In a paper presented to the 2013 Fenner Conference in Canberra, I described the economic opportunities and environmental risks that may arise from the proposed agricultural expansion in Australia’s tropical north. I had blogged on that same topic back in Nov 2011 (Northern Australia’s fascinating wetness) and this was a chance to update my own thinking, based on extensive scientific and economic analysis undertaken by CSIRO and the Northern Australia Task Force. The new liberal (read ‘conservative’ for foreign readers) government in Australia had just re-activated calls for a new Australian ‘food bowl for the world’ in our tropical north, built on the perception of abundant water and fertile soils, so it seemed most timely to do so.
The proceedings of that Fenner Conference will be out soon, so I thought it might be useful to highlight here some of the key points I make in that paper, broadly addressing two main questions:
As Australia looks increasingly to its tropical northern lands as a prospective ‘food-bowl for Asia’ we should reflect on two important questions:
(i) Have we gained sufficient knowledge and wisdom from a century of unsustainable irrigation practices in southern Australia to do things differently in the future?
(ii) Is Northern Australia really the agricultural utopia that some in the community argue, and do the potential rewards justify the risks to our largely pristine and biodiverse tropical river basins?
In part one in this series (from the conference paper), I first describe the environmental consequences of water resources development in Australia’s south – in the Murray-Darling Basin.
Part 1. Understanding and managing the causes of environmental damage arising from irrigated agriculture
1.1 Landscape and Catchment impacts
The sustainable management of land and soils has always been a fundamental challenge for Australia and, unfortunately, it is something we were poor at for a very long time. The combination of unconstrained clearing of native vegetation, particularly in hilly terrains, combined with the erosive power of high-intensity Australian rainfall led to massive soil erosion problems in many farming areas.
This led to a double-whammy outcome where (i) farmers lost fertile top-soils and damaged their lands through gullying, and (ii) the river environment down-stream of these agricultural areas suffered as farm soils and sands were washed away during heavy rainfall, muddying river waters and smothering stream-bed habitats essential for the survival of riverine biota.
Along with soil and fine sediment, fertilisers and pesticides may be transported from farms into rivers, wetlands and coastal water, especially when they are applied poorly or in excess of crop needs. These too have a negative impact on receiving waters – either by stimulating unwanted algal blooms or by being toxic to native animals in the river system.
The positive news is that thanks to a combination of good scientific and agronomic research over the past two to three decades, combined with extensive on-ground trials by land-holders (funded and carried out through various programs such as Landcare and various regional National Resource Management groups) we now have a reasonably good handle on the most appropriate agricultural practices for stopping erosion and retaining soils on-farm, and for minimising fertiliser and pesticide run-off. For example, in the Great Barrier Reef coastal catchments, where restoration and prevention programs are in now place to improve farming practices which will minimise soil and fertiliser wash-off.
Another major problem in Australia agriculture has been soil salinisation. This has two different settings and causes, arising separately on dry-land and irrigation farms. In both cases, mobilised salt can travel from farms back into adjacent rivers – by natural run-off processes or via irrigation drains – causing river salinity problems tens or hundreds of kilometres downstream. This was the situation in which the people of Adelaide found themselves in the 1960s and 1970s, eventually necessitating huge engineering interventions[i] to stop the salt from reaching the River Murray and Adelaide’s water supplies which are drawn from it.
From a river ecosystem perspective, the biggest impact arising from the development of irrigated agriculture in southern Australia, indeed throughout the world, has been the building of large dams on rivers to store and distribute water . Dams on rivers have two major types of ecological impact. First, there are physical impacts – they are a barrier to the necessary upstream and downstream movement of aquatic animals, especially fish. Second, there are hydrological impacts – by capturing water for irrigation, dams reduce downstream flow volumes and velocity, and also change the timing and pattern of flows.
The hydrological changes can be highly detrimental to river biota, especially to the plants and animals living or breeding on the river’s floodplain. They rely on regular flooding to sustain their growth or to stimulate seed germination or animal breeding. Small to medium sized floods, occurring every year or so are of particular ecological benefit and it is these that are most reduced by dams (large floods pass through a river system more or less unaffected).
The dampening of the natural variability in downstream flows by dams also impacts on fish in the river channel, which rely on certain flow velocities or water depths eg. as a cue for migration. Water released from large dams is also often much colder than that naturally flowing in a river and this may also impact negatively on fish breeding.
On-farm dams – large and small – can also cause serious eco-hydrological problems, even if individually they are much, much smaller than on-river dams. When there are many in a catchment, across many farms, their combined impact on run-off and river flows can be significant (Nathan and Lowe 2012).
Further, in the so-called unregulated reaches of the northern Murray Darling Basin where there are no large dams on rivers, large on-farm dams harvest river waters during flood times for later use on crops, mostly cotton (these are often known as ‘ring-tanks’ because of the way they are constructed). While this might sound harmless enough, perhaps even beneficial, the combined impact of large ring-tanks on downstream river flows and biota can be serious.
Smaller weirs are built on rivers to provide a local head of water to allow gravity supply of water for irrigation (and for some towns). We learned early on that fish cannot move up and down the river to feed or breed if there are weirs blocking their path. Some weirs are removable or have gates that can be fully opened at certain times of the year to allow fish to move past. In other weirs, fish ladders were built to allow fish to move past them. Unfortunately our original designs were taken from Europe and were based on the behaviour of salmon. Salmon are fish that jump but our sluggish Australian fish are not much when it comes to jumping!
After some good local research in the 1980s and 1990s, we realised that ladders could be designed to better suit Australian fish (fig 4), and even fish lifts have been built where fish can swim in at the bottom, get lifted up in a cage and swim out upstream at the top.
