Environmental flows, river salinity and biodiversity conservation: managing trade-offs in the Murray Darling basin

Transcription

1 CSIRO PUBLISHING Australian Journal of Botany, 2003, 51, Environmental flows, river salinity and biodiversity conservation: managing trade-offs in the Murray Darling basin Kevin F. Goss Murray Darling Basin Commission, GPO Box 409, Canberra, ACT 2601, Australia. Abstract. The Murray Darling basin s river system suffers from over-allocation of water resources to consumptive use and salinity threats to water quality. This paper draws attention to the current state of knowledge and the need for further investigations into the biological effect of river salinity on aquatic biota and ecosystems, the threats of dryland salinity to terrestrial biodiversity, and managing environmental flows and salinity control to limit the trade-offs in water-resource security and river salinity. There is growing evidence that river salt concentrations lower than the normally adopted threshold have sublethal effects on species and ecosystems, over a longer time period. Further knowledge is required. There is no agreed process for incorporating terrestrial biodiversity values at risk into a strategic response for dryland-salinity management. This is a public policy issue to be addressed. Recent studies have quantified the trade-off in surface water flow and river salinity from refforestation and revegetation of upland catchments to control salinity. The potential losses or benefits to environmental values have not been quantified. Such improved knowledge is important to the Murray Darling basin and relevant to other river basins and catchments in Australia. BT0303 K. River F. Gsalinity oss and biodiversity conservat ion i nthe Muray Dar ling basin Introduction Salinity in Australia has damaged natural resources and infrastructure. Dryland salinity has an impact on terrestrial biodiversity in situ. Aquatic biodiversity is at risk from salt washed off the land surface or seeping directly into streams and rivers from rising groundwater. Inland river systems may redistribute this salt from upland catchments to flood plains, affecting biodiversity values and consumptive use of water downstream. Most salinity-management strategies in Australia require adoption of plant-based land-management systems that reduce groundwater recharge, thus tackling the cause of salinity rather than the symptoms. However, re-introduction of deep-rooted perennials in particular can result in a further rise in river salinity in the short term, at the expense of water-resource security and river salinity. The Murray Darling basin s river system has been highly regulated and its water resources over-allocated for consumptive use, prompting a current policy initiative to enhance environmental flows. In this specific case, managing salinity, environmental flows, water-resource security and biodiversity conservation requires careful consideration. In this context, this paper draws attention to current state of knowledge and the need for further investigations in the following areas: (1) biological effect of river salinity on aquatic biota and ecosystems; (2) threats of dryland salinity to terrestrial biodiversity; and (3) managing environmental flows and salinity control, to limit the trade-offs in water-resource security and river salinity. Murray Darling basin: a special case Given the high degree of river regulation and diversion in the southern Murray Darling basin and that the river system is highly inter-connected through anabranches and watersupply channels, there is a close relationship between dryland salinity and irrigation salinity. While ~40% of river flow arises in the largely forested alpine catchments above the Hume Dam, the remainder comes from tributaries with their head waters flowing through cleared farm land to the west and north on the inland slopes of the Great Dividing Range. The salt mobilised in these catchments and moving to the rivers is predicted to double in the next 100 years. The best estimates are that ~60% of this induced salinity will be CSIRO /BT /03/060619

2 620 Australian Journal of Botany K. F. Goss re-stored on the floodplains, in wetlands and irrigation areas (Murray Darling Basin Ministerial Council, MDBMC, 1999). For example, the amount of salt exported from the Coleambally Irrigation District to the Murrumbidgee River in (through elevated groundwater and drainage) was exceeded by the amount of salt entering that system (Coleambally Irrigation 2001). In short, irrigation agriculture faces a double threat on-farm salinity and rising salt loads from dryland catchments. That most of the mobilised salt never leaves the basin is easily explained by its hydro-geology (Evans et al. 1990; Williams and Goss 2002). It is a large, sedimentary basin with its groundwater system draining to a point just west of its centre. The southern outlet to the sea is quite restricted. Once tributaries leave the uplands, their fall is very slight: 200 m in 2000 km. The