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UNIT 2

Saltland Basics

 

2.4 The case for preventing recharge

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Most land affected by secondary dryland salinity in Australia is caused by the planting of agricultural crops and pastures which use less water than the previous native vegetation. The rainfall in excess of crop and pasture use leaks below the root zone (recharging the groundwater system) and causing saline groundwater to rise with salt mixed throughout the soil profile rather than being stored at depth below the root zone of the native vegetation. At low points in the landscape, this saline groundwater might come close enough to the soil surface to enter the root zone, or even discharge directly at the surface.

The predominant emphasis of research and management during the 1990s and early 2000s was to reduce this recharge, thereby preventing dryland salinity by removing the cause of the rising groundwater. Indeed, the landmark report from the Prime Minister’s Science, Engineering and Innovation Council in 1998 recommended that – “Dependent on the value of the areas under consideration, and on the costs and benefits of treatment, the emphasis should move from treatment of symptoms to treatment of causes”.

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Where does recharge occur?

When rain falls on a paddock, there are essentially three things that can happen to it. Some will be stored; in the plants, on the soil surface, or in the soil itself and then eventually be evaporated back into the atmosphere either directly or through plants. Some might leave the paddock as surface run-off. Some might seep through the soil beyond the plant’s roots until it reaches the watertable and so recharges the groundwater. This is a very simple definition of the ‘water balance’, and much more comprehensive analysis of recharge and its impacts on catchment hydrology are available.

Recharge can occur virtually everywhere (including on discharge areas), as agricultural systems based on annual species rarely use all the water that falls. However, recharge is much more likely when annual rainfall is high and winter dominant (reducing evaporation); the surface soils are permeable; the landscape is relatively flat; the rooting depth of crops and pastures is shallow; there are no impermeable layers (rock or heavy clay) to prevent the drainage reaching the watertable; and when long fallows are used. Though every catchment is different, significant recharge is often associated with fractured rock systems or with deep alluvial sands, rather than with more fertile soils that are often found lower in the landscape.

Not all recharge is bad – indeed, some is essential, particularly when the groundwater has low salinity. Recharge replenishes groundwater systems that provide base flow into rivers and creeks so that they flow between rainfall events. Recharge also provides the water that replenishes Australia’s artesian basins. However, when the water balance is upset so that recharge exceeds the natural outflow of the water from the groundwater system, flooding may result. When the groundwater is saline, this discharge can have even more serious consequences for stream water quality, for pastures and crops, and for native bushland, riparian vegetation and wetlands.

There are, however, some key problems associated with controlling recharge as a mechanism for salinity prevention discussed in section 2.4.

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What plants reduce recharge?

The native vegetation prior to European settlement was ideally suited to preventing excessive recharge as the climate and the vegetation had established a balance over a long time period. Whilst seasonal conditions (particularly rainfall) varied from year to year, the water balance in the catchment was essentially stable and watertables did not rise or fall significantly. Indeed, in landscapes receiving less than 900 mm per year, the native vegetation systems allowed very little recharge and only occasional run-off. Replacing the native vegetation with annual crops and pastures upset this balance, and restoring it now by large scale revegetation would eventually restore the ability of those catchments to prevent excessive recharge. However, this environmental gain would be very slow to materialise and would be impose enormous economic and social costs, and the success and time scale would depend on the extent of the particular groundwater flow system where the catchment is situated. 

Commercial or farm forestry can generally make use of as much of the rainfall as the native bushland, but the dramatic reduction in the volume of fresh surface water flows can be a serious issue in some catchments. Furthermore, farm forestry options are only likely to be profitable in high (and occasionally moderately) rainfall zones.

Of the widely-used, agriculturally important species, only lucerne approaches the recharge prevention ability of native bushland and commercial forestry. However, there are real limits to the proportion of a farm over which the growth of lucerne will be profitable. The CRC Salinity has identified those areas of southern Australia where lucerne has agronomic potential and has determined the optimal economic levels of adoption for different farm enterprises.

The vast majority of agriculturally important crops and pastures are annuals that only use water from late autumn to late spring, and these are the primary cause of the increased recharge across the landscape. Perennial pastures, based on either introduced (e.g. phalaris, cocksfoot, fescue) or native grass species provide some recharge control and can be can be utilised on a landscape scale, at least on grazing properties where the groundwater flow system is local.

Case Study 1 - Kamarooka, Central Victoria

The National Land & Water Resources Audit examined two options for reducing recharge and overcoming salinity in this catchment in central Victoria – either a 50% reduction in recharge through conversion of annual pastures to lucerne or a 90% reduction in recharge through extensive reafforestation.

The Audit report concluded that a 90% reduction in recharge would give a 50% reduction in the area of saline land within 20 years and a complete reduction within 100 years. However, this was unlikely to occur because of an expected 40% reduction in net farm incomes over a 50-year period.

The 50% reduction in recharge was predicted to give a much slower response, but would still eliminate salinity from the catchment after 100 years. Indeed, the modeling predicted that such a change (annual pastures to lucerne) would also be about 40% more profitable than current systems. Again though, the Audit concluded that such a change was unlikely because of constraints such as the complexity of lucerne not fitting well in farming systems dominated by small holdings and off-farm income.

