State of the Environment Report 2007
Inland waters have unique natural water and flow regimes influenced by climate, surface runoff, catchment size and geomorphology (Boulton & Brock, 1999). In some WA catchments, water regimes have been dramatically altered from their natural state. This has occurred through artificial increase or decrease of water levels, or alteration to the volume, velocity, duration, timing or frequency of flow events.
Many types of human activities influence water regimes within catchments. Widespread changes to vegetation cover and land uses can affect both surface water and groundwater. Direct interference to a waterway, such as the construction of dams and weirs for water storage and flood protection structures (e.g. drains, levee banks, river training) can modify surface water regimes. Extraction of water for domestic, agricultural, mining or industrial reasons (e.g. pumping, irrigation channels) can also have profound impacts on surface and groundwater regimes.
Alteration of natural water regimes is now recognised as a major contributor to loss of biodiversity and functionality of aquatic and terrestrial ecosystems. It can modify the values of inland waters and lead to other land and water problems including floods and drought-like conditions, waterlogging, salinisation, eutrophication, acidification and erosion. The maintenance of biodiversity and productive land and water systems depends on ecosystem services that in turn rely on maintenance of natural water balances and flow regimes. In severe cases, excessive alteration of natural water regimes leads to widespread loss of whole ecosystems and water supplies.
In the context of allocating water to users, the Environmental Water Provisions Policy for Western Australia (Water and Rivers Commission, 2000b) identified a primary objective for managing natural water balances:
However, for broader land uses another objective is needed:
Major alterations to water regimes for surface water and groundwater are evident across most of the South West (Figure IW2.1). Moderate to major levels of change can be largely attributed to widespread land clearing, damming of major waterways, high levels of water extraction and replacement of perennial native vegetation with low water-using pastures and crops. Some major rivers near populated and agricultural areas (e.g. the Avon, Vasse, Collie and Gascoyne rivers) have had their watercourses significantly altered (i.e. straightening and levee banks constructed) to reduce the risk of flooding. Widespread artificial drainage, both surface and deep, is also evident across developed areas.
Varied water regimes of wetlands and waterways in the rangelands reflect the extreme ranges in the climate, due to episodic rainfall, cyclones, droughts and floods. However, impacts beyond their range of tolerance will result in significant ecological impacts. Most of the rangelands has minor to moderate changes to natural water regimes, which may include flood protection measures, mine dewatering and construction of water reservoirs.
![Figure IW2.1: Extent of altered water regimes in Western Australian river basins. [Data source: Derived from Morgan (2001); Analysis: EPA; Presentation: EPA.]](/site/files/images/Figure-IW2.1_Large.jpg)
Demand for water is rapidly growing. Gnangara Mound, a major groundwater formation near Perth, has been under extreme water demand in the last 30 years. Groundwater is managed by government agencies according to specific boundaries or management units. Currently 19% of groundwater management units in this area exceed their allocation limit (see 'Water supply'). This means in some areas groundwater extraction is exceeding that required to maintain the environment, and the water resource is being mined. Watertable levels on the mound have been declining over the last 20-30 years, falling by up to 6 m near the crest of the mound (Salama et al., 2002; Department of Environment, 2005a). This is considered to be the cumulative result of reduced rainfall, increased groundwater extraction and reduced groundwater recharge by pine plantations. This problem was highlighted in 2000-03 during a period of consecutive low rainfall years where large sudden drops in the water table occurred over most of the mound, placing considerable stress on some cave and lake ecosystems (Figure IW2.2). It been highlighted over several years that environmental conditions relating to environmental water provisions on Gnangara Mound have been breached (Department of Environment, 2005a; EPA, 2007). Due to water extraction, water levels in the deeper Leederville and Yarragadee groundwater aquifers are also declining, with falls in those aquifers ranging from 5-15 m and 5-25 m, respectively (Salama et al., 2002).