rivers have little energy, and meander across the plains with a complexity of anabranches, effluent streams and flood runners. The low and variable rainfall at the basin s central and western areas is another reason for the sluggish rivers. The sedimentary rocks offer little capacity to store additional water, and water tables quickly rise under increased recharge following land-use change. By way of contrast, many of the coastal rivers on which we are dependent for urban and irrigation use, have their head waters in forested catchments with much steeper gradients and flow to the sea with considerable energy. Under current policies, water for consumptive use in the Murray Darling basin is a scarce commodity. An audit of water use (MDBMC 1995) identified a threat to security of water supply unless growth in river diversion was halted. This resulted in a Cap on diversions effective from the irrigation season; from then on, new development must acquire water from efficiencies and savings or through water trading. Today the median flow of the River Murray to the sea is 27% of natural flow. Basin-wide the mean annual diversion is GL (including 1080 GL from the Snowy Scheme), compared to the mean annual runoff of GL, and the total capacity of major storages is GL or 1.4 times annual flow. Other inland river systems do not have the same high level of diversions and regulation for consumptive use. A review of the operation of the Cap on diversions concluded that it was not sufficient to prevent further decline in the environmental condition of the River Murray system (MDBMC 2000). The flow has been inverted to run constantly full supply in the summer months and in winter inflows are held back in storages. The many structures on the rivers impede fish migration, and water released from dams is colder and lower in oxygen than under natural conditions. Snags have been removed to allow navigation. The River Murray mouth experiences a higher frequency of drought conditions; currently an internal delta of accumulated sand is being dredged to prevent the mouth closing altogether. Restoring environmental flows The declining state of the riverine environment has been widely reported, following a snapshot report commissioned by the Murray Darling Basin Commission (MDBC) (Norris et al. 2001) and the National Land and Water Resources Audit (NLWRA, 2001a). Nearly the entire length of the river system (97%) is modified, with substantial or severe degradation (29% of river reaches) limited to southern tributaries. Sediments have built up and threats of algal blooms are imminent in many rivers. As an indication, current population levels for native fish are estimated to be 10% of pre-european settlement levels and 8 of 35 species in the basin have been declared threatened (MDBC 2002a). Under the direction of the Murray Darling Basin Ministerial Council, the Commission is now assessing the environmental benefits, economic costs and benefits, and social impacts at the following three reference points for additional flow in the River Murray: 350, 750 or 1500 GL per year on average (MDBC 2002b). By way of comparison, the agreed program for restoration of flow to the Snowy River is to add 210 GL per year on average. For both initiatives water will be found from re-development of irrigation supply systems ( savings ), but in the case of the Murray it may also come from reduction of consumptive use. The three reference points are based on ecological science that may have wider applicability. An expert reference panel advising the Murray Darling Basin Commission concluded that a river requires two thirds of its natural flow characteristics (in average volumetric flow and in flow variability) if there is to be a high likelihood of returning or maintaining it as a healthy, working river. At one half of its flow characteristics there is a moderate likelihood of achieving this level of ecosystem functioning (Jones et al. 2002). For the River Murray the 1500 GL per year reference point is roughly equivalent to half its former natural flow, and hence offers a moderate likelihood of a healthy, working river. Clawing back water for the environment is necessary but not enough for such an outcome. The Commission is planning and designing a works program to restore fish passage through all structures, alter structures to reintroduce variable flow and wetting drying sequences for wetlands, purchase easements for managed, moderate-level floods, and to rehabilitate priority river reaches. A River Murray Environmental Manager has been appointed to direct river operations for environmental benefits, and an environmental water-accounting system is being developed. All these potential changes are called The Living Murray initiative, with a proposal for a first-step decision on flow restoration to be considered by the Ministerial Council in November An initial five-year program of acquisition, works and management is being developed. Although, the water diverted from rivers was capped in 1995, the environmental state of the Murray Darling River system has

