Wickham’s Lane – Tooborac
Phil Dyson & Associates P/L
An inspection of the property on 6 September 2007 revealed an interesting landscape present within the headwaters of Wild Duck Creek catchment. The owner requested the assessment in an effort to better understand the geomorphic nature of the land and to gain a better appreciation of the management implications arising from an appreciation of the landscape-groundwater processes active in the development of both dryland salinity and soil erosion.
Geology and geomorphology
The property is located immediately north of the metamorphic aureole known as the McHarg Range that surrounds the granitic rocks of the Cobaw Batholith. The terrain comprises moderately steep hilly lands interspersed with deeply dissected valleys. This hilly land is formed from folded, fractured and partially metamorphosed marine sedimentary rocks (metasediments) of Lower Ordovician age (figure 1). These rocks form the ‘spine’ of the Great Dividing Ranges in Victoria. They crop out throughout the property as prominent north-south trending sandstone ridges in almost all regions other than in the valley floors where they are overlain by alluvial sediments that infill the drainage depressions.
The soils on the rocky slopes are for the most part stony and skeletal and generally quite shallow in depth. They are typical of soils found on ‘rocky ridges’ through northern Victoria and they are known to have high permeability and to be naturally acidic. Low pH inspires both aluminium and manganese toxicity issues that are most often problematic for introduced (non
native) pastures and other vegetation. The alluvium found within the valley floors carries yellow duplex (texture contrast) soils that comprise fine sandy loam topsoils overlying sodic silty clay subsoils. The sodic subsoils are dispersive and, accordingly, prone to soil erosion.
Figure 1 Image depicting folded fractured metasediments outcropping on the shores of Lake Eppalock
Extensive gully erosion occurs within the valley floors. This is a consequence of a runoff regime imposed by relatively high long-term annual rainfall (about 700 mm), sodic soils, and extensive dryland salinity resulting from saline groundwater discharge.
Figure 2 Image depicting extensive rock outcrop and shallow stony soils in the catchment headwaters north of the metamorphic aureole (McHarg Ranges).
The Ordovician metasediments underlying the property form a fractured rock
aquifer that for the most part outcrops or partially outcrops throughout the sloping lands. The aquifer is formed from fractures in the rock that that are open to depths ranging from fifty to a hundred metres. Aquifer recharge is greatest where the rock mass outcrops or where it carries thin highly permeable soils. In this instance almost all of the property other
than the groundwater discharge zones within the drainage depressions can be classified as having a propensity for very high groundwater recharge. Evidence of elevated groundwater recharge is apparent from the presence of extensive saline groundwater discharge zones within most valley floors in the immediate region of the sloping lands. Groundwater recharge occurs on the hills causing extensive zones of groundwater discharge in adjacent valley floors. Zones of valley floor saline groundwater discharge are apparent from infestations of Spiny Rush (Juncus acutis). This plant is a salt tolerant volunteer weed that colonises moist areas where groundwater lies within capillary reach of the land surface. In the clay soils of the valley floors this is most likely to occur where the watertable is less than 1.5 metres deep.
Figure 3 Localised groundwater flow from steep hills realising saline groundwater discharge in adjacent breaks of slope and valley floors
The relatively high (long-term) annual rainfall together with high rates of aquifer recharge suggest the groundwater salinity is likely to be at the lower end of the range of values that normally cause dryland salinity. Indeed it likely that the salinity of groundwater occurring within the sloping terrain may be as low as 2,000 to 4,000 mg/l. In this sense the groundwater may have some
potential as a resource in times of extended drought and/or climate change.
Land degradation issues
Active zones of saline groundwater discharge contribute markedly to the sodicity of soils found within the valley floors. The soils are naturally sodic but this condition is exacerbated where they are bathed in the groundwater that has a chemical composition high in sodium. This ion is readily absorbed onto the exchange sites on the surface of the clay adding to the potential for excessive dispersion and consequent erosion issues.
Land management – role of vegetation
It is difficult to consider land management strategies within this terrain within any great certainty because most of the options are intimately linked to climate, and we live within times of considerable climatic uncertainty. However, given an appreciation of the terrain outlined above some general principles that can be established. The discussion of land management fits within the soils and water balance framework for the property and reflects
(a) options in terms of the management of lands in which groundwater recharge occurs and
(b) options for areas in which groundwater discharge occurs.
For the most part the groundwater system appears to be constrained locally to the immediate hills that surround the valley floor groundwater discharge zones. Accordingly, conventional wisdom suggests the salinity problem is best tackled at the cause by reducing excessive groundwater recharge. Whilst this philosophy may be appropriate the practical difficulties in achieving it should not be understated. The difficulty stems from the extensive occurrence of high recharge landscapes carrying thin acidic soils of low water holding capacity, in a
climatic regime subject to relatively high winter rainfall. Under these circumstances it is very hard to ‘drive’ the water balance of the catchment in order to mitigate groundwater discharge.
