Lake McLarty¦Rainfall Change, Hydrology and Management

KRM Branch March Meeting

The speaker in March was Peter Muirden, a DBAC Hydrologist; he was accompanied by Bob Paterson from the Friends of Lake McLarty.

Lake McLarty is an ephemeral wetland just to the east of the Peel Harvey Estuary. It has long been a significant site for waterbirds, with the numbers for several species sufficient to maintain its status as a Wetland of International Significance within the Peel Harvey – Yalgorup Ramsar Site. However, prior to the high rainfall in the late winter and spring periods of 2018 that saw the lake fill to high water levels, changes to rainfall patterns had resulted in several years of low water levels that lead to an earlier drying out of the lake. This has resulted in lower numbers of birds using the lake. Peter explained that he has investigated the hydrology of the lake in order to understand what is happening and how conditions can be improved. The first few slides of his presentation outlined trends in historical rainfall levels at Pinjarra. A historical rainfall chart showed that rainfall figures were relatively stable until 1970 when there was an 11 per cent decrease and a further drop to 22 per cent less in 2000. It should, however, be noted that this significant decrease in rainfall is not evident for the majority of Australia, it is only seen in the south west and south east of the country. A further breakdown of the rainfall data indicates that for the south west of WA the middle region (including Lake McLarty) shows the 1970 and 2000 decreases, the southern region only shows the 1970 decrease, the northern region only from 2000 while the Wheatbelt shows no change. A seasonal breakdown of the Pinjarra rainfall data shows that the drops in annual rainfall from 1970 and 2000 can be explained by a drop in early winter rainfall (April – July) whereas there is no change in late winter rainfall totals (August – October). A study of water levels in the lake between 1995 and present show that, in 1999 and 2000 (big rainfall years), water levels reached 1.35m and 1.60m relative to sea level, respectively. Years 2005 (1.3m SL), 2008 (1.20m SL) and 2013 (1.00m SL) also saw good water levels. In the period 1995 – 2005 the lake was dry for between one and three months of the year. After 2005 this has changed to between three and seven months dry per year. For example, in 2017/2018 the lake was dry between November 2017 and June 2018. Prior to 2005 there were mudflats across the lake, which were full of macro invertebrates on which the birdlife were feeding. From 2006 onwards there have been fewer mudflats (with no macro invertebrates) and more Couch grass and Samphire. The roots of these went down into the substrate and caused cracking of the lake surface when dry leading to the release of acid sulphate soils. This can result in a drop in water pH and algal blooms. A PowerPoint slide showed images that highlighted the change.

In June 2018 the Technical Advisory Group for Lake McLarty prepared funding submissions for a continuation of surface water monitoring, groundwater bore monitoring, and an acid sulphate soils study to further understand conditions and processes at the lake that will enable management strategies to be developed. Peter then showed an aerial photo of the lake after the winter of 2018 with water well back into the surrounding Melaleuca woodland. This was the result of 925mm of rainfall in 2018 at Lake McLarty (compared to 659mm at Mandurah). The lake had a volume of 1,840ML (an inflow of 1140ML from the drain that feeds into the lake at its southern end plus rainfall of 700ML) and a water level of 1.23m SL and Peter has calculated that it will hold some water until at least late April 2019. Surface water monitoring showed TDS (Total Dissolved Solids) levels in the centre of the lake at 1000mg/L whereas at the eastern edge TDS levels reached 9700, suggesting some interaction with groundwater (groundwater under the lake is almost the same as sea water). Concerns that the cracking evident in some regions of the lake’s floor—and exposure of acid sulphate soils—may lead to lower pH levels in surface water were allayed when monitoring results indicated a pH range of 7.7 to 9.4, suggesting there is sufficient acid neutralizing capacity in the lake’s surface sediments. However, an acid sulphate soils study of the lake has commenced and will continue in 2019.

The last part of Peter’s presentation discussed water management at Lake McLarty. The main input to the lake is via a channel off the southern drain. To direct more water into the lake some barriers have been placed in the main drain. This drainage water can be impacted by nutrient run off from farming properties as was the case in 2017 when a local farmer heavily fertilised some land to increase feed for stock. The increased nitrogen and phosphorus levels will increase growth of couch and other undesirable plants at the lake. Peter explained that the offending land has now been purchased by DBAC. There are plans to move the drain south to loop through the newly acquired land. The drain will be planted with plants that will aid in stripping nutrients from the drainage water. It is hoped that this will help to control couch growth on the lake surface during dry times.

Peter finished his presentation by stating that there is also the possibility that other drainage water, currently passing directly to the Peel-Harvey Estuary, may be harvested to increase water flow into the lake and return the time when the lake is dry to the 1 – 3 month range. It is hoped that lower nutrient inputs together with shorter dry periods will see a return to the lake’s floor providing mudflat conditions attractive to migratory and resident water birds. Bob Paterson then provided details of the number of water birds on Lake McLarty during the annual Shorebird 2020 count on February 10, 2019. A total of 16,000 birds were counted, with 44 species represented. This is the highest count for a number of years. It is hoped that the management strategies outlined in Peter’s presentation will see these numbers sustained in future years. The audience thanked Peter and Bob for a most informative presentation.

Colin Prickett