3 River salinity and biodiversity conservation in the Murray Darling basin Australian Journal of Botany 621 continued to worsen. Returning, or keeping, an as yet undecided annual volume of water in the River Murray will have major consequences for current levels and patterns of use. Controlling salinity The Murray Darling basin salinity audit (MDBMC 1999) predicted significant increases in river salinity, salt-affected land and in costs borne by irrigators, urban water users, and regional communities. The National Land and Water Resources Audit (NLWRA 2001b) soon followed it. In summary, the area of salt-affected land will not rise to the same proportions as in Western Australia, but salt concentrations in some tributary rivers and streams will rise in years to levels too high for continued consumptive use. End-of-valley wetlands and in-stream ecosystems in tributaries of central New South Wales and western Victoria are threatened. Salinity in the Lower Murray will exceed the World Health Organization standard for desirable drinking-water quality (800 µs cm 1 or 500 mg L 1 ) ~40% of the time, and incur additional costs of AUS$ per year for every electrical conductivity (EC) unit rise in salinity. Full costs to regional infrastructure, utilities and services are difficult to estimate but total $250 million per year for eight catchments (NLWRA 2001b) and as much as $1 billion per year for the whole basin in the future (MDBMC 1999). Since 1989 the Commission has administered the River Murray Salinity and Drainage Strategy for the purpose of reducing river salinity, while allowing for land drainage and other works to rehabilitate irrigation areas under the threat of salinity. Under this strategy median river salinity has been lowered and exceeding of the 800 µs cm 1 standard fell from 42 to 13%, although the goal was 5%, measured at Morgan in South Australia (MDBC 1999; P. Sharma, pers. comm.). The improvement came about through a combination of additional dilution flows (115 µs cm 1 ) and bore fields intercepting saline groundwater before reaching the River (80 µs cm 1 ), while providing 30 µs cm 1 buffer for land rehabilitation. Public investment was targeted through Land and Water Management Plans, with major gains from irrigation system upgrades, technological changes and improvement in on-farm water use efficiency. Some badly salt-affected areas have been retired from irrigation, and water trading into high salinity hazard areas is prevented. In general terms, the threat that ha of irrigated land would have high water tables by 2015 has been averted and river salinity improved (MDBMC 1999; MDBC 1999). However, the Murray Darling basin salinity audit concluded that major intervention to limit salt mobilisation from the dryland catchments was required or the benefits of the Salinity and Drainage Strategy would be lost in years. The Basin Salinity Management Strategy (MDBMC 2001) addresses this matter. While not going into the complexities of the new strategy, its key features are the following: (1) river salinity in the Lower Murray (Morgan, South Australia) is to be kept at its current improved condition up to 2015; (2) river salinities from the tributaries will continue to rise but not beyond agreed targets in 2015; (3) an expanded salt interception program will remove the additional salt downstream of the end-of-valley targets (61 µs cm 1 in the first seven years); (4) land-management actions under regional catchment plans are required to address the long-term threat, and these include farming systems changes, targeted reforestation and vegetation management (10 µs cm 1 ); (5) little more can be gained from dilution flows; and (6) living with salinity will be part of our future, including commercialising salt as a resource. The greatest uncertainty and therefore the greatest risk to this strategy meeting its objectives, is the land-management actions. Salinity and biodiversity in water supply catchments The experience of public policy and knowledge to manage salinity and protect biodiversity in the Murray Darling basin, with its 19 tributary catchments, provides useful pointers to the same task in any of Australia s water supply catchments. While the scale of the threats and management responses may differ, their acute and urgent nature in the basin mean that here there is the opportunity to observe and learn. Clearly, the onset of dryland salinity is a threat to terrestrial biodiversity, and salt mobilisation to rivers from dryland and irrigation areas has an impact on the aquatic environment. While the objectives of the Basin Salinity Management Strategy cover the range of values at risk, including terrestrial and aquatic biodiversity, it has a monitoring, reporting and accountability requirement based on river salinity targets an agreed cap on salt concentration and load at year While the end-of-valley target for each catchment is finalised through a community engagement process accounting for what s achievable, it is informed by knowledge of threshold salinities for consumptive use and environmental damage. For consumptive use there is a widely acknowledged threshold 800 µs cm 1 or 500 mg L 1 is the upper limit for desirability of drinking water A threshold of 1500 µs cm 1 was adopted for river salinity damaging environmental values, taken from the Australian and New Zealand Guidelines for Fresh and Marine Water Quality (ANZECC 2000). The guidelines largely draw on reviews of the biological effects of saline discharges into freshwater systems, across the spectrum of microbes, macrophytes, micro-algae, riparian vegetation, invertebrates,

