Menindee lakes, 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 7500 5000 2500 0 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 1600 1400 Jan-80 1200 Jan-85 1000 Apr-92 Jun-01 800 Sep-05 600 Jun-13 400 200 0 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 The huge variability in volume and source of Darling river 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 1000 900 800 700 600 500 400 300 200 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). 1000 800 600 400 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 800 600 400 200 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 irrigation upstream from Menindee. Most of the increase in NSW storage and irrigation occurred from the 1960s (Keepit dam 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 dams 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 Lake Wetherell is only about 2 meters – almost irrespective of the water height at the main weir.