Kamarooka Update November 2017
The rainfall trend for 2017 is presented below (figure 1) as cumulative monthly values. The values for Bendigo are about 20 percent higher than those for Kamarooka, however, the rainfall pattern is very similar.
The start of the 2017 growing season was very promising. By the end of April we had recorded 231 mm of rain, and almost half of this fell in April. The rain, however, failed in May and June with only 17 millimetres recorded. Good rainfall occurred in July and August, but since then the year has been quite dry. It would have been a very dry year without well-above average (113 mm) in April.
The all too familiar loss of seasonal rainfall in the spring is with us yet again. The total rainfall for September and October was just 44 mm. The very wet spring of 2016 now appears as a short lived somewhat anomalous event and it seems we have reverted to the erratic patterns experienced over the past two decades.
The trend analysis presented in figure 2 demonstrates the erratic nature of the climate we now experience. This trend to below average rainfall commenced in 1996 and it became more intense in 2002. The rainfall deficit relative to the average continued through to 2009. This period is sometimes described as the 'millennium drought', however the term drought is inadequate in describing the climatic phenomena experienced over the past two decades. Droughts are dry periods in which rainfall is less than expectations. It is generally understood they have a beginning and an end, largely reflecting ocean - atmosphere interactions that play out in the Pacific and Indian oceans. The post-1996 climate appears to be something more sinister. Indeed, that word 'drought' now seems redundant. Instead, we should be using language that recognises 'climate change or climate shift' given we have clearly moved on from the climatic conditions that prevailed throughout the latter half of the last century.
Figure 1 - Cumulative rainfall for Bendigo
2 - Rainfall trend for Bendigo plotted as cumulative residual mean (deviation
from the average with annual values summed over time)
Figure 2 needs a little explanation. The data is from the Bureau of Meteorology climate station at the Bendigo Airport. The trend represents the difference between the annual rainfall and the average rainfall over the period of record, in this instance from 1991 through to 2016, summed each year. The technique illustrates progressive changes over time. If rainfall is less than average over several years the graph will shift downward as it does for the dry times between 1996 and 2009. Where the annual rainfall is greater than the average over for a few years the opposite is true with the graph rising. This is clearly evident in the response to the exceptionally wet years of 2010 and 2011. The post 1990s rainfall pattern seems to be one in which extended dry times prevail between infrequent wet years that occur perhaps once every four to six years.
The Kamarooka Watertable
The watertable at Kamarooka in areas that lack plantations or other woody vegetation reflects the erratic rainfall pattern. It rises in response to the occasional episode of abundant seasonal rainfall and falls during the intervening dry conditions.
The catchment above the saline land continues to sustain a shallow watertable in the region of the break of slope between the foothills and the plain beyond, however, it is no longer a significant player in the salinity story. There is insufficient recharge to sustain the old salinity inducing water balance. Instead, the water balance on the saline land is now a function of local episodic recharge at the upland-plains interface. Groundwater is 'topped up' within the saline land when it does rain and recedes when it does not rain
Salinity at Kamarooka has, accordingly, crossed a threshold that now has catchment processes giving way to super-local processes within the degraded land. In the plantations, however, these local processes are interrupted by an alternative water balance as the trees are forced to source their water from the saline groundwater below their root zone.
Figure 3 - Watertable at Kamarooka over the life of the project
The watertable at Kamarooka reflects the climatic trend shown in figure 2. When annual rainfall is depleted relative to the longer term average the water balance shifts and the available water is used by evaporation and transpiration by vegetation. Recharge is eliminated and the groundwater recedes through a combination of down-based flow or in the instances of a shallow watertable through evaporation at the land surface. The hydrographs in figure 3 depict the watertable under the plantation established in 2004 falling from about one metre below land surface to about 3.5 metres by early 2009. The process is repeated in the dry times following the wet years of 2010 and 2011.
