Manage Mallee seeps at the source for best results
Growers dealing with waterlogging caused by Mallee dune seepage should treat the problem at the source for best results.
Growers learned the best methods of managing Mallee seeps at a GRDC-funded workshop in Karoonda, South Australia, which presented the results of deep drilling, soil characterisation investigations, land unit mapping and monitoring well installations.
The investigations, performed across three SA Murray Mallee sub-catchments near Wynarka, Karoonda and Mannum, were conducted by the SA Murray–Darling Basin Natural Resources Management Board through funding from the Australian Government’s National Landcare Programme.
Juliet Creek Consulting principal consultant James Hall says the work has demonstrated the mechanisms of Mallee seeps, including how seeps form (Figure 1) and how best to minimise their impact.
“Initial soil and drilling investigations in the sub-catchments assessed to date have shown that the main cause of seep formation is excess water draining through sandy dune soils, which is then held-up when it reaches the low-permeability layer known as Blanchetown clay,” Mr Hall says.
“This excess water then forms a ‘perched’ watertable upon the Blanchetown clay, which can be at considerable depth (for example, up to 10 metres). However, where this low-permeability clay occurs at shallow depth, especially in swales and on lower slopes, water can be forced to the surface, causing severe waterlogging. Prime cropping areas can become non-arable swamps.”
The best way of managing the issue, and to improve farm production, is to attempt to use more rainfall where it lands, and prevent or minimise the amount of seepage water that feeds into seep discharge areas. Other management options include only managing the seep area itself in an attempt to prevent spread and degradation, or doing nothing and waiting to see what happens.
“To use more rainfall at the source, growers can work on improving crop agronomy, such as better management of water-repellent soils and soil nutrition,” Mr Hall says
“Another option is to improve the soil, which could mean alleviating soil compaction by the use of spading or other methods, or the use of clay spreading to improve water-holding and nutrient-holding capacities. Other options to improve water usage include incorporating deep-rooted perennials into the farming system or growing summer crops.”
Growers should seek advice from appropriate specialists before undertaking soil amelioration or selecting suitable perennial plants.
Mr Hall does not recommend growers adopt a wait-and-see approach.
“It is better that growers attempt to maximise the water use by productive plants across their land, rather than hoping a seep problem might resolve itself. Moreover, once seep areas become degraded by erosion or increasing salinity, they are much harder to rehabilitate back to productive farm land.
“Based on anecdotal and other evidence, it seems most seep areas in the SA Murray Mallee formed after wet periods in and following 2004 or 2010. However, there is no evidence that once formed, seeps will go away by themselves. In fact, many seeps seem to be increasing in size.
“Research is continuing into methods of improving the fertility and water-holding capacity of dune sands, to increase crop production and reduce seepage,” he says.
“For instance, initial trials of spading chicken manure to a depth of 40 centimetres are showing promising results.”
Tube wells (peizometers) and water-depth-monitoring devices have been installed to monitor the effect of land management changes on seepage-induced watertables.
0477 400 092,
Murray Bridge Natural Resources Centre,
03 8532 9100;
GRDC Project Code AEA00005