Menindee , droughts and record low inflows

Recent issues of the excellent drought communiqués issued by the NSW ‘Dept of Primary Industry’ (DPI) note that cumulative inflows to the Menindee lakes system since June 2013 were lower than comparable periods in the millennium drought. In October 2015 they become the lowest on record - for total (cumulative) inflows over an extended period. The communiqués include a chart showing cumulative inflows for the last three dry periods – starting on June 2001, Sept 2005 and June 2013 respectively. They show that cumulative inflows in the 28 months from June 2013 to September 2015 were under 300 gigalitres (GL), slightly less than the cumulative total in the 28-30 months after June 2001. In the absence of major rains in coming months, the post June 2013 total will fall further below previous record lows.

Given the great volatility in flows to Menindee it is worth looking in more detail – over the longest possible time span. The following chart shown annual (calendar year) inflows to Menindee since 1900 – together with a (centred) 3 year moving average. (The plot for 2015 assumes no more flows apart from the few gigalitres currently in transit down the Barwon Darling system).

12500 Inlow to Menindee 3 year average 10000

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1901 1905 1909 1913 1917 1921 1925 1929 1933 1937 1941 1945 1949 1953 1957 1961 1965 1969 1973 1977 1981 1985 1989 1993 1997 2001 2005 2009 2013

Despite the fact that annual data can sometimes obscure some features shown by monthly data, the graph seems to suggest that the 3 periods shown in the communique are the three lowest extended periods of low flows on record. In fact, if one looks at monthly data and calculates 24, 30 and 36 month moving totals for the last hundred years, the same 3 periods show up as the lowest on record for both 30 and 36 months. The worst 24 month period on record is the 2 years from June 1918 to June 1920 – but was preceded and immediately followed by very large flows. The annual chart (or detailed monthly data) also show that the next two worst periods are in the early 1990s and the 1980s – followed by the period of the 1965-68 drought.

1 The following chart shows cumulative inflows for the both the three periods mentioned in the DPI communiques, and, the other recent periods of low inflows beginning on Apr 1992, Jan 1985 and Jan 1980. Together, these periods cover 6 of the 7 extended periods of lowest inflows in the last 100 years (missing only the 1965-1968 drought). This suggests that extended periods of low flows are becoming more common and more severe.

Cumulative inflows (Gigalitres) to Menindee - months from dates shown

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The huge variability in volume and source of flows means it can be hard to separate the underlying cause of low flows – whether an episodic major drought (eg 2001-09), ‘climate change’ or the increased extraction of water from the Darling’s tributaries. All three factors have probably been important.

Most flows to Menindee arise from significant ‘rainfall events’ that usually are evident in rainfall data for several centres in the Darling basin. The following graph shows data for a representative range of places which can impact on Menindee – for which rainfall data is available for at least 100 years (which eliminated many centres). Glen Innes is close to (and west of) the highest part of the great dividing range and near the source of the Gwydir, Macintyre – and, to a lesser extent, the Namoi rivers. (Because Glen Innes data only goes back to 1910, nearby Emmaville was used for 1892-1909). Mudgee is in the upper Macquarie catchment. The other seven centres are widely spread along the main rivers in the Barwon Darling system. Because annual (calendar year) rainfall varied so much from year to year, a five year moving average was used. Plots for rainfall observations for 2015 assume no rainfall in the December quarter. Despite the large amount of short term fluctuation, there seemed to be sufficient similarity in broad movements over long periods to make it worthwhile to calculate an average for the 9 centres – which is shown by the thick blue line.

2 Darling basin annual rainfall – 5 year moving average in millimetres

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200 Glen Innes Mudgee Moree Mungindi 100 St George Collerenibri Brewarrrina Louth Menindee Average of 9 0

1894 1899 1904 1909 1914 1919 1924 1929 1934 1939 1944 1949 1954 1959 1964 1969 1974 1979 1984 1989 1994 1999 2004 2009 2014

The lower graph shows the ‘9 centres’ average from the top graph, togetherwith the annual (calendar) year average for the 9 centres, and the long term average of the same 9 centres. The purple line shows the 5 year average for the 6 of the 9 centres for which data was available back to 1892. The black line shows the 5 year average inflow to Menindee. To get comparability of scaling with millimeters of average rainfall, Menindee inflows were shown as gigalitres per quarter (ie 5 year average annual inflow divided by 4).