![Figure IW2.2: Trends in superficial groundwater levels for Gnangara and Jandakot mounds, 2000–03. [Data source: Department of Environment (2005); Analysis: DoE, EPA; Presentation: EPA.]](/site/files/images/Figure-IW2.2_Large.jpg)
Much of WA's coastline has a freshwater superficial aquifer that overlays a thin wedge of saline water that may extend several kilometres inland. Excessive extraction of water from the superficial aquifer may result in a rising saline watertable, rendering water production bores salty and killing native vegetation. This problem is particularly apparent in Carnarvon where close management of groundwater abstraction is required to prevent salt water intruding into horticultural bores and to prevent damage to fringing vegetation along the Gascoyne River.
Superficial groundwater levels across the South West agricultural zone are generally rising and no trends of decreasing groundwater have been detected on a broad scale (Figure IW2.3; Nulsen, 1998; McConnell & Short, 2001; Ghauri, 2004). The rate of rise of groundwater varies from less than 5 cm/yr to 100 cm/yr, with many areas rising at 20-30 cm/year (George, 2002). This rise is caused by historical widespread clearing of native vegetation and replacement with annual crops and pastures, resulting in reduced water uptake (see 'Land salinisation'). However, there are local instances where revegetation of land with perennial shrubs (e.g. tagasaste) and agroforestry has been associated with local groundwater declines of 40 cm/yr to 80 cm/yr, respectively (Nulsen, 2000).
Notwithstanding other shortcomings, artificial deep drainage in the Wheatbelt has also been effective in some areas in reducing rising saline groundwater levels. For example, a 70 km network of drains in Narambeen removes about six megalitres (ML) of excess groundwater per day from the agricultural landscape (equivalent to six Olympic size swimming pools). Disposal of this water has proven very contentious as it often affects natural waterways and wetlands.
![Figure IW2.3: Modelled long-term trends in superficial groundwater levels for the South West agricultural zone. [Data source: Department of Agriculture [ver. 2005]; Presentation: EPA.]](/site/files/images/Figure-IW2.3_Large.jpg)
It is well recognised that dam construction severely modifies natural flow regimes in waterways. For example in the Kimberley, construction of Lake Argyle on the Ord River has turned the river from a seasonal to a permanently flowing waterway, reducing its flows by 20% and severely limiting the frequency and intensity of high flows and flood events (Water and Rivers Commission, 2003). In the South West, damming of the Canning River has captured about 95% of its flow, turning downstream reaches into a trickling stream (Australian Academy of Technological Sciences and Engineering, 2002). In the Pilbara, damming of the Harding River has captured 80% of its flow and severely modified downstream ecology (Water and Rivers Commission, 2004).
Trends in waterway and wetland water levels are often reflected in trends in catchment groundwater levels. For example, there is anecdotal evidence that many wetlands in the Perth metropolitan area are starting to dry up, including Lake Jualbup, Perry Lakes, Lake Gnangara and Jandabup Lakes, amongst others. This may be the result of a combination of factors including increased superficial groundwater use (from bores) and the effects of climate change.
Some smaller estuaries, such as the Margaret River estuary, are slowly diminishing due to climate change and increasing extraction of groundwater for a growing population and the vineyard industry (Brearley, 2005). Other long-term changes, including a 50% reduction in volume being discharged by some South West waterways, have been linked to climate change (Indian Ocean Climate Initiative, 2005a; Water Corporation, 2005).
In other inland areas, there are estimates that a two to four-fold increase in the area of shallow water tables in the Wheatbelt (caused by widespread land clearing) will lead to at least a two-fold increase in waterway flood flows (George et al., 1999). Long-term monitoring of forty-one wetlands in the South West conservation estate has shown that 29% have increased in depth between 1977 and 2000 and 2% showed a fall in depth (Lane et al., 2004). Excessive amounts of water in wetlands can be just as detrimental as not enough water to ecological systems, through drowning wetland associated vegetation.