4 622 Australian Journal of Botany K. F. Goss fish, amphibious reptiles, mammals and birds (Hart et al. 1990, 1991). This information has since been expanded and incorporated into a salt sensitivity database that now has 1500 entries covering 1200 different taxa. The majority of data is from field-monitoring reports in which presence or absence of taxa was reported along with the salinity of the water column at the time of sampling. The salt-sensitivity database is maintained under the National Rivers Contaminants Program. While 1500 µs cm 1 has proved a useful trigger for monitoring river salinity and setting targets, the salt-sensitivity database has its limitations. The earlier reviews highlighted the dearth of information on the sensitivity to increasing salinity, particularly sublethal and long-term effects, synergistic and antagonistic relationships, and potentially more sensitive life stages. Experimental data more reliably estimates effects than presence/absence data, yet experimental studies have been few and covered limited taxa and sites. Now an alternative information source is available to river managers, one that predicts the loss of aquatic biodiversity from changes in salinity (Kefford et al. 2003). While still in development under the National Dryland Salinity Program, it ascertains the relative salinity tolerances of species for specific geographic areas, drawing on a combination of experimental manipulation of salinity by observing the salinity range over which species are found in the field. The intent is to predict short-term salinity concentrations that cause mortality and longer-term sublethal effects at more moderate but elevated salinity levels. These effects include impaired physiology such as impaired movement, feeding, escape response and drift. Already results have indicated a high, short-term salinity tolerance for invertebrates, but a lower threshold concentration for survival over the longer term. For the time being, the 1500 µs cm 1 generic threshold stills stands as a benchmark for setting river-salinity targets. From a policy viewpoint, this is defensible until further data and predictive capacity comes to hand. However, for the river operator, full knowledge of salinity tolerance ranges for sublethal effects is required, particularly where there is the capacity to manipulate flow for dilution purposes. However, there is not the same capacity to predict salinity impacts on terrestrial biodiversity; the research challenges are obviously greater and to date there is less work to call upon. Arguably, Western Australia is most advanced in predicting salinity impacts at the land surface, surveying plant and animal species at risk, and experimenting with a process for valuing ecosystems and habitats at risks and setting priorities for action (Wallace 2001). A recent biodiversity survey of the wheat belt of that state (Keighery et al. 2001) found that ~1500 plant species occur in the lower parts of the landscape, that are most vulnerable to future salinity impacts, and of these 450 species are endemic and in danger of extinction over the next 100 years. Also, 31 of 125 small, ground-dwelling vertebrate fauna and 210 of more than 500 invertebrates have their distributions centred on the wheat belt or are endemic to it. Drawing on considerable research on distribution and connectivity of remnant vegetation in the central wheat belt of Western Australia, and the population status and behaviour of fauna, there has been an attempt to apply the focal-species approach to community government planning and investment in habitat protection and restoration (Lambeck 1999). Acknowledging that the focal-species approach is one among a number of models for deciding on scale and priority for management action, it is clear that more work of this nature is required Australia-wide. There is a growing consensus, initiated by the Western Australian Salinity Action Plan (Government of Western Australia 1996), and confirmed by the National Land and Water Resources Audit (NLWRA 2001b), that the threat of dryland salinity to terrestrial biodiversity is the strongest driver for public investment in salinity control. This is particularly the case for inland regions where potable water is not a consideration and rivers are ephemeral. Salinity management and trade-offs in water-resource security and river salinity For at least a decade