Irrigation effects on ecosystems in the Murray-Darling Basin

The Murray-Darling Basin provides about 40% of Australia's agricultural output by value. As I posted yesterday irrigators in the MDB face immediate difficulties because water allocations might be set at zero over the current year unless dramatic rains occur over the next 8 weeks.

I now comment on issues of water management in the MDB in relation to ecological concerns - specifically to the promotion of the second major class of outputs in the MDB, biodiversity outputs.

Most of these comments I base on an unpublished article by Terry Hillman, ‘Ecological Requirements: Creating a Working River in the Murray-Darling Basin’ (to be published later this year in L. Crase (ed) ‘Water Policy in Australia: The Impact of Change and Uncertainty’).

I found Hillman's article of considerable value in trying to understand water resource management implications for ecology.

Hillman is concerned with the relationship between ‘working rivers’ – river ecosystems supporting a water resource used by humans. Natural resource managers have a duty of care with respect to such ecosystems if the water resource is to be maintained in a productive condition.

The MDB covers 14% of Australia but because the continent is so dry it is a ‘small’ river system in terms of water flows. Diverting water to agriculture reduces the average supply of water to the river system. Volumes of water flowing to the sea are now less than 30% of those under natural conditions. This has put pressure on the complex estuarine/freshwater ecosystems of the Corrong and the decline of River Redgum sites in the Chowilla area.

Water flows are also naturally highly erratic and unpredictable – left unregulated the Murray-Darling Rivers can be expected to cease flowing several times per century. There is also high seasonal variability in water supplies. This has meant that native flora and fauna in the MDB have developed capacities to adapt (and even to depend on) unpredictable hydrological outcomes. Hillman argues that it is these effects rather than volume effects which do most damage to ecosystems.

Irrigators need water and they particularly need water when it would be naturally scarce namely in the hot dry months. The diversion of water to human use and the smoothing of supplies are drivers for ecological change. Thus floodplains contain a mosaic of biodiversity types that reflect hydraulic heterogeneity which creates distributary channels, backwaters, billabongs, wet meadows and hence a variety of plant and animal communities. Hillman emphasises this perspective by reversing the standard view that the MDB’s primary role is to deliver maximise values for irrigators and to look instead at how well modern river management serves ecosystems.

Seven effects of smoothing the availability of water supplies:

1. Inter-Basin transfers. Not common in the MDB with the exception of water diversions from east-flowing rivers to the Murray and Murrumbidgee. Potentially there are ecological effects of such transfers and damage to the Snowy system has been one well-recognised effect of this arrangement.

2. Physical barriers. Weirs and dams are used to smooth seasonal water availability but the implied barriers prevent native fish and other marine invertebrates from migrating either seasonally or in response to flow conditions. The disruption of natural flow also restricts species dispersal creating significant spaces between population and hence habitat fragmentation. Sediment movements downstream are also inhibited. Reduced longitudinal connectivity limits the capacity of biological systems to adapt to such things as climate change.

3. Depressed summer temperatures. In deeper water bodies, during summer months, a stratification occurs whereby deeper layers get colder and lose oxygen content. When dams release water from low levels these effects can be detected for hundreds of kilometres downstream. This can inhibit or prevent fish and insect breeding. Reaches of the river system where temperatures are depressed problem depend on tributaries to maintain viable populations.

4. Inverted seasonal flow. Ironing out seasonal variability to synchronise water delivery with crop demand is achieved by capturing runoff in winter and early spring and releasing it during the hotter months. This inverts seasonal flows in areas downstream of major storages but upstream of major irrigation diversions. In conjunction with long periods of constant flow, high summer flows suppress the establishment of riparian plants and thereby pose a serious threat to bank stability. These damages are exacerbated by cattle grazing and turbidity.

5. Modified short-term flows. Loss of daily short-term river flow variation reduces a river’s productivity by reducing the zone through which sunlight can penetrate thereby limiting photosynthesis and diversity in this community - the biofilm. Constant water flow also concentrates water action at one stratum creating erosion notches resulting in bank slumping.