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Average 9 centers, annual 5 yr average 9 centres 200 Long term average 9 centres, 5 yr average 6 centres 5yr average MENINDEE INFLOW 0

1894 1899 1904 1909 1914 1919 1924 1929 1934 1939 1944 1949 1954 1959 1964 1969 1974 1979 1984 1989 1994 1999 2004 2009 2014

3 The ‘9 centres’ average rainfall was also calculated monthly over the last 25 years – ie to cover the last 4 extended periods of low flows to Menindee. Because the northern areas of the basin have more rain in summer, the monthly averages were summed to 12 month moving totals. This also makes scaling comparable to the two previous graphs. Thus the following graph shows the same variables in the preceding graph – but on a (twelve month ended) monthly basis.

1400 Rain (mm) in prev 12 months, 'average 9 centres' 1200 Long term average rain 9 centres (mm) 1000 Quarterly average low to Menindee, last 12 months, Gl

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0 Jan-90 Jan-91 Jan-92 Jan-93 Jan-94 Jan-95 Jan-97 Jan-99 Jan-00 Jan-01 Jan-02 Jan-03 Jan-04 Jan-05 Jan-07 Jan-09 Jan-10 Jan-11 Jan-12 Jan-13 Jan-14 Jan-15 Jan-96 Jan-98 Jan-06 Jan-08

Close examination of the last three graphs shows,

• Although parts of the Darling basin are experiencing extended dry spells, the current record for low flows over an extended period has occurred without record lows for rainfall. Although below average, rainfall in recent times has (so far) been noticeably higher than in several earlier dry periods – including the droughts of 2002-4, 1965-68, the mid 1940s and the ‘Federation drought’. • there seems to be a trend for inflows to Menindee to be progressively lower for a given amount of rainfall – especially after the 1960s. In the first half of the 20th century there are several examples of reasonably strong inflows to Menindee continuing despite rainfall being below average for extended periods. In more recent times, significant flows to Menindee seem to require ‘above average’ rainfall.

Obviously a major factor in the shift in the relationship between ‘basin wide average rainfall’ and ‘flows to Menindee’ is the increased storage and upstream from Menindee. Most of the increase in NSW storage and irrigation occurred from the 1960s (Keepit 1960, Burrendong 1967, Copeton 1973 and Split rock 1987). Together these major storages capacity is over 3300 GL. – and the initial filling of the two larger would have contributed to the

4 observable fact that the high average rainfall years of the early 1970s was associated with a smaller flows to Menindee that the high rainfall years in the 1950s. There are some gaps in the readily available data for the volume held in the major NSW upstream storages on Darling tributaries – however what data is available suggests that filling these storages is usually only associated with ‘above average’ rainfall – the last of which was 2010-12. Importantly, all major dams are currently at very low levels.

All this suggests that extended periods of low flows to Menindee are now much more likely than they were a generation ago – unless full implementation of the Murray Darling Basin Plan has a major impact. A range of data suggest, that implementation of water buybacks and environmental flows from the progressive implementation of the Basin Plan has already helped the Murray. However this is not yet apparent in data for the lower Darling – nor would it be expected, as ‘environmental releases’ to places like the Macquarie marshes or Narran lakes would never reach Menindee. Thus, it would seem prudent to plan on the basis that extended periods of low flows are now more likely than they were prior to the 1980s.

