The Gordon Gnohm

Play along at home, read Future Farmer in The Braidwood Bugle every Wednesday. 

Passive Land Hydration Pt 4 Abatement Swales

If you’ve spent any time looking out the window of a moving car you are probably very familiar with Abatement Swales, although you might not know it. They are very similar to Contour Trenches with one significant difference. 

There are plenty of examples in our area. The most visible being at the Doughboy overtaking lanes on the Kings Hwy. On both sides of the road, Abatement Swales can be seen. They wind across the landscape like dirt snakes catching runoff. They assist the filling of dams in dry times and guide overflows to less fragile areas of the paddock, just like contour trenches and Yeomans furrows.

The grass colonised the mound decades ago, and reeds are evident in the low points of the structure. There is some habitat as described last week, so I was excited when I first saw these structures in the landscape.

How did they get there?

Abatement Swales were installed in the 1970s to protect the Welcome Reef Dam area. They were created to remedy erosion. Significant rain events were damaging paddocks and waterways. Runoff was taking a lot of earth with it after crossing distressed landscapes and dumping it in the Shoalhaven River. Something had to be done.

Stopping the water at the top of the system is paramount to reducing its power. A series of structures slowing the flow is the key to keeping topsoil in your paddock, be it rocks, logs or trenches. The Abatement Swales have done a good job of reducing erosion, but they have missed an opportunity. One key function. 

Abatement Swales are constructed with a 5% fall, as opposed to contour trenches that have 0% fall. This means that abatement swales are more accurately abatement drains. The trenches are filled only when the water is running, akin to a seasonal creek. When the rain event halts so do the full trenches.

As we have learnt, the limiting factor for water soakage is the time it spends in contact with the earth. The 5% fall means that the water will slowly but surely drain away. An Abatement Swale does a good job at draining without damage but is missing a key component for soil health in paddocks. 

Remedy

The soaking benefit of standing water is the best way to improve your soil moisture. Hydration is the key to soil life and productivity. The abatement drains have provided a noble service in the face of erosion, and some minor tweaks to the structure can reap even more benefits.

You could get machines in to modify the trench for a significant change, or you could use simple techniques to a lesser effect. The idea is to create small weirs in the trench. To hold water back before overflowing down the drain and into the next section.

A weir every 10-20m down the Abatement Swale would hold water for soaking in sections along the structure. Not the same capacity as a true contour trench, but enough to improve your paddock.

If a machine was on hand, the trench could be cleaned out on contour and the spoils used to create a weir downstream. This would take some management while grasses re grow, with specific attention paid to the weir during and after rain events. Perhaps it would be best to scrape the trench intermittently down the line, so some vegetation is still present in the system providing some hydraulic roughness.

A simple way to build a weir would be to use stones from the paddock. Sediment will get caught up in the grass and stones. Initially creating a leaky weir and as time goes on, the weir will become impervious, holding more water for longer.

Best use

When the land gets steep, a contour trench can fail under the pressure of the standing water. On steeper land, more loose material in the downhill mound, will come under pressure. On shallow land the trench does most of the work holding back the water. 

Sepp Holzer is famed for growing in the Austrian alps, on steep country. He specialises in projects from 1000m to 1500m above sea level. His use of modified Abatement Swales has brought him great success over the past 70 years, in conjunction with other harvesting systems.

An advocate of the ‘pay attention: copy nature’ philosophy, first coined by his country man Viktor Schauberger at the end of the 19th century. Holzer has created productive polyculture farms where most folks wouldn’t bother, opening up marginal country to regenerative agriculture. 

He raises animals, grass, fruit and vegetables in the wild. He creates productive vegetable garden and orchard systems that thrive in extreme conditions. The growing season for Sepp is typically less than 90 days between frosts/snow. A much more challenging environment than Braidwood, Canberra or Goulburn. He managed these feats using trial and error and rejecting accepted practices, all the time being led by natural systems. Food for thought.

This week

In addition to the trees changing colour, Currawongs are another great indicator of the onset of the cold season. They have just changed their call, signalling the start of their departure. Have you noticed? It’s time for them to move on for winter and they are telling anyone who will listen. 

The Currawong is traditionally a migrant and likes to leave for a warmer climate when it gets cold in the hills or tablelands. Interestingly, evidence is building in cities and towns along the Great Dividing Range, where Currawongs are resisting the urge to migrate because they have an abundance of food, and the human habitat is warm enough in winter. 

The next wave of sauce tomatoes has been harvested and there is way more than the rats and birds can eat. This bounty means its Passata time. Getting an efficient setup takes experience and it evolves every season. Where are all the items I need? I haven’t seen some of these things for a year. More likely, they have been used for other duties, so the setup gets an upgrade. 

Its been a great growing season this summer, regardless of the cooler temperatures. I’m grateful I didn’t have to hand water the garden. The load of produce to store will max out our modest space. There is more than we can possibly eat, so the chickens get to feast too. 

The contribution from the flock is always appreciated, so they get the best tucker to fuel their work. Sure, they provide eggs and entertainment, but I’m in it for the scratching and the poop. 

Remember, every day is a school day.

Stay awesome.

The Gordon Gnohm

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