Post flood investigations at Kamarooka

Summer Storms

On Thursday 19 December a cool change swept across northern Victoria bringing storms that had been predicted up to a week earlier. The first storm dumped 19 mm of rain on Kamarooka between 4.30 and 5.00 pm.  During the same event in Bendigo 10 mm of rain fell in just 8 minutes generating spectacular flash flooding.

On Friday 20 December a further 100 mm of rain fell at Kamarooka flooding the land already soaked from the rainfall of the previous day. To the west of the NUFG project site the broad low lying floodplains of the Myers Creek were inundated and a vast lake developed.  Large dry depressions that were once ancient wetlands filled overnight transforming the area into a series of lakes and islands.

When the rain stopped Kamarooka had recorded 132 mm in just 24 hours. The NUFG Project site was flooded. On Saturday 21 December 2007 floodwater was observed draining from the marginally higher ground in the southeast of the site through to the lower lands in the north and west. Floodwater had mostly  receded from the saltbush on the more saline land apart from a few large pools. Further north in the grasslands large pools of water persisted. Further north again, the entire NUFG plantation remained under a sheet of water 5 to 10 cm deep.  This deepened to the west forming a large lake beyond the plantation. Wilkinson Swamp Road remained above the floodwater bisecting the lake into eastern and western sectors.  

Impact of flooding on watertables

The widespread flooding over such a short period of time produced  an immediate groundwater response. By the Saturday the watertable across the entire NUFG trial site had risen to within 1 to 1.5 metres of the land surface. The impact of ten years of drought and the draw-down established through revegetating had been cancelled out in one single 24 hour event.  The watertable under the NUFG 2004 plantation which had fallen to 4.5 metres was now back within 1.5 of the soil surface and posed a serious threat  to tree survival.  

Kamarooka Groundwater Hydrograph January 13 2008

In the weeks that followed the floodwaters receded and the discussion returned to the elevated watertables under the plantation. Would the trees persist or would they succumb to the combined impacts of both watertlogging and salinity?  Would the shallow groundwater recede rapidly recede leaving the land as it has been before the storm?  Would the roots of the trees be pruned to the level of the new watertable?  Would the trees use the new water and rapidly lower the watertable? 

Some of us reasoned that the floodwater responsible for the rapid elevation of the watertable might produce a layer of fresh groundwater rested on top of the more saline groundwater.  Although this view was regarded as somewhat speculative it had long been discussed within the Australian salinity fraternity and was regarded as credible.  If such a layer  existed it might help explain why the NUFG plantation continues to grow rapidly in an  environment of high salinity shallow groundwater.

The investigation

The discussion over the post-flood elevated watertables resulted in resulted in a research proposal that sought to investigate the salinity of the groundwater at the watertable. The investigation was made possible with financial support from the North Central Catchment Management Authority. 

The plan was to sample groundwater by constructing large diameter wells at sites adjacent to a selection of the existing groundwater monitoring sites across the project area. The depth of each well would need to be slightly greater (0.5 -1.0m) than the known depth of the groundwater level in the adjacent monitoring bore. 

Ian Rankin Excavating drilled the six wells on 13 January 2008 with a 600 mm diameter hydraulic auger. The sites chosen were adjacent to bore 10 in the NUFG and NCCMA biodiversity trial plot, two bores within the NUFG forestry plantation, and three bores within the saltbush on the most saline land.  

The large diameter wells were left for 24 hours after construction to allow sufficient  groundwater to seep into the holes and be collected for laboratory sampling.  Five of the six wells contained about 50 cm of groundwater, however bore 8 (within the central area of forestry plantation) was still dry after 24 hours.  Free groundwater eventually collected in the well adjacent bore 8 and it was sampled 4 days after construction. 

The images on the left below shows one of the wells shortly after excavation. Saline groundwater can be clearly seen seeping in from both the base and lower sides of hole. The image on the right shows one of the wells some 24 hour after construction partially filled with saline groundwater. Note that with closer inspection the zone of groundwater saturation is clearly evident in the wetted zone immediately above the groundwater within the well.  

Building shallow bores from the large diameter wells

In late January 2008 NUFG constructed shallow watertable bores from the large diameter wells.  PVC casing was inserted into each well and coarse sand packed around the base allowing groundwater entry into the bore.  The annulus between the casing and the well was then backfilled with the clay that had been excavated from the hole. Thumbnail Images of this operation can seen below. Click on each to see the larger image.  

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Findings

Groundwater was bailed from each well and duplicate samples were placed in 250 ml bottles. One set of samples was sent for laboratory analyses of electrical conductivity and the other refrigerated in anticipation of any further consideration. The samples were analysed for electrical conductivity by 'Ecowise' laboratories in Bendigo. The results can be seen in Table 1.   

