Fill your ring dams with used plastic drink bottles!
Rural Australia has a vast number of ring dams. When flying you can sometimes see a dozen or more of them winking up at you. Farmers need water storage and ring dams are generally the cheapest option. Unfortunately, they are fairly inefficient and lose a lot of water to evaporation. In most of Australia a ring dam will lose over 1,000 liters of water per square meter of surface area a year. In some places, twice that. Dams can be covered but this is expensive and requires ongoing attention and maintenance. Thousands of floating balls could be thrown into a dam as a low maintenance approach, but there is probably no way they could currently be made cheaply enough to be cost effective for farmers without a subsidy. So I thought, why not fill up ring dams up with floating used plastic drink bottles?
Provided enough were put in a dam they should cut evaporation by more than half. They may need to be partially filled with water to increase the dam surface area that is blocked from contact with air and stop them being blown out in a gale. Used drink bottles are very cheap. Currently people just throw them out. They are also made of materials that are known to be safe for long-term contact with drinking water. Supermarkets in rural areas could collect used bottles and donate them to farmers for ring dam covering as a green public relations exercise.
Now there may be problems with this idea, apart from the aesthetic one of filling dams with rubbish, but if there are problems I've overlooked, hopefully people will comment and let me know.
Update: Okay, one thing I've overlooked is bottles could, on average, create more turbulent airflow over the open bits of water offsetting some of the benefit. But I don't know just how much of an effect this will have. I asked a friend to model turbulance for me and that was three years ago and she hasn't finished yet. And now whenever I see her she has a crazed look in her eye and constantly mutters, "I need... more... processing power!" Apparently she spends most of her time online trying to steal FLOPS. So I told her she should stop trying to hack into Chinese supercomputers and instead hack whoever it is that makes Fantasic Four movies.