Finally, one other thing that that has been learned is that clearing vegetation right down to the water line of the river is not a good idea. The stream-side or riparian vegetation plays many key roles in maintaining a healthy river. It filters out (some but not all) nutrients running off the land before they reach the stream, as well as stabilising river banks, shading smaller streams and otherwise providing habitat for animals, aquatic and terrestrial.
Now that we properly understand the importance of river riparian corridors and the impacts of clearing, much restoration work has been undertaken. This includes physical works to reshape river banks (where badly eroded) and the re-establishment of endemic vegetation (and the removal of invasive, exotic species such as Willows where required).
One other catchment-scale impact, while largely out of sight, that should not be ignored is the (unsustainable) use of groundwater. Where pumping by farmers exceeds the rate of replenishment,[ii] the groundwater level decreases, making pumping more expensive or even impossible in extreme cases. At the same time, unsustainable pumping can negatively affect so-called ‘groundwater-dependent ecosystems’ (Murray et al. 2003). These could include certain types of wetlands where water supply from below is important (other wetlands rely on surface run-off only), some woodlands and the mound springs of the Great Artesian Basin.
1.2 Local or Habitat impacts
The range of local impacts of agriculture on river and floodplain habitats – the places where plants and animals live, feed and reproduce – is broad. Many are directly linked to the catchment-scale impacts outlined above, while there are others that arise due to distinctly local factors. Habitat degradation linked to agricultural practices in a river’s catchment include:
(i) Sand smothering of river bed habitats – impacts particularly on invertebrates that live and feed on the bottom of streams, and which are also the food source for many fish and other animals (eg. platypus) (fig. 2b).
(ii) Fine sediment run-off – makes water more turbid with lower penetration of sunlight into the water. In turn, this affects the ability of plants to grow below the water’s surface. These submerged plants are an important part of healthy river ecosystem.
(iii) Fertiliser run-off – stimulates the growth of nuisance, filamentous algae that grow on submerged logs and rocks – these crowd-out the formation of natural microbial biofilms which are a more palatable food source for river animals.
(iv) Changed local water depth, flow velocity or water temperature caused by upstream dams – may impact directly on the local habitat suitability for many animals including fish, turtles and mussels.
Habitat degradation that is not linked to upstream catchment condition, but arises due to local farming impacts include:
(i) Edge habitat destruction – caused by cattle given direct access to the river for watering. Many riparian restoration programs across Australia now fund farmers to fence off their lands from the river and to provide alternative watering points. Cattle defecating in streams under these circumstances also create local pollution problems as well as further downstream (including potential human health problems from drinking water which is contaminated by animal intestinal parasites such as Cryptosporidium and Giardia).
(ii) Levees and block-banks on the floodplains – farmers with lands adjacent to rivers may construct levees to divert minor flood-waters away from, or towards, certain parts of their property. While the local ecological effects of this may be minimal, there are situations where such works cause the drying out of wetlands or woodlands further along the floodplain.
1.3 Lessons learned
The upside of this wide-range of impacts that have arisen through the development of irrigated agriculture is that, as scientists, managers, farmers and concerned citizens, we have learned a great deal about how not to go about developing and maintaining a large, productive agricultural system! For any new irrigation development, including any proposed for Northern Australia, we can reasonably claim knowledge of the impacts of past agricultural practices and that we have learned ways of carrying out irrigated agriculture far more wisely.
To summarise, we have learned that sustainable irrigated agriculture should include the following catchment and farm-scale practices:
(i) Clearing land sufficient to grow crops and no more, retaining as much native vegetation on-farm as possible
(ii) Protecting vegetation along riparian zones, including fencing where necessary, and on steeper slopes or other areas of higher erosion risk
(iii) Adopting modern tillage and other agronomic practices that maximise water and soil retention on farm (and that enhance soil fertility)
(iv) Applying water sufficient to meet crop needs and no more, using modern high-efficiency irrigation delivery systems (viz. micro-irrigation, pressurised supply, etc.)
(v) Controlling the application of fertilisers and pesticides, at the lowest practical levels, and retaining any drainage waters on farm (unless otherwise proven safe to discharge)
(vi) Avoiding or minimising the need for dams, especially large dams. If dams must be built (and, to be clear, this is not desirable) the combined storage volume of dams on a river system should be much less than the mean annual run-off upstream of where the dam is to be built. (Note: in the Murray-Darling Basin the combined dam storage volume is 1.5 times mean annual run-off, hence the huge magnitude of their ecological impacts.)
(vii) Implementing regulatory controls on the construction of farm dams and floodplain banks and levees
(viii) Adopting the combined and sustainable use of surface water and groundwater – for example, between wet and dry seasons. This should be optimised to maximise supply reliability and to minimise impacts on all water-dependent ecosystems.
(ix) Applying ecologically-defined limits on the total amount of water that can be withdrawn for irrigation from a river or groundwater system in any season/year.
There are many other sustainable practices that should be adopted – this list is meant to be illustrative rather than inclusive. The key message is that ecologically-sustainable irrigated agriculture is technically feasible, if we have the intelligence and the conviction to implement it fully.
Whether or not politicians have the motivation or the will to fully implement the required approaches and practices is another question. Ecologically-sustainable irrigation comes at a cost – in increased system development and operating costs, in reduced area of agricultural farmlands or water available for production, or both. But the costs of environmental degradation itself are economically real – not just unforeseeable externalities – although they may not be observed for years or decades, certainly well beyond the life-time of most politicians.
Herein may lie the dilemma for irrigated agriculture in Northern Australia.
[i] These consisted of a network of salt interception bores and discharge/evaporation basins. See: <http://www.mdba.gov.au/annualreports/2009-10/chapter3-6.html>.
[ii] Replenishment of ground-waters occurs by local rainfall infiltration or transport from further afield via connected aquifers.