there has been a consensus among salinity researchers, consultants and practitioners that the planting of deep-rooted, perennial vegetation is essential to any preventative strategy (Stirzaker et al. 2000; Williams et al. 2002). What is still debatable is the scale of the revegetation required to significantly impact the rise of dryland salinity. This very much depends on the nature of groundwater flow systems, the values at risk and the economics of such control measures. However, in water-supply catchments there is an additional factor: reforestation can result in a decrease in security of water supply and a temporary rise in river salinity. It is now well established that reforestation of cleared catchments reduces surface water run-off (Vertessy 2001). Both native forests and plantations have much higher evapotranspiration rates than annual crops and pastures, with an associated decrease in recharge to groundwater and in run off. This is largely to do with their perennial properties, the architecture and water-extraction ability of their rooting systems. For instance, in a subcatchment of the Tumut River, New South Wales, the average reduction in annual run-off for 5 years after plantation establishment was 182 mm, or 21% of the mean annual rainfall of 850 mm. In small catchments, there is likely to be a greater impact at times of low flow when reliability of water for consumptive use is already under threat. Considerable current work is under way to more precisely and reliably predict the effect of different afforestation scenarios on river flows and their allocation to irrigation use (Vertessy et al. 2002).

5 River salinity and biodiversity conservation in the Murray Darling basin Australian Journal of Botany 623 The Murray Darling Basin Salinity Management Strategy requires changes to land uses and farming systems that vastly reduce groundwater recharge, if its targets are to be met. The optimal zone for reforestation and revegetation is between the 500- and 800-mm rainfall isohyets. However, hydrological modelling developed by CSIRO, and now called the Biophysical Capacity to Change Model, shows that surface-water response to reforestation is much quicker than groundwater flow, resulting in a temporary rise in salinity (salt concentration). It is only after the groundwater discharge starts to slow that salt mobilisation (salt load) likewise decreases (Vertessy et al. 2002). To better understand the economic consequences of these trade-offs in water-supply catchments, the Murray Darling Basin Commission has initiated, or assisted, several studies to assess the impact of reforestation scenarios on river salinity and the net benefits for water resource use. The Australian Bureau of Agricultural and Resource Economics (ABARE) developed and applied a scenario assessment model to 25 tributary catchments in the basin to estimate the direct effect of land- and water-use changes on benefits and costs of water users downstream (Heaney et al. 2001). It concluded that the direct economic costs of increased river salinity are modest, that the impacts vary across tributary catchments and that widespread reforestation may not be a cost-effective option in many catchments (Heaney et al. 2000). However, this conclusion should be qualified. First, as the ABARE authors acknowledged, the environmental values and some other public assets likely to be at risk in these catchments were not included as direct economic costs (Heaney et al. 2001). Second, targeted reforestation may be cost-effective in catchments where there is a short response time in groundwater flows and where groundwater salinities are greater. More specific studies of the potential trade-off between salinity control and water-resource security, through reforestation, have been done for the Macquarie River catchment in New South Wales. In a preliminary hydrologic modelling of the upper catchment above Burrendong Dam, the main water source for the region, replanting 10% of the land to trees reduced mean annual flow 18% and reforestation of 5% of land area reduced it by 11% (Davis et al. 2001). It is already known that small areas of plantings are unlikely to have significant impact on salinity at a catchment scale. However, there remained the question of a more tactical approach to reforestation, taking account of site-specific characteristics such as those identified in the ABARE study. This was investigated by applying the same modelling to the mid-macquarie catchment, an area draining downstream of the Burrendong Dam but upstream of Narromine where most irrigated agriculture is practised (Herron et al. 2001). By focusing the replanting on the highest salt-export areas, comprising 22% of the mid-macquarie land area, a 26% reduction in salt loads could be achieved in 100 years, compared to a 14% reduction in flows. Again biodiversity values were not taken into account, including the Ramsar-listed Macquarie Marshes, located downstream of Narromine. This three-cornered contest between salt, trees and water