For the purposes of clarity hydrographs post 2010 are presented in figure 4. The groundwater recession in response to growth of eucalypt plantations is very evident in bores 7 & 8 within the 2004 plantation as is the recession within the biodiversity plantation comprising acacia, bluebush and other native shrublike species.
Figure 4 - Groundwater hydrographs post 2010
We have been able to shift the water balance through the establishment of eucalypt plantations and other areas of woody vegetation. Consumption of both surface water and groundwater by the trees has lowered the watertable significantly relative to the adjacent non-treed terrain. The watertable beneath the plantation was only one metre deep in 2004, and in 2017 it is now deeper than seven metres (depth of our bores).
Figure 3 and figure 4 also demonstrates the difference in groundwater responses beneath the plantation and adjacent agricultural land during infrequent episodes of seasonal high rainfall. The difference is particularly evident during the wet spring of last year (2016). The watertable across most of the site had receded to four or five metres following a series of very dry years. In the second half of 2016 Bendigo received 451mm of rainfall bringing the total for the year rose to 677mm or about 130 mm above the long term average. The wet spring saw the watertable rapidly rise within one metre of the surface in the all areas that did not have woody vegetation. Within the 2004 plantation, however, the watertable did not respond with groundwater remaining deeper than seven metres.
Thirteen years after the establishment of the 2004 plantation the trees are sufficiently large to control the water balance causing the elimination of groundwater recharge even during the wettest years. In this regard our 2004 plantation now emulates the hydrology of the native box-ironbark forests of the region.
It is interesting to note, however, that the 2005 plantation along the eastern margins of the site has not had the same impact on the watertable as witnessed in the 2004 plantation. The watertable remains at a depth of about two metres. The reason for this is the difference in plantation design. The 2004 plantation has a much larger width compared with length and this affords a much more effective groundwater drawdown than is possible within the longer but much narrower (about 50 metres) 2005 plantation.
Figure 5 Groundwater hydrographs from 2005 plantation
The groundwater hydrographs from the 2005 plantation presented in figure 5 (bores 11, 12, and 13) clearly depict a different response to that recorded in the 2004 plantation. There is no lowering of the watertable and no groundwater depression. The trees thrive despite the shallow watertable but the 2005 plantation is not effective in groundwater control in the same way as the 2004 plantation. Note that bore 13 is on slightly higher ground than 11 and 12 and, accordingly, the depth to watertable appears a little deeper.
Drawdown adjacent the 2004 plantation
Another interesting aspect of the Kamarooka experience revealed through the groundwater monitoring is the impact the 2004 plantation is now having on the watertable in the adjacent saline land. This is evident from the hydrographs constructed from two monitoring bores extending form about 10 metres south of the plantation (bore 6) through to about 50 metres south (bore 5). The results are presented below in figure 6.
For the first seven years of the project a deepening depression in the watertable beneath the plantation did not extend into the adjacent lands to the south. In the dry times that followed the very wet 2010 and 2011 years, however, the depression grew considerably as the trees eliminated recharge and transpired some of the saline groundwater. Groundwater along the margins of the forested land began to fall relative the non-forested land within the interior. The results are reported below in figure 3 illustrating the separation of the two hydrographs post 2011.
Figure 6 - Groundwater drawdown adjacent 2004 plantation
I have added a conceptual diagram (figure 7) that helps explain the drawdown adjacent to the 2004 plantation. The forest behaves as a pump which removes surface water and saline groundwater through transpiration. This in turn creates a depression in the groundwater surface and as the trees mature the groundwater depression extends into the adjacent non-forested saline land. Lowering the watertable immediately adjacent the forest assists salinity management in this area.
7 - Conceptual diagram illustrating the deepening groundwater depression
under the 2004 plantation and adjacent lands
links below are to the recent videos of the Kamarooka Saltland Restoration
courtesy of Philip Ashton and his new drone.
Minutes of October Meeting
Minutes October 2017