Effectiveness of Current Farming Systems in the Control of Dryland Salinity

Effectiveness of Current Farming Systems in the Control of Dryland Salinity Glen Walker, Mat Gilfedder and John Williams W. van Aken © CSIRO W. Why do we need to worry about dryland salinity? Dryland salinity is a serious problem in many parts of Australia, including the Murray-Darling Basin. In 1998, the Prime Minister’s Science, Engineering limit of 800 EC units for desirable drinking water, and Innovation Council estimated that the costs and create concern for its long-term sustainability of dryland salinity include $700 million in lost for urban water use. In some northern parts of the land and $130 million annually in lost Basin it is expected that river salinity will rise to production. The effects of dryland salinity include levels that seriously constrain the use of river increasing stream salinity, particularly across the water for irrigation. southern half of the Murray-Darling Basin, and The enormous level of intervention needed to losses of remnant vegetation, riparian zones and deal with dryland salinity, and the landscape’s wetland areas. Salinity is degrading rural towns slow response to any changes, mean that now and infrastructure, and crumbling building is the time to devise new ways to manage the foundations, roads and sporting grounds. problem. The problem is not under control—we can expect The government of Western Australia is the effects of dryland salinity to increase developing a dryland salinity action plan for that dramatically. For example, if we do not find and State. The Murray-Darling Basin Commission is implement effective solutions, over the next fifty currently setting in place a process to develop years the area of land affected by dryland salinity new natural resource management strategies to is likely to rise from the current 1.8 million address salinity issues in the Basin. hectares to 15 million hectares. This report is a contribution to that process. 2 Stream salinity is also a major concern. It seeks to establish the role we can expect our Projections for the town of Morgan, a key current farming systems to play in salinity control location used to monitor the effect of salinity in the future. Further, it sets out what is required in the lower part of the Basin in South Australia, of new farming systems if they are to be part of illustrate the problem. Here, the salinity of the strategies to control salinisation by treating its River Murray is expected to increase by a further cause. 240 EC units (microSiemens/cm) over the next 50 years. This will bring salinity in this part of the river close to the World Health Organization’s Australia’s low rainfall means that we have one What is dryland of the lowest amounts of runoff in the world. As a result, most of our rainfall is used by plants salinity? where it falls and only very small amounts leak Australia is a dry country. Its landscape contains to the groundwater. In addition, poor drainage large amounts of salt from the ocean, introduced and older, less permeable landscapes are into the landscape through rainfall and the conducive to the accumulation of salt. Combined, chemical weathering of rocks, and built up these conditions mean that salts are not flushed over thousands of years. from the landscape by leaching. Vegetation further inhibits leaching because it uses the Our clearing of native vegetation for dryland rainfall. The consequence is that saline lakes, agriculture (such as grazing and cropping streams and land are a natural part of the systems) has allowed too much rainfall to leak Australian landscape. into the groundwater systems of these landscapes. This leakage causes watertables to rise and brings saline groundwater close to the land surface. The The way it was... movement of salt to the land surface with rising groundwater in non-irrigated lands is known as In most cases Australia’s native vegetation dryland salinity. comprises trees or woody shrubs. This perennial vegetation, with its relatively deep roots, has The warning signs of a landscape affected by become effective at taking full advantage of any dryland salinity include sick or dying trees, available water. As a result, it can use most of declining vegetation, the appearance of salt- the water entering the soil. The ‘leakage’ of excess tolerant volunteer species such as Sea Barley water into the deeper soil below the roots is Grass and Spiny Rush, bare salty patches, and usually quite small, if it happens at all. Various saline pools in creek beds. Any of these can affect studies have shown that over most of Australia’s crop productivity, the sustainability of agriculture current dryland grazing and cropping areas, and the quality of water in rivers and streams, or this leakage was commonly between 1 and crumble the foundations of buildings and roads. 