Recommissioning Nature’s flood fertigation infrastructure

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Recommissioning Nature’s flood fertigation infrastructure

Postby matto » Fri Nov 02, 2012 6:15 pm

Great posts at ... structure/ by Cam Wilson.

Cam is a great student of Peter Andrews and in my mind is one Peter's best interpreters, as far as he can take the complex issue of landscape function and make it easily digestable and understood.

Go to the link above for the full story and more on Natural Sequence Farming.

When building natural capital (including beef or wool), increased potosynthesis is the goal of any land manager. Available moisture is, of course, a key factor.

At the time Europeans settled in South-Eastern Australia, many broad upland valleys were described as chains of ponds or swampy meadows. There are a few of these well hydrated, very productive systems, effectively drought proof systems still remaining (for example the Hazell’s property), but the majority have been severely eroded and subsequently drained (click on the following for an outline of the degradation process, in diagrams or the scientific literature).

At Tarwyn Park, Peter Andrews demonstrated the potential primary production benefits from reinstating the original floodplain processes and rehydrating the surrounding landscape.

One way of doing so is by raising the alluvial water table through lateral infiltration (as described in the post Floodplain water storage). The speed this occurs depends on the soil type, but if it’s going to happen any time soon the main driving factor is a fairly constant supply of water from the catchment above.

High in the landscape, inflow from the catchment above is generally only available for a short period of time. Where this is the case, the effectiveness of relying solely on a lateral hydration approach is limited, as a severely drained landscape will take a considerable time (maybe several lifetimes) before the water table is raised high enough to enhance plant growth on the floodplain.

Where short sharp bursts of runoff are available, the fastest return can be achieved by reinstating the old flood flows. Water spreads out across the landscape once more, soaking into the floodplain for the extended use by the plants and soil life. Sediments are also deposited, the process which has made floodplains the rich production zones they are worldwide. Basically, it’s recommissioning nature’s flood fertigation system.

In an intact landscape, there are predictable locations where floodwater is more likely to top the banks, just as there are locations where it’s likely to re-enter:

On a macro-scale, floodplain flow patterns are often closely related to the ridges intruding into the floodplain (Tane, 1999)

Where multiple ponds exist between the major landscape features, braided flood flows (red arrows) generally exit the downstream half and enter the upstream half of a pond (P Hazell, personal conversation)

When siting structures, an understanding of these processes is the key to getting the most bang for your buck. A structure in an inappropriate location may get the water up onto the floodplain, but it will soon spill back into the gully, maybe even worsening the existing erosion. In contrast, a well positioned structure results in the flow heading away from the watercourse, spreading into a more passive flow and hydrating the floodplain surface before re-entering the stream sometimes hundreds of metres downstream.

On Gunningrah, Charlie and Anne Maslin have sited their structures as well as anyone I’ve seen with this goal in mind. Having taken inspiration from Peter Andrews on ‘Australian Story’ and attending a Natural Sequence Farming field day, Charlie has since constructed around 40 leaky weirs on Gunningrah (For more information about the Maslin’s farming prowess, see their profile in the Soils for life case studies).

There are a range of positive results which the Maslins have achieved depending on the landscape position of the works, but the following couple of examples are a good demonstration of utilising the original flooding processes mentioned above.

(Note: To avoid hefty fines, it’s important to adhere to local watercourse regulations. In many places there are few restrictions on ‘dam walls’ within first and second order streams other than the harvestable rights of the property)

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