Loss of storage water through evaporation with particular reference to arid and semi-arid zone pastoralism in Australia (Working paper no. 19, the WaterSmart™ Literature Reviews)

With increasing environmental concern and focus on irrigation water use efficiency, there is now considerable pressure on us all to optimise, as far as possible, the use of our most precious resource: water. The rate of evaporation is in excess of 2 m per year over most of Australia’s landmass and mean rainfall in Australia is less than 500 mm per year and falling. It has been estimated that up to 95 per cent of the rain which falls in Australia evaporates and does not contribute to runoff. When harvested, water is commonly stored in small storages and dams, but it is estimated that up to half of this may be lost to evaporation. This represents a huge waste of our resource. The price and value of water are increasing dramatically and the scarcity of water is the main \nlimiting factor working against agricultural production in Australia. Australia has approximately 500 large dams with a combined capacity of 80 000 GL, roughly equivalent to four times the annual amount of surface water diverted (NLWRA 2001a). Australia has several million farm dams which account for an estimated 9 per cent of the total \nwater stored, or approximately 7000 GL (Environment Australia 2000). Assuming that these small dams on average contain water only 50 per cent of the time, and assuming that 40 per cent of this is lost to evaporation, it can therefore be roughly argued that the total agricultural \nwater lost to evaporation is probably around 1400 GL. There is great uncertainty in this figure however, as the last two quantities are largely unknown. A scoping style research study is urgently required to obtain more accurate and quantitative data in this area. The amount of water lost to evaporation from storages depends on many factors including atmospheric evaporative demand, the size of the water storage and storage method. There have been many attempts to reduce evaporation losses by altering how the water is stored. Water loss from storage dams can firstly be managed by increasing their depth and secondly by \ninstalling a good quality liner to prevent seepage. Circulation of cold bottom water (destratification) has been used successfully in deep storages (storages greater than 20 m) but is inappropriate in most agricultural storages which are generally less than 7 m deep. Windbreaks \ncan also be used in certain circumstances, but their overall effect in reducing evaporation is likely to be small, because solar radiation, rather than wind, is the key driver of evaporation. A realistic management option is to invest in a cover over the dam to reduce evaporation. The \nNational Centre for Engineering in Agriculture (NCEA), University of Southern Queensland (USQ), has been recently involved in a DNR (RWUEI) funded project to assess the relative effectiveness and economic viability of different types of cover for storage evaporation control. The control methods investigated include chemical monolayers, floating covers and shade structures (Craig & Hancock 2004). To evaluate the relative strengths and weaknesses of each \nevaporation reduction method, accurate methods for measuring actual evaporation loss were developed as part of the project. A brief description of the method is also included in this review.