Widespread land clearing (see 'Loss or degradation of native vegetation') and replacement of native vegetation with impervious surfaces associated with buildings and roads can increase water runoff and reduce recharge to groundwater. Where land is cleared and not adequately drained, excess water may build up and contribute to a rising watertable. Similarly, replacement of native vegetation with agricultural crops and pastures that use less water leads to a gradual rise in groundwater levels. Use of drainage channels to prevent build-up of groundwater also modifies receiving waterways and wetlands by increasing or prolonging flows. Therefore, altered water regimes will occur if changes to catchment water balances happen at faster or slower rates than would occur naturally.
Climate change may also be having an effect on natural water regimes. For example, a 10-15% drop in South West rainfall since the 1970s has produced a corresponding 50% reduction in average annual flows in some South West rivers and streams (Indian Ocean Climate Initiative, 2005a). Anecdotal evidence over recent decades suggests that some of Perth's lake systems are being converted to swampy flats. Some wetlands associated with climate-sensitive groundwater systems have also experienced more frequent dry states. In drier parts of the north-eastern Wheatbelt, groundwater levels have declined in upper to mid-slope areas as a result of reduced rainfall. Groundwater levels in the central and eastern Wheatbelt are not rising as quickly as they initially did in response to widespread clearing, and in some areas with shallow watertables (less than 8 m) levels are stable or declining (Indian Ocean Climate Initiative, 2005b).
River systems that have water storages are known as 'regulated rivers'. Major storage structures (including dams, weirs and pipeheads) provide water primarily for urban, industrial and agricultural use. Water supply dams, while benefiting humans, impact waterway and floodplain ecology by affecting natural flows and the volume of water reaching downstream areas. Environmental water provisions (water allocated back to the environment from dams) can be made to reduce the environmental impact of river regulation.
In 2005 the State had 48 major storage dams, with most situated in the South West (Figure IW2.4). Some major river systems are under significant pressure, with multiple dams contributing to major alteration of natural water balances and flow regimes. These include the Canning (six major dams), Murray (five), Harvey (six), Collie (four), and Blackwood (nine) rivers, and Lake Argyle on the Ord River, which is Australia's second largest reservoir. With the Harvey and Ord rivers as exceptions, most rivers do not have environmental water provisions determined. It is estimated there are several thousand smaller dams and weirs constructed on both regulated and unregulated rivers by local authorities, government agencies and individuals. In these cases the effects of altered water regimes may be relatively minor, but cumulatively the environmental impacts become significant.
![Figure IW2.4: Number of registered major dams by drainage basin. [Data source: Water Corporation [ver. 2001]; Analysis: EPA; Presentation: EPA.]](/site/files/images/Figure-IW2.4_Large.jpg)
If water allocations are not managed properly, water extraction from the environment can begin to exceed sustainable yields (i.e. the amount of water that can be taken without damaging the environment in the long term; see 'Water supply'). At this point ecological systems begin to be impacted. In 1997, 36% of river basins had at least one management unit exceeding the sustainable yield and 25% of surface water management units (i.e. monitoring sites) were over-allocated (Table IW2.1). This information is now ten years old and requires urgent updating. As of 2006, about one-third of groundwater management areas are approaching or exceeding their allocation limit. Overall the number of management units that are over-allocated is relatively small (11%) (Table IW2.2). In many places, unregulated water extraction and inadequate monitoring, has made it extremely difficult to accurately calculate sustainable yields and effectively manage water resources.
Surface drainage in urban areas provides flood protection but also modifies natural water regimes in catchments. In urbanised areas the excessive use of drains has contributed to mass depletion of water from landscapes and prevented wetland and groundwater recharge, even during high rainfall. Some wetlands are being drowned by excess water from drains and some groundwater aquifers are not being adequately recharged. Drains also rapidly transport nutrients and other pollutants from the catchment to waterways and wetlands, with little opportunity for filtering by natural ecosystems. It has been estimated that in the Perth metropolitan area alone, there are approximately 830 km of drains managed by the Water Corporation and 3000 km managed by local governments (together, about the distance from Perth to Brisbane).