Full case study

Given the relatively low profitability of these two strategies for controlling recharge, it is not surprising that the Kamarooka farmers have now focused on the growth of trees and saltbushes on land at risk of salinity as their main amelioration strategy. This area now has a strong reputation as a showcase for these solutions.

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Positive outcomes?

There are in fact, few examples of positive catchment scale outcomes from interventions that aimed to reduce recharge. This is partly because in some cases it took decades for salinity to appear and a similar time, at least, can be expected before the problem is reversed. In fact, the time lag is effectively greater because salt affected land seldom returns completely to its unaffected condition.

However, the main difficulty is that recharge occurs over large areas in most catchments, so that large areas of perennial vegetation need to be established to intercept recharge if a significant reduction in total catchment recharge is to occur. Even this would need to be matched with patience to wait, in some cases for hundreds of years, to fully see the desired result.

In the Denmark River Catchment on the South Coast of WA, large-scale, profit-driven adoption of blue gums has transformed the region, and reversed salinity trends in the river, but there are few other cases to match this. In this case, the area had a high rainfall, and was close to a port so the widespread adoption of farm forestry was viable. Furthermore, as mentioned above, the impact of forestry on run-off water can be a serious detriment to catchment health.

Given that there are few perennial options that are commercially viable, there are few examples of catchments that have had sufficient intervention to materially impact on the catchment water balance. The CRC Salinity and the Future Farm Industries CRC were established largely to overcome this lack of commercial options.

At the individual farm level however, there are many examples of positive outcomes where the groundwater systems are small and local, so that the recharge is adjacent to the discharge, and the time between action to reduce recharge and the impact on discharge is relatively short. SALT Magazine has featured many of these, including some where quantitative data appear to support the claim. For more information, see Challenge of rehabilitating a saline site ultimately rewards or Salinity management in a variable landscape. However in the absence of controlled experiments, it is difficult to confidently differentiate between the impact of recharge abatement tactics and factors such as other land use practices, the impacts of climatic change, and influences elsewhere in the catchment.

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Difficulties with reducing recharge

Reducing recharge has major emotional appeal – it deals with the cause rather than the symptom, and it fits well with sustainability and the general notion of restoring landscapes to a less damaged condition. Who would not want to overcome salinity or ‘win the war’ against salinity? Unfortunately, there are four significant problems with the approach:  

  • Many of the proposed high water use systems (such as agroforestry) are less profitable, and/or more complex, and/or require major up-front investments compared to the annual crops and pastures they potentially replace. Farmers are unlikely to voluntarily adopt such systems. Presently, the economic incentive is to keep growing crops at the expense of a potential increase in the area of salinity.

 

  • Vegetation options that reduce recharge (lucerne, forestry, native bushland) also reduce run-off – in fact they usually cause a greater reduction in run-off than in recharge. This has a range of economic and social impacts from dry dams on farms, through to impacts on urban communities and other agricultural systems that rely on drawing water from rivers and steams. Major urban and irrigation industries and infrastructure have been built up on the expectation of significant good quality surface water flows. Recharge management cannot proceed independently from consideration of these water demands.

 

  •   The long time lag that can occur between recharge occurring and the eventual appearance of salinity in the landscape also works in reverse. This means that while in the long term the outcome for river water quality might be positive, in the short term saline discharge into streams continues unabated, while surface run-off (i.e. fresh water to dilute the saline inflows) declines. This long lag time, as well as the associated uncertainty of ‘success’, makes it difficult to secure a competitive return on the investment of public or private dollars.

 

  • In many situations it is not possible to accurately identify the exact location where the recharge is occurring. This is particularly the case for intermediate and regional groundwater systems – see groundwater flow systems. The extent of groundwater flow systems underlying a catchment has significant impact on the potential success of plant based systems in controlling recharge. If the groundwater flow system is local in a moderate rainfall area there is reasonable chances of success. However, with intermediate and regional groundwater flow systems recharge control is beyond the capacity of vegetation especially where recharge and discharge occur on the same area.
     

 The result of these issues has been a significant rethinking of the approach to dryland salinity. It is very difficult to confidently make either public or private investments that reduce recharge when the cost is high, large areas need to be revegetated, the lag time between the investment and the elimination of salinity may be 100 years, and there is a significant chance of associated negative outcomes.

Case Study 2 – Mid Macquarie NSW

Researchers in the CRC for Catchment Hydrology modeled the effect on water run-off and salt load of different reafforestation scenarios in the Mid-Macquarie. Two scenarios involved tree planting on 22 % (‘focus’ areas) or on 2.6 % (targeted locations within ‘focus’ areas) of the catchment. They found that:

  • Under the 22 % planting scenario, salt loads would decrease only marginally 50 years following reafforestation.
     
  • Due to the long time required for changes in salt load to be expressed in stream salinity, salinity would get worse before it got better.
     
  • Under the 2.6 % planting scenario, salt loads would continue to increase and be only marginally less than the ‘do nothing’ scenario.
     
  • An associated economic analysis showed that none of the ‘best bet’ options for salinity control were economically viable for landholders in the absence of subsidies to implement the options.

Full case study

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