Where has the water gone?
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. This chart 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!
The chart below 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.
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. The table below shows the number of days in each quarter where the Fiery Creek has not been flowing, measured at Streatham. 1 = jan to mar, 2 = apr to jun, 3 = jul to sep, 4 = oct to dec.
Stream flow and rainfall
The relationship between rainfall (right hand axis) and water flow at Streatham (left hand axis) is displayed for two different time periods in the figures below. 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. It can be seen that the variation in annual stream flow (Ml/year) is linked to the variation in annual rainfall (mm/year). Stream flow in the recent past is much reduced compared with the flows of 70 years earlier under similar seasonal rainfall regimes.
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.