The Murray–Darling Basin has a highly variable climate and its hydrological conditions often differ each year and across regions. Planning for wet and dry conditions is a major challenge for river operators, water resource managers and the community.
A history of extended droughts followed by floods highlight this challenge. We have seen new extremes in both drought and flood in many parts of the Basin.
Changes in rainfall patterns and warming temperatures leading to higher rates of evaporation add to the intensity and impacts of both floods and droughts. These changes will continue to increase the complexities and uncertainties faced by water managers.
The challenge of drought
The need to plan for drought has been understood since the early days of water management in Australia. Before dams were built along the River Murray, dry times meant parts of the river and its tributaries would sometimes be reduced to a series of pools, or cease to flow at all.
These experiences highlighted the need to manage the Murray in a more cooperative manner and build the dams, weirs and other structures to provide a more reliable flow. The resultant 1914 water sharing agreement put in place between the Australian and state governments enabled water to be allocated to help to secure water in dry periods.
The creation of the Snowy Hydro Scheme during the mid-20th century provided more water to communities by adding additional inflow to the River Murray system from outside the Basin. Water from the Snowy River continues to be critical to the Murray system, particularly during drought.
Recent initiatives include strategies and actions resulting of the Millennium drought from 2001 to 2009. Foremost among them is the Murray–Darling Basin Plan, which rebalances water entitlements between consumers and the environment. It also includes tiers of water sharing arrangements and protects water for critical human needs.
What does drought mean for the River Murray system?
Predicting and even defining drought has presented another challenge. Drought conditions are caused by sustained periods of lower than normal rainfall, however, their nature and effect varies across the landscape and each drought is different from the last. Other factors that impact drought include soil moisture, evaporation, temperature and cloud cover.
In Australia, the Bureau of Meteorology has led the way in understanding drought and its causes. The concept of rainfall deficiency leading to acute water shortage is used by the bureau to define drought, and a better understanding of the climatic drivers of rainfall is improving predictions.
However, in river systems, the nature and effects of drought are different. The amount of rain that ultimately flows into rivers and storages is heavily influenced by the pattern of rainfall over the year and across the landscape. River regulation and the soil moisture level across catchments are critical.
In a regulated river system with large headwater storages like the Murray, an acute water shortage can occur when both storage levels and tributary inflows are low.
A single dry year can impact landscapes and activities like dryland farming, but will have a limited impact on water availability if storage levels started out high. On the other hand, when long periods of below-average rain and low inflows fail to replenish storages, we see water shortages being felt across the landscape.
When catchment conditions are dry, water recovery requires more than a single burst of heavy rain. Long periods of steady rain at particular times of the year are needed to wet the catchments. When the catchments are soaked, rain will generate run-off. Even then, storages may not be full until flooding rains arrive. The storage capacity of Dartmouth Dam is close to 5 years of average inflow, and even with flooding, it can take a few years to top up following a drought.
Planning ahead
Drought management is about planning ahead, determining future needs and continually assessing risks.
This means testing dry scenarios in our future planning and day-to-day computer modelling. This modelling helps us to understand the effect of inflows receding towards very dry conditions. We also only allow for improvements in water availability as they occur. We routinely manage the system to maximise water availability. This includes storing water as far upstream as practical to reduce evaporation and allow space for downstream storages to capture inflows.
Predicting drought
Our knowledge of the possible extent and intensity of drought is informed by looking at past events. We look at rainfall and flow patterns, long-term averages and historic highs and lows to predict future rainfall, stream flow and water storage levels.
The Murray–Darling Basin Authority (MDBA) is making greater use of the Bureau of Meteorology’s growing forecasting capabilities and the increased performance of climate and rainfall outlooks.
As weather and climate records continue to be broken, we are constantly reviewing and adjusting our assumptions around future inflows, evaporation losses and water demands.
Managing extreme dry
Extreme dry events trigger special management and operational responses to conserve remaining water supplies and limit the impacts that are otherwise unavoidable. Our responses include preserving water for critical human needs, reducing weir pool and flow target levels and changing state water sharing rules.
Management plans are implemented in collaboration with the Australian and state governments and water management agencies. Most individual water entitlement holders are able to actively manage their own risk by using water trade and carryover where possible.
No guarantees
Even with better forecasting, improved planning and new policies in place, we cannot drought-proof the Basin. Droughts will continue to impact communities and the environment.
The demand for water by all river users, and the need to conserve water means there will always be some tension in managing drought.
By improving the way we plan and manage risk, river operators and water users can build resilience and knowledge to help prepare and adapt for the difficult conditions of the future.