5 mm/year. Over thousands of years the minimal leakage has Australian landscape allowed the salts introduced through rainfall or rock weathering to build up in the soil below the and salinity depth of plant roots. Differences between the Australian landscape and salinity landscape in most other parts of the world mean that agricultural systems that are sustainable elsewhere do not necessarily transfer to our unique conditions. One major difference is that most of our groundwater and surface water systems are poorly drained, leading to the storage of enormous amounts of salt in the landscape. Dryland salinity does occur in other countries, but to a lesser extent. A combination of natural 3 factors has led to Australia’s significant problems with dryland salinity: the continent is geologically old and stable, and the climate is very dry (that is, there is a low rainfall compared to potential evaporation). Our native vegetation has adapted to these unique conditions. W. van Aken © CSIRO W. of rainfall), the permeability of soils and subsoil, Changing the and vegetation characteristics. For example, any water leaking beyond the root zone does not natural conditions always end up in groundwater, since in certain European settlers have changed the Murray- situations it can move laterally through the soils Darling Basin to a remarkable degree in a and end up in surface streams. In other situations, relatively short time. Large-scale clearing of native leakage can occur from the base of the streams vegetation and its replacement with crops and into groundwater systems. agricultural systems have substantially increased the amount of water entering the groundwater systems of the landscape. There is too These increased amounts of water now entering much leakage the groundwater under current agricultural Leakage can be much greater under current production systems greatly exceed the capacity agricultural systems than under natural of the groundwater systems to discharge the vegetation. It occurs when the plant/soil system additional water to the rivers and streams. cannot cope with the amount of water that has As the input to the groundwater exceeds the fallen over a period of time. Leakage is caused by output, the watertable must rise. As it rises, more a complex combination of climate and rainfall water is discharged to the land surface as seepage patterns, vegetation cover and soil properties, surfaces (usually at lower positions in the landscape). Wherever this groundwater contains Climate salt or intercepts salt stored in the landscape, salt is mobilised to these seepage faces, and hence to The effect of climate and rainfall on leakage to the land surface, rivers and streams. groundwater can be simplified into two different types, as follows. The amount of water leaking into the groundwater system depends on various factors 1. In wetter areas, normal rainfall can exceed that include the climate (particularly the amount potential evaporation for a period of the year, leading to leakage when the excess water cannot be stored in the soil. 2. In drier areas, leakage is likely to occur mainly as a result of exceptional circumstances, such as intense rainfall and flooding that may only occur once every 3–20 years. In determining leakage, the distribution of rainfall is as important as the total amount of rain. The sequence of rainfall events is critical, particularly for the episodic nature of deep drainage and recharge. Seasonality—the particular time of year when most rainfall occurs—is another major climatic factor that affects leakage amounts. In the winter rainfall dominated southern parts 4 of the Basin, most of the rainfall occurs during the cooler part of the year, when evaporation and hence the amount of water the vegetation is using is likely to be low. Under these circumstances, the rainfall infiltrating the land must be stored in the soil if leakage is to be prevented. If the Replacing native vegetation with shallow rooted annual crops and pastures has led to substantial increases in the soil already contains water that was not used by amount of water ‘leaking’ into the soil. The consequences agricultural plants in the growing seasons, leakage are rising groundwater levels and dryland salinity. will occur more readily. For example, a water balance study at Katanning in south-west Western Australia showed that shallow rooted clover and deep rooted lucerne both dried out the shallow soil (0–45 cm) to the same extent, while the deeper rooted lucerne was able to dry out the deeper soil (45 cm–1.2 m) to a much greater extent. Leakage from 80 mm under the clover was reduced to 30 mm because the lucerne was helping to dry the deeper soil. Even with deeper rooted crops, leakage can occur during the early growing stages when they have established only their shallow roots. A study at Wagga Wagga showed that in the first year of lucerne in rotation with wheat, there was a similar amount of leakage past 1 m to that under wheat, but in its second and third years, when the deep root system became established, leakage was dramatically This map shows the differences in rainfall seasonality in reduced.

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