Table 1. Groundwater salinity within large diameter wells established 13 January 2008

Groundwater sample

Electrical Conductivity (uS/cm)

Approximate concentration 
(mg/l)
Groundwater salinity as a percentage of seawater
Bore 4 34,000 21,760 62%
Bore 5 43,000 27,520 79%
Bore 6 28,000 17,920 51%
Bore 7 32,000 20,480 58%
Bore 8 Not yet avail N/a N/a
Bore 10 26,000 16,640 48%

The results show that the groundwater ranges in salinity concentration from about half to three quarters of the salinity of seawater.  No fresh water layer formed over the saline groundwater in response to the flooding. Importantly the results also demonstrate that a fresh water layer never exist prior to the flood. 

The vigorous growth of the forestry plantation and the health of the trees in such a challenging environment of shallow saline groundwater at Kamarooka is not supported by the presence of a layer of fresh water resting above the more saline groundwater.

What caused the watertable to rise?

The intense rainfall event of 19  and 20 December 2007 flooded the entire NUFG project site to depths ranging from 5 to 15 centimetres and induced a large groundwater recharge event that brought the watertable to within 1 to 1.5 metres of the land surface. This single event  in a 24 hour period reinstated the extensive shallow watertable conditions that had not been witnessed in the area for more than a decade.  Despite this  dramatic accession to groundwater and the 2 to 3 metre rise in the watertable the groundwater remains highly saline.  

The key to understanding the lack of change in groundwater salinity lies in appreciating the aspects of the water balance associated with the high rainfall event. First there is a need to understand how much water might have actually reached the watertable.  In the moist conditions within the capillary fringe immediately above the watertable most of the clay subsoil is filled with water. The tiny pores (micro-pores) are filled with saline water drawn up by the 'blotting paper' effect from the groundwater below.  Only the larger macro-pores that drain under gravity remain unsaturated. As a result the filling of macro-pores by water percolating down from above is all that is needed for the watertable to rise.  

At Kamarooka we understand, from previous work, that macro-pores occupy about 3 percent of the subsoil volume. This means that for soils subject to the immediate influence of the underlying shallow watertable we only need to add 30 mm of water to realise a one meter rise in groundwater.  The rise in watertable of 2 -3 meters experienced during the flood event can, therefore be equated to as little as 60 to 90 mm of flood water seeping down to the watertable. The volume of groundwater actually reaching the watertable was most probably of the order of 0.6 to 0.9 megalitres per hectare (60 to 90 litres per square metre). The volume of water entering the groundwater system, thus, was relatively small compared with the volume of saline groundwater stored within the saline  landscape.  

The second thing we should account for is that water percolating down from above must travel through salt laden soils and subsoils that normally exist within an environment of salt accumulation.  Hence it is not surprising that the deep percolation following excessive rainfall is saline when it finally reaches the underlying body of saline groundwater.

Perhaps the only conditions under which a shallow layer of fresh water might exist over the underlying saline groundwater would be during a time when seepage from a fresh surface body was sustained over a lengthy period of time that allowed for considerable salt leaching. Since this is unlikely to ever occur within the saltland at Kamarooka it is reasonable to suggest that the potential for fresh water layer is extremely remote.

Trees and summer storms and watertables

It is instructive to consider the response of the watertable beneath the NUFG forestry plantation in the weeks that followed the storms of 19 and 20 December 2007.  Bore 8 at the centre of the plantation is particularly interesting. The bore hydrograph (link below) shows  that the groundwater fluctuations are extreme. Substantive recharge induced by the flood resulted in a dramatic increase in watertable elevation and is followed by significant groundwater recession. No immediate explanation is available beyond increased transpiration fuelled by the summer climate and the moist soil conditions afforded by the flood.  

Groundwater hydrograph - central NUFG forestry plantation

Groundwater levels within the forestry plantation began to fall almost as soon as the flood water receded.  In the first few weeks they fell as much as 70 cm in a single week, and more recently they have fallen by 33 cm in a week. There is no hydro-geological explanation for this rapid watertable recession. The only explanation that makes any sense is for the trees to be transpiring the saline groundwater. 

The results of our research leave us, once again, confronting previous scientific assertions that claim that trees and salt do not mix. We contend that native trees are well  adapted to saline soils, but are poorly tolerant of sustained water-logging in high salinity environs. The drought of the past decade affords the former but not the latter perhaps explaining why the NUFG 2004 forestry plantation has thrived.

If we are entering a time in which climate change robs us of our winter/spring rainfall, and subjects us to great summer storms perhaps we are well placed to consider the role of trees as phreatophytes in areas in close proximity to shallow groundwater in land not subject to sustained waterlogging. Perhaps the extremes of climate in these regions  support fluctuating watertables that leach and remove salt from the immediate root zone of plantations. 

The overwhelming conclusion of our work to date is that eucalypts and acacias have more salt tolerance then they are given credit for.  In most instances it is not the salt that kills them, it is the combination of salt and waterlogging. This would seem to indicate some need to consider and perhaps re-design the location of tree plantations relative to groundwater discharge zones and conceptual models of landscape-groundwater flow systems.

We will learn much more of the dynamics of trees relative to shallow watertable and saline groundwater at Kamarooka over the coming weeks and months. The summer flood of  December 2007 affords considerable opportunity to do the same.   

Kamarooka farmer Andy Hay (above) says he has never seen 132 mm of 
summer rainfall occur in 24 hours in his lifetime.