6. Removal of a flow class. Smoothing out very high and very low water flows and diverting excess flows into off-river storage for irrigation has important ecological effects. Maintaining stable low flows can create bank notching while diverting ‘excess’ flows limits inundations of floodplains damaging species dependent on such events. Periodic inputs of organic litter from floodplains help to restock the fertility of a river system. Reducing lateral connectivity between the river and its adjacent floodplains reduces the ecological productivity of a river system.

7. Changes in the frequency of significant flow events. A number of ecological processes (fish migration, water bird breeding) are cued by high water flow events and the timing of such events is made less frequent by the current water management regime. The lifespan of some bird species which depend on extreme flow events to induce breeding is now less than the interval between maximum flow events.

The effects of these interventions on biota can be classified into four groups:

1. Fish. There are about 40 species of native fish in the Murray/Darling although in catchments surveyed less than 50% of these species are being sighted. Overall fish numbers are 10-12% of their pre-1750 numbers. The reduction in numbers is due to in-stream barriers, depressed temperatures, seasonal shift of high flows and reduced frequency of high flow events. Some species are cued to migrate and breed by high flow events while others depend on high flow for successful movement. Alien species have impacted on numbers but poor flow management compromises the resilience of native fish populations.

2. Waterbirds. Waterbird rookeries occur throughout the MDB in non-permanent floodplain water bodies. Most have significantly declined over the past 50-100 years. Some wetlands must fill up and then remain full for long periods while others need to be fed from adjacent rivers. If these requirements are not met breeding success will be low. Increased modification of flows will reduce such successes.

3. River Redgum. This is the major floodplain tree along the Murray. It sets deep tap roots that reach into the groundwater and even where this water is quite saline the tree can survive for years. Flowering and seed set however require fresh water to feeding roots and this requires periodic inundation in an appropriate season. Lack of such flooding through stream-flow smoothing or continuous inundation will kill the tree. Reducing the frequency of flooding also allows competition from species such as Black Box.

4. Aquatic plants. These plants live under varying degrees of water cover – from totally submerged to emergent but capable of surviving for periods without water. They provide habitats for fish and invertebrates as well as protection from the erosive effects of river flow. Seasonal inversions and constant flow patterns have a negative effect on the extent and diversity of plant communities – particularly in conjunction with high turbidity.

Consequences for management.

1. One aspect of improving the ecological productivity and resilience of a working river is to reduce the direct human uses of water from the system. Another source of improvements is just to change the way the river system is managed without changing the quantity of water supplied.

2. Fish ladders can improve longitudinal connectivity by improving upstream migration opportunities. Other connectivity failures will not be addressed by such moves.

3. Lateral connectivity can be improved by periodically directing water supplies to floodplain areas.

4. Temperature changes induced by drawing water from the bottom of reservoirs can be dealt with by expensive retrofitting which allows for water to be drawn from other levels.

5. Seasonally inverted flows are damaging particularly when combined with inter-basin transfers such as in the Tumut River. These damages can be offset by fitting regulators to billabongs and larger floodplains to restore their ecological function.

6. Retiring water from human productive use is a last resort and should be achieved by increased efficiency if possible. Such efficiencies might involve agriculture or support for the environment. Allocations to improve the ecology require an investment in achieving good ecological knowledge.

7. Ecological allocations will need to be adaptive, strategic and flexible. Fixed rules won’t work though environmental reserves should be maintained. Management protocols need to direct water saved for ecological purposes to its best uses.

Given the likelihood that water availability will fall and temperatures will rise in the MDB over the next 20 years, due to climate change, the case for the policies suggested by Hillman will presumably intensify.

For the most part they are 'strengthening resiliance' policies although it would be interesting to contemplate policies that specifically address climate change concerns and which account for complementarities with agriculture. For example addressing salinity issues dirtectly promotes both agricultural and biodiversity outcomes and has a direct link with water supply policies.