One unfortunate consequence of extended periods of low flows is that (apart from about 10Gl in Copi Hollow) most of the remaining water in the Menindee system is the shallow lakes (Wetherell, Tandure and sometimes Pamamaroo) that are upstream of the main weir. Paradoxically, although the deepest water in the Menindee system is in the river channel upstream of the weir, the average depth of Wetherell is only about 2 meters – almost irrespective of the water height at the main weir. This is demonstrated in the following chart, which

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6 W&T Level above 53m AHD 5 W&T average depth 4 Wetherell average depth 3

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0 Jul-12 Jul-13 Jul-14 Jul-15 Jan-13 Jan-14 Jan-15 Sep-12 Sep-13 Sep-14 Sep-15 Nov-12 Nov-13 Nov-14 Mar-13 Mar-14 Mar-15 May-13 May-14 May-15 covers the full range of levels from when the lakes are ‘supercharged’ (August 2012 and mid 2013), periods of river channel ‘bank full’ (59.7m AHD, 6.7m on the chart) in early 2013 and mid 2014 - and the recent low point in May 2015.

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The top line shows the monthly average lake level of Lake Wetherell (including Tandure) above the ‘no-flow’ river level below the main weir (ie actual lake level less 53m). The red line shows the average depth of Lakes Wetherell and Tandure derived by dividing the volume in the lakes by the average area implied by the respective observations for changes in ‘level’ and ‘volume in storage’. The green line shows the similarly calculated average depths of Lake Wetherell in the period in 2015 when the lakes Wetherell and Tandure were separated by an embankment and water pumped from the shallower Tandure into Wetherell. Paradoxically, the average depth actually falls when the lake Wetherell level rises above 59.8m AHD, and water spills out of the river channel onto the flood plain.

Evaporation around Menindee averages more than 2 meters a year – often much higher in years of above average heat and wind - which generally co-exist with drought and water shortages. In such circumstances it seem absurd to attempt to store water at an average depth of only 2 meters. Within 12 months the only water left will be that stored in the small area that that was previously deeper than 2 metres (a few square kilometers in the case of Wetherell). Although storing water 2 meters deep may have been less rediculous when the average period between major inflows was quite short – it is crazy when major inflows are frequently more than two years apart – as has seems to have increasingly been the case since the 1980s.

I commend the NSW government’s 2014 decision to separate the shallower Lake Tandure from the deeper areas in Lake Wetherell – and reduce the surface area of water (the critical factor to reduce evaporation – and generally a corollary of raising average depth). Nevertheless, notwithstanding important environmental and heritage constraints, I find it astounding that more has not been done to concentrate water in the Menindee system to reduce surface area and limit evaporation losses – especially when water is scarce.

There are many reasons for a major effort to raise the average operational depth of Lake Wetherell (including Tandure) – by a mix of excavating to increase the area of deep water and embankments (to prevent spreading shallow water - except when there are strong environmental grounds – or needed for flood control). Reasons include, • Wetherell is ‘first filled’ (to 59.8M) and ‘last emptied’. Hence any increase in average depth will have on-going permanent benefits for reducing evaporation loss (in summer, every 3 square km reduction in surface area in the system saves about a gigaltre a month in evaporation loss), • Wetherell seems to have more areas of lesser environmental or heritage importance than other lakes of high environmental significance, • the deepest parts of the system are in Wetherell, and the difference between its ‘floor level’ and ‘average depth’ is much greater than for other lakes. It is also much better connected to downstream uses than Cawndilla – which has the next deepest areas, • the absence of other reasonable options when the Menindee system has fallen to under 2 meters average depth (and holds under 200GL) – and is receiving small inflows. When refilling from low levels, the average depth

6 actually falls when Wetherell and Tandure are filled to 59.8M AHD (average depth about 2.2m) and water starts flowing to Pamamaroo. It does not regain an average depth above 2 meters till the system holds about 200Gl. • excavation (and building embankments) is generally easiest in dry ground – and involves less disturbance to existing stored water. The river channel within Wetherell only amounts to about 10 sq km – meaning around 90% of the lake is frequently dry (ie when the lake level is 59.8m AHD or less), • the graph of depths shows that separating lake Tandure raises the average depth of Wetherell by about a metre (at other depths as well as the 59.8 ‘bank full’ – when it leaves an area of about 10sq km at an average depth of 3 meters). This suggests Tandure should be separated as often and for as long as environmental considerations allow.

