The Huge Challenge of Dryland Salinity
GroundCover™ Issue: 26
Stories this page and next are edited versions of material published in a special feature'Dealing with Dryland Salinity' in Ecos 96, reported by Bryony Bennett and reprinted courtesy of CSIRO Publishing.
RISING SALT: THE BIG PICTURE Why and what now?
Dryland salinity affects almost 2.5 million hectares of Australian farm land, and is expanding at a rate of 3-5 per cent a year, at an estimated annual cost of $270 million.
Pressure is mounting for farmers to address land degradation issues such as dryland salinity, a consequence of Australia's variable climate, unique geology, and the failure of agricultural systems to perform the ecological function of native vegetation.
Australia's geology has been relatively quiet for the past 60 million years. This has resulted in old weathered soils, many of which have low conductivity, and a generally flat topography with low hydraulic gradients. Water movement through the landscape is so slow that sea salts carried inland by wind and rain have accumulated in the soil. (In more mountainous and wetter continents, salts are regularly flushed back out to sea.)
Before land clearance, most watertables were deep below the surface, and the undissolved salt was immobile in the soil. But in agricultural landscapes, deep-rooted perennial vegetation has been replaced with shallow-rooted crops and pastures.
Native vegetation is attuned to Australia's variable rainfall patterns because over a whole year it has the potential to transpire more water than actually falls. It doesn't matter if rainfall peaks in winter, or is dumped in severe storms. The vegetation's deep root systems even out the variation by drawing on the water year round.
In the case of agricultural crops and pastures, however, any water percolating deeper than about 2 m will not be retrieved. This excess water moving below the root zone has thrown natural hydrological cycles out of balance, causing watertables to rise beneath catchments, much like the filling of a bathtub. In the process, salts are redissolved and carried with the water to the surface, concentrating in topsoil and leaching to waterways. Salinisation can take decades to emerge, and to reverse.
It will get worse before it gets better
Dryland salinity affects almost 2.5 million hectares of Australian farm land, and is expanding at a rate of 3-5 per cent a year, at an estimated annual cost of $270 million. By the time it stabilises, it is expected to have affected some 12 million hectares. Off-farm effects will be felt by all Australians through declines in water quality, habitat degradation, damage to buildings and other assets, and biodiversity loss.
Strategies for salinity management aim to reduce the amount of water recharging into watertables. They include increasing water use by crops and pastures, strategic tree planting, switching from annual pastures to perennial systems and enhancing remnant vegetation. Sometimes, drainage and pumping are used to collect and reuse or dispose of excess ground and surface water.
The choice of strategy, however, and its potential for success depend on a baffling array of interconnected site factors. These include climate, soil type, the extent of salinisation and its position in the landscape, the size, geology and topography of the catchment, and the depth and salinity of the watertable.
Because these conditions vary widely across southern Australia, remedies must be individually tailored. Controlling dryland salinity is therefore a daunting prospect not only for landholders, but for governments, communities and scientists.
|state||area salt affected in 1996 (ha)||potential area affected at equilibrium* (ha)|
|Western Australia||1 804 000||6 109 000|
|South Australia||402 000||600 000|
|New South Wales||120 000||5 000 000|
|Queensland||10 000||74 000|
|Total||2 476 000||>11 783 000|
*The potential area affected at equilibrium is the likely area to be affected if the current levels of salinity are not treated. (Source: LWRRDC 1997.)
A 1997 review of the National Dryland Salinity Program (NSDP) (see pl4) pinpointed a number of reasons for Australia's failure to manage dryland salinity. Chief among these was the lack of technically efficient and cost-effective solutions for the variety of hydrological imbalances for a range of locations.
According to the technical committee of the NDSP, the ability of scientists to prescribe effective remedies is hampered by a lack of spatial and temporal data on the extent and risk of salinity at almost every scale. Compounding this problem is the absence of 'short-cut' approaches.
The NDSP review found an even bigger obstacle to management may be the physical distance between the causes and effects of salinisation.
Areas where rainfall enters groundwater systems (recharge zones) are often in elevated parts of catchments, well away from the visual impact of salinisation. Discharge zones are where watertables are close enough to the soil surface for water (and dissolved salts) to be drawn upward by evaporation and capillary rise.
Discharge can occur many kilometres from the source of recharge, and its costs, in addition to reduced farm productivity, are spread across the wider community. As a result, efforts aimed at reducing recharge would not necessarily benefit the landholders required to make them.
A success story
A model project of dryland salinity control comes from the 900-hectare Burkes Flat catchment in central Victoria, which by 1983 was 12 per cent salinised. A salinity-control strategy in the mid-1980s — which introduced trees and perennial pasture in recharge zones — effectively drew down saline watertables by 1-4 metres.
A major reason for success was the catchment's small size. Only four landholders were responsible for managing it. They all contributed to the salinisation, and they all experienced its effects, so their incentive to tackle the degradation was strong.
