Drought seems to be fully on, and if you are in Sydney you start noticing it when newspaper articles start reporting on possible water restrictions. Water stored in the dams, and most notably in Warragamba Dam, is dwindling. This is critical for Sydney’s drinking water supply. Warragamba Dam sits at only 55% full, a number that has been rapidly going down over the last several months. It is now very likely that the level will fall below 50%, and this is critical not only in terms of water supply to the City, but also in terms of water quality.
Our research based on long term data has shown that episodes of eutrophication (a state of poor water quality often paralleled by algal blooms) in the Warragamba Dam are much more likely to occur when the reservoir is less than 50% full. More importantly, the likelihood of these poor water quality events increases dramatically when the nearly empty dam starts filling rapidly when the drought finally breaks. The runoff from the surrounding catchment areas that occurs as a result of drought breaking rains carries significant sediment that is rich in nutrients, and specifically phosphorus. This adds to the already high concentration of nutrients of the water in the dam that has been increasing as a result of the lowering of the water level during the drought. Consequently, it is very likely that eutrophication event might eventuate, dramatically worsening water quality, and adding significant cost to drinking water treatment. This was the case during the drought breaking events in 2007, and earlier in 1998.
What can be done to try to prevent eutrophication in situations like this? Typically, all the attention when the dam is empty is towards water quantity, and water quality seems to be put on a side. In contrast to this practice, our research has shown that activities to prevent load of runoff rich in nutrients into the dam should be stepped up at times when the dam is nearly empty, and when it could be expected that sooner or later drought breaking rains will come. This means that investment in nutrient abatement practices (such as raising and maintaining buffer strips, or fencing off cattle to prevent their access to catchment areas) should increase at these times. Rather than investing in abatement at a constant rate over time, it is optimal to step up investment at times of drought, and ease off when the dam is full of water.
Drinking water supply is critical for functioning of a large city like Sydney. Managing dams and other sources to provide sufficient water quantity is of course of key importance. Nevertheless, managing for water quality is as important. At challenging times like this, when the drought threatens Sydney dams, managers should not forget to guard against water pollutants. The relief in terms of drought breaking rains will come, but if preparations are not undertaken those rains will also bring pollutants in the dams, and while there will be water it will be of poor quality. We can and should avoid that!
