IRRIGATION IN THE AUSTRALIAN WATER RESOURCES CONTEXT

Dr Wayne S Meyer
Program Leader, Sustainable Agriculture
CSIRO Land & Water
PMB No 2, Glen Osmond, Sa 5064

Water Distribution In Australia

TABLE 1

The most recent complete accounting for water in Australia is based on 1985 data and a summary is given in Table 1. Total runoff from the continent was estimated at 397 km³, with most of that water being located in northern Australia associated with the summer rainfall dominant tropics and semi-arid tropics, and in temperate Tasmania. The total runoff from the continent is equivalent to that coming from the much smaller countries of New Zealand and Japan and represents only a fraction of the water which runs off continents of equivalent dimensions, such as the United States. Another feature of our water supplies is the high variability in availability, especially of the major inland rivers of the Australian continent (see Meyer, 1998).

The 1985 compilation made an estimate of the volume of "divertible resource" or exploitable water. The method of deciding what water was exploitable is arbitrary and no doubt will change as our concepts and judgements of exploitable water change. The major emphasis of the National Land and Water Resources Audit currently underway is to audit total water availability and how it’s distributed. This will also include estimates on groundwater resources. Again, recent information should give us much better estimates of both volumes and exploitable amounts which are available in many of our groundwater resources. In the next iteration of defining what is exploitable yield, the factors associated with water quality, principally salinity and, in the case of surface waters, sediments, will be considered, as will seasonality of flow and the reliability of flow. In addition, the emerging concern for environmental flows will feature as part of our consideration of exploitable water resources.

Irrigation Water Use In Australia

Total water used within the Australian context was estimated by Woods and Banks (1991) to be about 14.6 km³ of which 10.3 km³ was used for irrigation. This was based on mid 1980s to late 1980s data. We know this is an under estimate because the Murray-Darling Basin audit of 1993/94 indicated a diversion of about 10.7 km³ for rural purposes, the majority (95%) of which was for irrigation. It would not be surprising if the audit indicates a considerable growth in the water use within Australia, perhaps as high as 18-25 km³ of total water use, of which 75% is likely to be used for irrigation. If Australia follows world trends in water use, then we would expect total water use to increase but the proportion used for irrigation to decrease into the future.

Scientific Irrigation Practice

The distribution systems that convey water from impoundment to farm gate are a major source of water loss. Distributing water in earthen channels over long distances should not be practised. Losses of 10% to 90% have been documented within Australia, while a world survey indicated a conveyance efficiency of about 75% (Table 2). Seepage and evaporative losses can be significant although pipes or lined channels do not guarantee high conveyance efficiency if they are not well constructed and maintained.

The purpose for irrigating is to provide water to plants, to optimise growth for either quantity and/or quality production. Unfortunately, there is an attitude that the purpose of irrigation is to wet the soil. As a result, poor attention is given to getting water where it is needed, in amounts it is needed, for plant production. The amount of water diverted to produce plants is still far too large compared with the productivity and, from an energy point of view, the energetics of capturing solar energy into plant products for the use of water is still very poor. An analysis of irrigation productivity over thirty years showed evidence of an increase but energetically irrigation returns as food energy much less than 1% of the total enetrgy involved (Meyer, 1997).

There are many instances where surface irrigation is inappropriately used. It should not be used on soils with high infiltration characteristics and, even on other soils, we need much better matching of input flows with slope and surface conditions to get water much more rapidly across bays and down furrows to minimise the amount of water which is saturating the soil profile and moving past the root zone. We must continue to develop controlled irrigation systems. It is totally inappropriate for high value crops to be planted today and irrigated with surface irrigation. The control on quality and quantity which needs to be exerted on these high value crops should also be reflected in the care and precision with which water is used on them.

Irrigating in semi-arid or arid environments will accumulate salt. In these environments with uncertain rainfall, the need for drainage is acute. Irrigation without drainage is not sustainable. In dispersed irrigation areas, matching irrigation induced deep drainage to natural drainage is critical. There are some areas of the landscape where it is possible to match water loading from irrigation with natural drainage processes of deep drainage and lateral dissipation. We should use to advantage an understanding of the deep drainage situation under any irrigated area which will only be reached with good measurement and consistent monitoring.