Pasture based options
It is unlikely that conventional perennial pasture options will afford a salinity benefit in this landscape. The long-term rainfall is too high for effective pasture based control of groundwater recharge, and the soils are too acidic for conventional introduced grasses. Native grasses adapted to the soils probably afford the best option given their propensity to cope with the soil constraints. Even native perennial grasses, however, will not afford sufficient recharge reduction to make a meaningful difference to the sub-catchment
Targeted tree planting on ridges
Targeted tree planting on upper slopes and ridges in regions where there is significant rock outcrop affords a good option for groundwater recharge reduction where the area planted is substantive relative to the area of the catchment.
Aquifer recharge in these lands, however, is unlikely to be significantly greater than the surrounding areas of cleared agricultural land and, accordingly, expectations need to be managed. It is likely that the recharge reduction within a plantation will be substantial, but the consequent reduction in groundwater discharge within adjacent valleys will be a function of the area of the plantation relative to the area of the sub-catchment. For example, if five percent of a sub-catchment is planted to trees it is unlikely that this will realise much more than a five percent decrease in groundwater discharge.
Break of slope plantations
Low salinity groundwater lying close to the land surface on breaks of slope between rocky hills and valley alluvium may afford opportunities for plantations. Trees may have some potential to behave as phreatophytes using groundwater to supplement their transpiration needs. There is, however, limited experience with this concept in this terrain so there can be
no certainty that it will be effective. It would seem, however, to be worthy of some assessment, particularly if the desired outcomes is for plantations that are less inhibited in their growth by summer drought. The expectation of break of slope tree planting also has to be measured. The option is best deployed in anticipation of optimum tree growth rather than a
sense that it will deliver outcomes that are substantive in terms of realising mitigation of groundwater discharge and salinity.
Break of slope tree planting in the fractured rock terrain provides some opportunity for vegetative groundwater harvesting, but the water taken comes only from the uppermost sections of a moderately transmissive and quite thick aquifer. The capacity for groundwater to flow beneath the plantations is, thus, considerable.
Plantations within groundwater discharge areas
Plantations established in the midst of the valley floor groundwater discharge areas are unlikely to perform well. The sodic soils lack the soil structure required for optimum growth and the saline groundwater is likely to be likely to be anoxic (depleted of oxygen). Salt tolerant grasses are most likely a better option in these environs.
Amelioration of sodic soils
The yellow sodic soils found within the valley floors should respond well to gypsum. Gypsum is a natural salt that comprises calcium sulphate. When it is incorporated within the soil the calcium displaces the sodium on the exchange complex of the clays imparting considerable improvement in soil structure. This can be very useful in providing young trees with the optimum conditions at the time of planting.
Summary and conclusions
The property is located in the headwaters of the Wild Duck Creek catchment immediately north of the metamorphic aureole that comprise the McHarg Ranges. The geology comprises folded and fractured metasediments that behave as a fractured rock aquifer permeable to depths of fifty to one hundred metres. The groundwater is expected to be saline, but at the low end of groundwater of the salt concentrations that normally cause dryland salinity. Most of the cleared landscape has a high propensity for groundwater recharge because it carries thin highly permeable soils of low water holding capacity. These soils are known to naturally acidic with consequent aluminium and manganese toxicity issues for non-native pastures. Sodic yellow duplex soils form on valley floor alluvium and sodicity issues are exacerbated by saline groundwater discharge. The dispersive nature of these soils makes them prone to soil erosion.
Under the normal long-term climatic regime aquifer recharge would be quite high on most of the hilly lands. Groundwater flowing down the landscape out of this terrain realises extensive groundwater discharge in adjacent valley floors. There does not appear to be significant variation in soil type on the sloping lands thus it is probable that most of the sub-catchment above each valley floor groundwater discharge zones will contribute equally to the problem. The present practice of establishing plantations on upper slopes and crests is an appropriate form of recharge management in this terrain. Substantive areas of plantations, however, will be required to establish to have an impact on the water balance that would result in mitigation of the valley floor groundwater discharge zones. It is unlikely that perennial pastures will have a role in salinity management within these lands as the long-term average3 annual rainfall is too high and the water holding capacity of soils too low for this strategy to be effective. Break of slope plantations along the margins of valley floor alluvium may afford opportunities for plantations that have some capacity to transpire water from the underlying shallow groundwater system. Anoxic groundwater and low soil permeability may inhibit plantation establishment within the central regions of valley floor groundwater discharge zones. The suitability of sodic valley floor soils for tree plantations may be improved
by the addition of gypsum and organic matter.