was considered in a ground-breaking conference in 2000 (O Loughlin and Nambiar 2001). Importantly, it put the trade-off issues into perspective: (1) Plantations will have their maximum effect on surface water flows in high-rainfall areas (greater than 850 mm). (2) Reforestation targeted on groundwater-responsive catchments in moderate-rainfall zones ( mm) will have far less an impact on surface water flows. (3) Optimum results for salinity management requires a much greater understanding of catchment-specific hydrologic process and the local and regional environmental values at risk. More recent work has given further pointers to how reforestation programs can minimise trade-offs through planting from the top of a catchment towards the river, through selecting high-productivity sites, and through phasing plantings over time (Vertessy et al. 2002). From a public policy perspective, regulation of plantation forestry in the high-rainfall zone should be considered before dismissing the case for salinity target reforestation in the medium-rainfall zone. Conclusions The onset of dryland salinity in the Murray Darling basin, although not the same level of salt-affected land as in Western Australia, is clearly a long-term threat to agricultural land, water supplies, regional infrastructure and biodiversity. The basin is considered a special case because there is such a high value on consumptive use of water, with river salinity a specific threat to aquatic biodiversity values. By adopting the river salinity threshold of 1500 µs cm 1 for protection of freshwater ecosystems, river reaches at risk in the basin have been identified. From a public policy perspective, this knowledge is useful for directing investment and action in salinity amelioration, such as dilution flows and salt-interception schemes. The threshold is well supported by databases. However, there is growing evidence that salt concentrations over a longer time period have sublethal effects on species and ecosystems. Further knowledge is required if river operations are to be modified to off-set salinity s impacts. In the Murray Darling basin, and in Western Australia, the nature and scale of the dryland threat to biodiversity requires further investigation. There is no agreed process for incorporating environmental values at risk into a strategic

6 624 Australian Journal of Botany K. F. Goss response, and from a public policy perspective this needs to be addressed. In the basin, with its over-allocated, highly regulated, and connected river systems the management of dryland salinity and protection of aquatic and terrestrial biodiversity values must be considered in the context of other threatening processes and responses. For instance, dryland salinity is a long-term threat to water quality for consumptive use and the threshold adopted is half that used for protection of aquatic ecosystems. For the time being, the more immediate threats are from over-allocation and regulation, and a policy response is being developed. Long-term management of salinity outside of irrigation areas requires reforestation and revegetation of a considerable portion of upland catchments. While the re-introduction of deep-rooted perennials can assist to protect terrestrial biodiversity values or create them, there is a trade-off in surface water flow and river salinity may rise temporarily until there is a reduction in groundwater flow and salt mobilisation. Recent studies suggest that targeting areas of high salt export in the mm rainfall zone, to achieve at least 20% cover with reforestation, can minimise the trade-off over the longer term. There is predictive capacity for assessing the economic benefits and costs of changes to land and water use at a basin scale, and the hydrologic responses for catchments and subcatchments. However, these studies have not incorporated potential losses or benefits to environmental values. In the interests of terrestrial biodiversity protection and effective vegetation-management strategies there is an urgent need for catchment-specific knowledge on groundwater flows, salinity hazard and biodiversity values at risk. This paper has described the issues, outlined the policy and management responses, and identified knowledge needs for the Murray Darling basin. They are relevant to other river basins or catchments where there is high dependence on surface water flow and river diversions for consumptive use. Acknowledgments I thank David Nicholls of the Murray Darling Basin Commission who contributed to the section on salinity impacts on aquatic ecosystems. References Australian and New Zealand Environment and Conservation Council (2000) Australian and New Zealand guidelines for fresh and marine water quality. Colleambally Irrigation (2001) Annual report Colleambally Irrigation District. Davis JR, Herron N, Jones R (2001) Is there enough water? Modelling the combined effects of government policies and climate change on flows in an Australian river. 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