The dollar costs and benefits of salinity management in larger catchments are less clearcut. A CSIRO report to the Department of Primary Industries and Energy showed that for many regions drastic land-use changes will be needed to significantly reduce areas of salinity.
The economic view may need to be broader. If policy and infrastructure considered regional benefits (downstream environments and users of water) and wider community values (rural communities, aesthetics, wildlife habitat), then (strategies like tree) plantations would be more attractive to land managers.
It seems that ultimately communities will have to determine socially acceptable rates of salinisation as a basis for setting regional management targets. They have to achieve a balance between salinity prevention and appropriate uses of saline land.
High-tech advice for farmers on watertight cropping
Hamish Cresswell and his colleagues must have harvested enough fodder in the past three years to fatten all the domestic livestock in Australia. Or they would have, if all their crops and pastures had been grown in paddocks rather than computers.
Dr Cresswell, a CSIRO soil physicist, is coordinating a team of scientists involved in deep-drainage research for the National Dryland Salinity Program. They hail from CSIRO Tropical Agriculture, CSIRO Land and Water, NSW Agriculture, NSW Land and Water Conservation, the Queensland Department of Natural Resources, and the Victorian Department of Natural Resources and Environment.
The team is using a modelling framework called APSIM (Agricultural Production Systems Simulator) to find out how much water drains beneath a range of cropping and pasture systems at a paddock scale. The framework was developed at the Agricultural Production Systems Research Unit in Toowoomba.
APSIM's agronomic prowess is awesome. It grows sorghum, cotton, sunflowers, maize, barley, cowpeas, peanuts, sugar cane, stylo, lucerne, and tropical and subtropical grasses, in a range of soil, water, nutrient and climate conditions. Under each scenario, cultural inputs such as fallow length, tillage and crop rotations can be varied, just as a farmer would in actual paddocks.
For each kind of production system, APSIM can predict many outcomes, including deep drainage and crop yields. This allows the associated costs and benefits, both economic and environmental, to be assessed. By repeating each production phase, cumulative effects over many years can also be calculated.
Knowledge gleaned from these model runs will help farmers in areas of rising groundwater tables across Australia to make land-use decisions that maximise production, but minimise leakage to groundwater tables. It will also help other scientists involved in groundwater modelling at a catchment scale.
Before the team will stake its reputation on APSIM, however, the framework is being subjected to a rigorous program of validation and development. Predictions are currently being reality-tested against the results of field trials on the Liverpool Plains in northern NSW. (This will yield a northern region cropping and wateruse scenario. See story next page — Opportunity Cropping — a win-win scenario? — Ed.)
"Once we're happy with its accuracy, we'll run the model over periods of 50 years of historical climate data, for example, to see what's happening in the long term," Dr Cresswell says.
above: Installing 6 m neutron moisture probe access tubes in lucerne pasture at research site on the Liverpool Plains. above left: The Liverpool Plains catchment.
above left: The Liverpool Plains catchment.
Effects of summer crops and no-till
Dr Cresswell says climate and soils can have a big effect on deep drainage, meaning the answer on one farm can be quite different on another. For example, water generally moves more slowly in soils with fine clay particles, but this depends on how well the soil is structured. Soils that are not ploughed are more likely to be well aggregated, retaining pore spaces through which water and air can move. The chances of reducing deep drainage through land-use change are better in summer-rainfall zones such as the Liverpool Plains than in regions such as southern Victoria, because significantly more water can be transpired during a summer cropping phase.
The research team hopes that increasingly APSIM will be consulted directly by farmers, perhaps through local agronomists trained in its use. Results of the modelling and fieldwork will also help determine whether changing agricultural management will make a significant difference to groundwater tables on a catchment scale.
"We're at a challenging frontier in methodology that requires both catchment and paddock models. The breakthrough we're seeking is to put the two together."
The Agricultural Production Systems Research Unit is a collaborative venture between CSIRO Tropical Agriculture, the Queensland Department of Primary Industry and the Department of Natural Resources.
WANT TO KNOW MORE?
These are edited versions of stories from CSIRO's quarterly magazine Ecos (No. 96) of July-September 1998. Back issues of Ecos ($8 each) or subscriptions ($25 p.a.) are available from CSIRO Publishing, PO Box 1139, Collingwood 3066, freecall 1800 626 420, fax 03 9662 7555, email firstname.lastname@example.org or on the Internet at http://www.publish.csiro.au
For more information about the National Dryland Salinity Program, contact Mr Nicholas Newland: ph 08 8204 9153; fax 08 8204 9144; email email@example.com
The National Dryland Salinity Program has a comprehensive website:
Six ways of putting 25% of the land back into deep-rooted vegetation. The length of the tree-crop interface on 1 hectare of land increases from 200m for block planting to 2500m for narrow alleys. The water use of trees in plantations is better understood than that from scattered trees or strips where advection, lateral root growth and subsurface lateral flow of water are important
(Diagram reprinted with the permission)
Region North, South, West