Surface and deep drainage are used in agricultural areas to intercept runoff and lower water tables, thereby improving soil productivity and enabling flood protection. However, increased drainage has severely altered water regimes in receiving waterways and wetlands. It is estimated there are 13 000 km of agricultural drains in the South West agricultural zone (four-times the distance from Perth to Sydney), with an estimated 1000 km being constructed every year (extrapolated from Dogramaci & Degens, 2003). Many farmers are increasingly seeing deep drains as a viable option to remove excess groundwater from agricultural land, but without any treatment of the water or assessment of environmental impacts. Consequently, there is a tension between the farming community and regulatory authorities, due mainly to a lack of understanding about the environmental risks and impacts of drainage water disposal options.
National Water Initiative: In 2006, WA became a signatory to a bilateral agreement with the Commonwealth government which aims to increase the productivity and efficiency of water use, enhanced service to rural and urban communities, and to protect the health of surface and groundwater systems by ensuring sustainable levels of extraction. A draft implementation plan has been released (Department of Water, 2007).
State Water Plan: was recently released and outlines the policy direction for the sustainable management of water resources, including the development of regional water plans, strategic water issue plans and statutory water management plans (Government of Western Australia, 2007). The plan will also integrate components of the Water Reform program, which aims to reform water entitlements and create a viable water trading system.
Environmental water requirements (EWRs): are water regimes considered necessary to maintain ecological health and protect environmental values, based on risk assessment and best available scientific information. The Department of Water takes the lead in establishing EWRs, although other research organisations and academic institutions are involved in determining EWRs for inland waters.
Environmental water provisions (EWPs): are water regimes provided as a result of the water allocation process in consideration of ecological, social and economic impacts. They may meet the environmental water requirements in part or in full. The Water and Rivers Commission developed a policy in 2000 which is currently being reviewed by the Department of Water. A significant proportion of heavily allocated water resources have not had environmental water provisions determined yet (Table IW2.3).
Water for Healthy Country Flagship: is an Australia-wide science partnership established by CSIRO which aims to provide information and management opportunities to improve the environmental, social and economic benefits from water resources. A number of research projects are being undertaken in the South West.
Groundwater investigation program: The Department of Water assesses and reviews the State's groundwater resources to ensure sustainable management of aquifer systems and dependent ecosystems. There are approximately 3000 monitoring bores distributed throughout the State.
Stormwater management: has evolved in recent years on the principle that stormwater is a resource providing social, environmental and economic opportunities. A Stormwater Management Manual for Western Australia builds on the traditional flood protection approach through the inclusion of water quality management, the protection of ecosystems and providing liveable and attractive communities.
Fishways: (more commonly known as 'fish ladders') are structures built in a stream to allow fish to move around barriers that impede normal flow regimes, such as dams, culverts and weirs. Fishways have been constructed in the Margaret, Hotham and Goodga rivers and Bennet Brook, with a project also underway on Lake Kununurra Diversion Dam to enable passage of barramundi upstream.
Modified water and flow regimes can have significant social, economic and environmental impacts. Too much water can result in flooding of ecosystems, infrastructure and communities, and damage productive land. It can also result in the waterlogging and drowning of native vegetation, salinisation, loss of soil health, loss of fauna habitat and a favouring of opportunistic weed species. Too little water results in drought-like conditions, often leading to loss of inland waters, degraded ecosystems, limited habitat, acidification and eutrophication problems, and widespread death where species are unable to adapt to modified conditions. Reduced water availability also limits provision for human consumption, agricultural purposes and recreation. Changes to the timing, duration and frequency of natural flows can be just as harmful, affecting biodiversity by altering natural migration and reproduction patterns of flora and fauna. It is now recognised that adequate water provision to the environment is critical for maintaining ecological functionality of inland waters and neighbouring environments.
4.8 Develop and implement statutory water management plans for priority areas and stressed ecosystems, with accompanying allocation plans and environmental water provisions.
4.9 Develop drainage governance arrangements for urban and rural areas.