This question has been an area of ongoing active interest from the beginning for the Beyond Bolac CAG. The drying out of Lake Bolac, and of the Fiery Creek which supplies it, for the first time in living memory highlighted the importance of water to our community for the amenity, social, cultural, economic and biodiversity values that it brings. We are trying to determine the relative importance of a number of contributing factors to stream flow decline including:
water extraction at the headwaters,
the effect of reduced rainfall on soil moisture content and runoff,
and the role of the reduction in pastures as broadacre cropping has increased.
Daily stream flow data has been recorded at Streatham for nearly 100 years. The seasonal pattern of zero flows has altered over time. In the past it was unusual for there to be periods with no flow in spring or early summer. This is now more common, especially after 2005.
This figure shows measurements of stream flow in megalitres per day (Ml/d) for the period 1920 to 2015. There is missing data for a ten year period during the 1930s and 1940s. Prior to 1973, measurements were not recorded for the periods when the creek stopped flowing. Measurements have been taken daily since 1973. You can see that the flood of 2010/2011 really was a 1 in 100 year event!
This was the Fiery Creek flowing past Streatham in January 2011.
This figure shows the total annual flow (megalitres) moving down the Fiery Creek at Streatham for the years 1973 to 2015. The annual flow plays an important role in the filling of Lake Bolac further downstream.
Over time there has been an increase in the number of days with no stream flow. This is shown in the chart below as the average annual number of days with zero stream flow per decade at Streatham. Periods with no flow results in increasing in salinity levels of remaining water, as water levels drop and pools contract. This causes stress on vulnerable remnant plants and animals, such as platypuses.
Stream flow and rainfall
Stream flow at Streatham is compared for two different decades in the figures below. These decades were chosen because they had roughly similar total amounts of rainfall in the catchment. Annual rainfall is shown on the right hand axis, and annual Fiery Creek water flow is on the left hand axis. Rainfall at Beaufort falls at the headwaters of the Fiery Creek, while Lake Bolac rainfall is broadly representative of rainfall patterns over the larger area in the lower catchment.
Creek flows in recent years are only a fraction of what they were for the same amounts of rainfall 80 years previously.
Salty groundwater is a feature of the Fiery Creek catchment. There are some salt lakes and also points where the topography and geological layers meet to produce salt water springs. This means that some salt water enters the creek at various points along the Fiery.
Water electrical conductivity measurements have been collected for many years along the Fiery Creek and its tributaries by members of the Upper Hokins Land Management group. These measurements give a guide to the salinity through the creek system.
This video shows the summarised measurements for each season across 15 years. (Click bottom right on the video for a full screen display.) The dates are shown at the top centre. Circles colour intensity increases for higher salinity levels. The creek dries out at various locations during dry periods. Those sites are then not able to be sampled, and in those cases will display as missing circles on the map.
It appears that the salinity levels in the creek increases during those seasons when there is limited runoff across the catchment, and so creek salinity fluctuates over time. During dry times the salinity in the water can become very concentrated at the lower end of the creek system as the water evaporates. Many questions remain unanswered at this stage about the long term changes in salinity, its impact on the system, and appropriate management options.
The Central Highlands Region Water Corporation (CHW) has a bulk entitlement to take up to 419 ML (Megalitres) of water per year from the headwaters of the Fiery Creek. The legal document detailing this entitlement is known as the Bulk Entitlement (Beaufort) Conversion Order 2005. A copy of the order can be found here. One of the requirements in this order is that actual annual extraction volumes be published each year. These numbers can be found in the CHW annual reports under the section titled Bulk Entitlement Compliance Report. The annual extraction has varied between 152 and 295 ML over the years between 2008/2009 and 2015/2016. This extraction amount represents a small proportion of the reduction in stream flow at Streatham.