In most irrigation areas, we will need to impose some form of artificial drainage. The critical issue associated with artificial drainage is how to minimise the amount of drainage which is generated. This can be done by managing water inputs. Next, is the need to concentrate up the drainage waters. As it’s presently practised, this happens by dumping dilute saline drainage water into evaporation basins. This is a short term solution. Disposal is not an option with respect to salt. We need to either reuse these drainage waters and see them as a recyclable resource, change their form or think of storage in a place in the landscape that is going to cause minimum contamination in the long term.

The best prospects for sustainable irrigation in Australia are in those areas close to the coast where controlled amounts of quality controlled drainage can be discharged to the sea. In inland areas, minimising drainage and looking at options such as serial biological concentration, where we are using solar energy to advantage and using the waste stream as a recycle resource, must be looked at.

To achieve irrigated practice which is scientifically sound, we need a major cultural change. This will need change in attitudes towards capital management, towards socio-economics, towards training needs and towards the practice of irrigation.

Increasing amounts of groundwater are being used for irrigation. It is almost always true that groundwater resources are exploited when significant groundwater dependent irrigation develops. Once a groundwater resource is exploited beyond its recharge rate, or is contaminated by inappropriate drainage, it is extremely difficult to rehabilitate or recover. Irrigation dependent on groundwaters must operate within the sustainable yield and to determine that there is a need for ongoing investigation, measurement and monitoring.

Offsite Effects Of Irrigation

Increasingly, irrigation (as with all agricultural enterprises) is being subjected to stringent environmental constraints. The withdrawal of water from rivers, particularly in the south east (Anon, 1995)and south west of Australia, for irrigation purposes, has been excessive, as indicated by the poor state of the river systems. There is no doubt that a decreased allocation from rivers is likely and, given the distribution of application efficiencies (see Thompson, 1995; Thompson & Schofield, 1998) that are demonstrable in all irrigation areas, a decrease of 10% in allocation would have little or no impact on productivity of irrigation areas because this would considerably tighten up the use of water and, in cases, the inappropriate and poor management of water. The effect of trading out some water from irrigation on these over allocated rivers to increase environment flows will very likely increase the variability of supply. The irrigation sector needs to develop plans for managing its irrigation practices with increased variability.

Drainage back into rivers from irrigation areas with waters containing salt, nutrients and sometimes other chemicals, presents a dilemma in terms of management of river systems. Rivers are drains but when people become highly reliant on these drains as their source of supply, the dilemma of running a supply and drain system in one conduit becomes extreme. Sensible resource engineering demands that we should separate our supply and waste streams. We should explore the resource potential of waste and generally concentrate it to reduce the diluted volume and find other uses for it.

Offsite Effects On Irrigation

The major changes brought about on the landscape, through perennial vegetation clearing and replacement with annual pastures and crops has caused major shifts in the hydrological balance of many agricultural areas. In the Murray Basin regional groundwaters have been affected by increased recharge in the higher rainfall area. This increased recharge results in increased discharge both into the Murray River and into the shallower aquifers. Unfortunately, much of the irrigated area of northern Vic and southern NSW is sited on top of a naturally occurring regional groundwater discharge area. Field and modelling studies show that trying to manage rising shallow water tables with improved irrigation practice i these areas is almost futile (Meyer, 1995; Poulton, 1996). In this situation, controlling regional groundwater pressures is the only way to secure the sustainability of the area.

Where shallow groundwaters interact with surface added irrigation water, as is the case for most southern Australia irrigation areas, there has been a push for conjunctive use schemes. That is, using both surface water supply and pumped groundwater as irrigation water. This practice is almost certainly a short term solution especially if water application is not controlled and deep drainage reduced and if there is no net salt export.

A Future For Australian Irrigation

Irrigated agriculture needs to continue development as a high productivity, diverse produce system. The prospects for irrigation in Australia are extremely good, given that the outputs that can be generated can be produced to specification in products which are in demand and which can be valued as exports. Irrigation has the advantage of being much less subject to the whims of seasonal variability. There is the chance to develop exemplary water use efficiency, to minimise losses in the supply systems and to improve greatly our design, implementation and management of controlled irrigation systems. We should set ourselves a goal of replacing Israel as the premier holder of world class irrigation practice. We have a range of environments, a range of leading equipment manufacturers and a well developed community and government agency process to foster the right balance for economic, environmental and social balance. The Murray-Darling Basin Commission, the total catchment management movement, the landcare movement, land and water management plans developed at community level, and site salinity action plans developed at community level, are all world-leading in terms of defining a dynamic balance to using our resource base. As a nation we can develop a presence as a provider of high quality and world leading irrigation science, irrigation practice and irrigation engineering.

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