Drainage channels are also important to reduce evaporation loss (and loss of native water creatures) because water has become isolated from exits. The SKM Water Savings project Final report of 2010 included a drainage channel in Lake Pamamaroo in ALL the long term options it found to be cost effective. It (and others?) should be put in place as soon as practicable.

If (as now seems possible) the Menindee system is going to have to operate for extended periods with less than 200 GL, it would seem sensible to quickly modify the infrastructure, so that when filling from near empty;

• the first 30Gl (about 10 sq km, average depth about 3 metres) goes into the river channel – before any water goes into Tandure (unless environmental considerations are compelling), • a further x? GL flows into more deep areas created in Wetherell by the best mix of excavation and embankments that allow the level to be raised above the current ‘bank full’ 59.8m AHD before spreading to Tandure or the flood plain1, • Tandure is filled to (at least) the same level as the modified Wetherell – with the capacity to be raised above Wetherell by pumping etc (or hold water above the Wetherell level in the drawdown phase), • Unless there are larger inflows in foreseeable prospect, water should not flow to Pamamaroo (hopefully with appropriate drainage channels and more efficient connectivity with Copi hollow) until the maximum possible level (and average depth) is reached in Tandure.

1 Excavation may be expensive for water storages but adding a square kilometer or two to Wetherell with a ‘floor’ below 53M AHD would provide a significant increase in the area of deep water that was protected from evaporating in a sustained inflow drought. But then in extreme shortages, the deepest parts of Wetherell will be the last available surface water in the whole lower Darling region for all native creatures.

7 Clearly there is scope for lower evaporation in a more efficient Menindee system to make a big contribution to the ‘water recovery’ still being sought under the Murray Darling Basin Plan. However, NSW and the Commonwealth need to come up with a major improvement to the 480/640 rule. The last time the rule ‘cut in’ what water left in (or on the way to) Menindee was in the wrong places. Clearly more efficient infrastructure (and better specification of water availability) should enable lower cut-off points – and facilitate some of the options canvassed in the SKM Water Savings project Final report. Recent moves to tap underground sources and provide additional security for ’s water supply add to the case for reducing the 480GL threshold.

Notes: Data on inflows was ‘Flow at Wilcannia’ (425008) obtained from NSW Water website. It was supplemented where appropriate by MDBA monthly data for ‘Inflow to Menindee’ from 1900. Some missing NSW water observations were filled by using the best available alternative (MDBA data, flow further upstream, change in Menindee volume less known outflows etc). There are some inconsistencies with the communiqué graphs – but they are not large enough to affect the conclusions. Estimates of average Wetherell (area and) depth used NSW water daily data (levels to 3 decimal places). Estimates of Tandure depth were derived from levels and volume data in DPI communiqués.

Rainfall data was obtained from the Bureau of Meteorology website. To avoid ‘missing observations’ affecting averages, the nearest appropriate site was substituted for the relevant period. The ‘averages of 9 centres’ does not purport to be a precise average for the basin – and have little significance beyond whether rainfall over various periods was above or below its long term average. While a larger or different selection would produce different averages, further sampling suggests similar outcomes for above/below long term average calculations. In fact, deleting Mudgee from the average (because it has a different weather pattern to the Northern basin – and very little water from the Macquarie ever reaches the Darling) and replacing it with a centre close to the Namoi dams (eg Manila) would slightly improve the correlation between ‘9 centre averages’ and ‘inflow to Menindee’. Similarly, including more centres in the Culgoa basin and upstream would make the sample more ‘representative’ - but only marginally change ‘above/below average’ results. There is also a problem getting data for earlier periods – especially for relevant places in catchment areas where dams (and associated recording stations) have only been established in the last 50 years or so – eg Copeton, Burrendong, Split rock, Pindari etc. I have not included places in the Castlereagh, Bogan and Warrego catchments because (as far as I can tell from readily available data) these rivers have only made sizeable contributions to Darling flows on comparitively few occasions – and when major rains and river flows have been widespread. One could argue that Wilcannia (where Menindee inflows are measured) should be used rather than Menindee – but the difference in rainfall is negligible – except over short periods.

Rob Foster

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