation surrounding playa lakes in south-west Australia with the natural variation in salinity and waterlogging that occurs spatially and temporally in natural systems. A pair of playa lake ecosystems, surrounded by primary production land, was modelled with hydro-geological data collected from March 2006 to March 2007 to determine the hydroperiods of vegetation communities fringing playa lakes and provide insight into vegetations tolerance or ‘coping’ ranges to waterlogging and salinity conditions.

These lakes are in the Lake Warden recovery catchment. They were chosen because the lakes are very close together but their groundwater and surface water systems are disconnected. As they are close it can be assumed they are subject to the same sort of conditions and degradation processes but that the varying vegetation health - one of the lakes has much more degraded vegetation - is a result of the different hydrological settings of the lakes which predisposes them to varying levels of waterlogging and salinity levels.

In 1999 and 2007 the Esperance town and region were severely flooded after large summer rainfall events which, under anticipated climate change scenarios, are expected to become more frequent. Hydrological data was collected over the year from March 2006 - March 2007 which included the summer flood of 2007. The study looked at the long-term effects of seasonal waterlogging and salinity and also the impact of the extreme rainfall events which occurred in 1999 and 2007. Large rainfall events such as those of 1999 and 2007 are hypothesised to have a significant deleterious effect on the ecosystems.

The study was designed to determine threshold or 'coping' ranges of vegetation communities using moderately extensive data over short temporal periods which will guide the design of potential engineering solutions that manipulate hydrological regimes to ultimately conserve and protect native vegetation.

The methodology was multi-faceted and included

• a detailed topographical survey;
• vegetation surveys;
• hydrological and hydro-geological monitoring over a 12 month period.

The hydro-geological data and vegetation data was linked with the topographical survey at a high resolution in a Geographic Information System (GIS) for spatial analysis to determine the degree of waterlogging experienced by vegetation communities over the monitoring period.

The study has found that the spatial and temporal variability of hydroperiods has been reduced by rising groundwater levels, a result of extensive clearing of native vegetation, and consequently populations are becoming extinct locally resulting in a shift in community composition. Extreme summer rainfall events also have a significant impact on the health of vegetation communities by increasing the duration of waterlogging over an annual cycle and in some areas expanding the littoral zone.

Vegetation is most degraded at lower positions in the landscape where communities are becoming less diverse and dominated by salt tolerant halophytic species as a result of altered hydrological regimes. Some species appear to be able to tolerate groundwater depths of less than 2.0 m from the surface, however there are thresholds related to the duration at which groundwater is maintained at this depth.

Potential engineering solutions include groundwater pumping and diverting water through drains to maintain sustainable hydroperiods for vegetation in areas with conservation value. The effectiveness and efficiency of the engineering solutions can be maximised by quantifying thresholds for vegetation that include sustainable durations of waterlogging.

The study has quantified short-term tolerance ranges to salinity and waterlogging but species may be experiencing a transition period where they have sustained irreversible damage that will result in their eventual mortality. With long-term monitoring, the methodology developed and tested in the study can be used to quantify the long-term tolerance ranges that are important for the application of conservation approaches that include engineering solutions.

The Lake Warden catchment is a recovery catchment in the state salinity plan. Two small lakes in the catchment were chosen because they are close together but have disconnected surface and ground watersystems.

Vegetation surveys were done across 9 transects, in total covering approx 2.91km.
The study used 1m x 1m quadrats across the transects because subtle differences in elevation can buffer the effects of salinity and waterlogging.

Recording vegetation health in small cells enabled the project to capture the sharp zonation which occurs along an
elevation gradient. Using small quadrats also enabled linking the high resolution GPS survey to the vegetation health in the cells.

A researcher collects rainfall and data from an automatic logging station.

Detailed topographical surveys were done using an amphibious wetland vehicle which communicated data back to its base station.

This project was undertaken by Tara Horsnell under the supervision of Professor Keith Smettem and Dr David Reynolds.

This project acknowledges funding from the Water Corporation, Centre for Groundwater Studies and financial support for field work from the Department of Environment and Conservation (DEC).

Thanks also go to Tilo Massenbauer (DEC), Daniel Winton (DEC), Nikki Cowcher (DEC), Dr E. Mattiske (Mattiske Consulting Pty Ltd) and Sarah Robinson (Mattiske Consulting Pty Ltd) for field support.