A Nutrient Dynamics Model for Australian Waterways

Land Use, Catchment Biogeochemistry and Water Quality in Australian Rivers, Lakes and Estuaries
Australia State of the Environment Technical Paper Series (Inland Waters), Series 2

Graham Harris
Department of the Environment and Heritage, 2001
ISBN 0 642 54823 4

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Executive Summary

Many natural habitats of Australia have been fragmented through land clearing. Rivers have been extensively dammed and their flows controlled. Much surface water is extracted for irrigation and urban supplies. This paper analyses the effects of this resource use by explicitly linking water quality data from State archives with land use. The analysis uses catchment biogeochemistry to illustrate the impact of land use on the movement of major ions and nutrients from the landscape and finds important relationships between land use, habitat fragmentation and water quality. These insights will influence how the Australian landscape is managed and restored.

Land clearing leads to far reaching cumulative, though non-linear deleterious changes to soil properties, vegetation and surface and ground water quality and quantity. At around 50% clearance, there is a sharp increase in export of salinity, suspended solids and nutrients to the waterways and groundwater (although not suspended solids for groundwater) with a corresponding decline in water quality. Stream power is related to slope and rainfall intensity so that once slopes are cleared the surface run off has sufficient power to begin to cut down into the soil and subsoil.

The increased problems stem from increased surface runoff and seepage to groundwater due to the reduced vegetation less effectively impeding water flow or retaining nutrients and particulates. It appears that deleterious changes will occur regardless of how sensitively the land is cleared, eg if care is taken to minimise erosion while clearing, changes will still occur, though perhaps less severe for suspended solids. Changes such as increased salinity can occur quite quickly, eg a few years to a decade after clearing. This has clear implications for the current high rate of clearing in parts of Australia.

This steep increase in runoff and export of nutrients, salinity and suspended solids after 50% clearance has considerable implications for Nation Action Plan for Salinity and Water Quality / Natural Heritage Trust (NAP/NHT2). The main implication is that meaningful NAP/NHT2 targets to improve water quality and landscape function should ideally aim to correct hydrological imbalances, likely to involve the equivalent water use of around 50% tree or possibly other deep rooted vegetation cover. Otherwise NAP/NHT2 targets, unless modest or interim, are unlikely to be met. Because of possible local effects, eg soils, topography and type and distribution of vegetation cover, it may be possible in some areas to get improvements to catchment exports and water quality at a somewhat lower percentage of deep rooted vegetation cover. The most appropriate targets could be determined by data and research on the best plant species (eg high water use) and their placement in the catchment, biodiversity goals and farming and grazing systems.

Compared with many other countries, Australia has low export of nutrients from catchments. This is due to the low rainfall, low relief, low fertiliser usage, low nutrient status of our soils, low population and lack of atmospheric deposition of nutrients (such deposition is a problem in Northern Hemisphere countries).

Aquatic ecosystems exist in two states, either clear and macrophyte dominated or turbid and plankton dominated. The switch between the two states is often abrupt and the ecosystem response to perturbation is highly non-linear and complex. The characteristic response shows strong hysteresis, which is when the ecosystem state shifts from one state to the other (eg clear to turbid) and is highly resistant to switching back. A waterway in a well vegetated catchment with clear water and dominated by macrophytes becomes replaced by turbid, more saline water that is algal dominated after clearing. Much of inland Australia corresponds with this situation. Ecosystems can show 'critical loads' or 'points of no return' which are of great concern for managers.

It is still possible to save many estuaries and coastal lagoons. Many estuaries are seasonally nitrogen limited and increased inputs of nitrogen from fertilisers, urban runoff and clearing, could impair estuarine water quality and cause seagrass loss through phytoplankton and epiphytic growth on the seagrass (seagrass is important for fish breeding). As long as abundant seagrasses are present, Australian estuaries have good water quality and nitrate is usually almost unmeasurable. Like impoundments, N:P ratios in surface waters are very low. In some cases, estuaries may also become phosphorus limited. The Port Phillip Bay study by CSIRO used the concept of interactions of the main biological groups to explain the behaviour of the bay and formulate recommendations for management.

Many rivers are past the point of no return unless very large amelioration is undertaken. Once the waterways become saturated with respect to incoming salts, nutrients and suspended solids, problems, such as a decline in in-stream biota become apparent. Such problems can be partly mitigated by measures to reduce erosion and prevent rise in groundwater levels.

Water quality in rivers is a function of land use and catchment geology as well as in-stream interactions, including position in the overall landscape pattern of regulated reaches, reservoirs and wetlands. Land use change in Australia has tended to increase flow regulation and the number of impoundments and weir pools of various kinds. Australia is losing wetlands rapidly because of flow regulation and drainage, which is a further problem for waterways and biodiversity.

Water quality reflects land use in other ways, eg sodicity and acidity of soils. Most Australian soils are poorly buffered and tend to be made acidic by the clearing of the land and extensive use of legumes such as sub clover, which generate nitrate in the soil profile, and certain fertilisers such as ammonium sulphate. Calcium leaching is associated with acid deposition and acidity alters the chemistry of both ground waters and surface waters. The acid sulphate soils of many Australian coastal areas are a special case and when disturbed can generate serious acid and heavy metal pollution of soils and waterways, with resultant loss of aquatic species including fish and infrastructure damage.

Salinity is a good indicator of perturbed major ion chemistry and, because the movement of ground water through the soil profile is a good integrator of the effects of land clearance and agricultural development. Ground water influences on water quality were clearly higher in cleared catchments.

Water salinity is not, however, a good predictor of other biologically relevant indicators of water quality (eg N and P). It is necessary to understand the forms, fluxes and transformations of N and P in catchments if we are to formulate a more complete theory of catchment biogeochemistry and its application to management questions. Dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) are better nutrient indicators than total nitrogen (TN) and total phosphorus (TP) and the ratio of DIN to DIP is a good predictor of algal blooms. They are constituents of TN and TP respectively and easier to measure than TN and TP. This does not preclude also measuring TN and TP.

The combination of flow regulation, impoundment of rivers and removal of wetlands has had a major effect on the ecology of Australian rivers. Clearance and land use change, increased erosion and increased sodicity, have flipped many Australian rivers from clear and macrophyte dominated, to turbid and plankton dominated. These effects are probably no longer reversible without massive, and unrealistic, landscape rehabilitation. While the rivers have been largely lost, most Australian estuaries are not yet past the 'critical load' point.

We should, therefore, take a landscape approach to understanding and managing water quality from hill-slope to estuary. Removal of more than 50% of the native plant cover results in increases in both the horizontal and vertical flows, so that runoff increases after rain and ground water becomes a more important influence on stream chemistry during low flow periods. Across wide areas of this continent stream salinity is a good integrator of the effects of landscape change in the catchment.

It is not just dryland salinity that results from land clearing. Changing land use over quite short time scales alters many aspects of water chemistry. Agricultural practices lead to soil acidity and sodicity, both of which are visible in the stream chemistry. To restore water quality and river ecology it will clearly be necessary to restore the land and its ecological function.

Citation

For bibliographic purposes, this document may be cited as:

Harris, G. 2001, A Nutrient Dynamics Model for Australian Waterways: Land Use, Catchment Biogeochemistry and Water Quality in Australian Rivers, Lakes and Estuaries, Australia State of the Environment Second Technical Paper Series (Inland Waters), Department of the Environment and Heritage, Canberra. http://www.ea.gov.au/soe/techpapers/index.html