Peter Hazell and Carolyn Hall presenting to Conference attendees on Day 2, overlooking the Lower Mulloon floodplain.

Wow. What a conference. Thank you to everyone who attended.

The preliminary data from the Mulloon Rehydration Initiative is now being released and some of the results are very promising.

My favourite takeaway is how this data seems to indicate that repaired landscapes containing leaky weirs do not decrease the flow of water down a catchment. Further, they seem to lead to increased flows through the system. To revisit the slides that show this data, see Peter Hazell’s blog post here.

A theory we are considering that explains the difference between the median flows at the two gauges in the first slide is that the Mid Mulloon floodplain pocket was already partially ‘sponge-filled’ during this period, due to the prior installation of leaky weirs in 2006. As this section was already ‘sponge-filled’ it was not taking water from the system, rather, it was likely to be contributing water. This water was then absorbed by the ‘dry sponge’ floodplain section of the Lower Mulloon floodplain (which was not ‘sponge-filled’ – it had no leaky weirs at this time), or it simply evaporated. This was a very dry period after all. As a result, an average of only 7.5 megalitres of water per day made it to the Lower Mulloon gauge. That is an average of 20 megalitres per day less than the flow that left the ‘already sponge-filled’ Mid Mulloon floodplain section.

The second slide (from 2018-2024) covers a ‘wetter period’ (when plenty of rain fell over the east coast of Australia). It also covers the period after we installed 35 leaky weirs along the 10km of Mulloon Creek between the Mid Mulloon gauge and the Lower Mulloon gauge.

Between 2018 – 2024, the median daily flow at the Mid Mulloon gauge immediately downstream of Mid Mulloon floodplain was 40.4 megalitres (an increase of 48% from the previous dry period – which would be expected during a wet period).

On the other hand, the lower-Mulloon gauge (below 35 newly installed leaky weirs) shows a median daily flow of 53.3 megalitres - an increase of 610% from the previous dry period median of only 7.5 megalitres per day.

So, how might we interpret this significant change?

It is possible, likely even, that the Lower Mulloon section has become ‘sponge-filled’ for much longer periods during this time due to the installation of 35 leaky weirs throughout this 10km section. This has banked water in the floodplain aquifers and created a pressure response from the raised water level of the stream, which is slowing recharged groundwater movement into the stream. As a result, the whole of the mid and lower Mulloon sections of the catchment are now more fully charged than prior to the installation of the weirs. This allows more water to flow through the system.

This data suggests the leaky weirs have had a significant and decisive impact. Even though it was a period of wet weather, if we compare the 610% increase in median flow at Lower Mulloon with the 48% increase at Mid Mulloon gauge, it is hard not to infer that these increased flows have something to do with the introduction of 35 leaky weirs into the Lower Mulloon floodplain.

So, what might we conclude from this?

The introduction of leaky weirs does not stop water flowing through a catchment. Given that leaky weirs are sometimes confused with dams, and some people believe they ‘impound’ water, this is important information.
Landowners who have installed leaky weirs appear to be giving the gift of more water to their neighbours further down the catchment while also hydrating their own land. How many more landholders might be in this situation if this method was scaled across the country!
Government regulation should be amended to encourage actions that rehydrate the entire landscape, especially our creeks and streams (see our draft Code of Practice and please spread the word!)

It is wonderful to see the fruits of the good decisions made by Tony Coote to support this ambitious experiment back in 2006.

Who says science is boring – these are exciting days!

Best to you all,

Matt

Matt Egerton-Warburton