Reducing evaporation losses6 October 2009
Annual evaporation losses from irrigation storage dams and channels are significant across Australia. Erik Schmidt gives an overview of the mitigation technologies that can be employed to reduce such losses
Australian agriculture is highly dependant on farm dams. Storage sizes range from a few megalitres (ML) for stock and domestic supplies to larger dams used for commercial irrigation. Conservative estimates suggest that in excess of 8 million ML is stored in farm dams (ie. 9% of total stored water), and that there are more than 2 million farms dams across Australia. Evaporation from bulk distribution channels in regulated irrigation systems also amounts to a significant loss. Annual evaporation losses from these storages are estimated to be around 1.32GL/yr and up to 2.88GL/yr. It is important to quantify evaporation losses from storages in order to assess the feasibility of investing in evaporation mitigation technologies.
Measuring losses from farm dams
It is difficult to accurately measure evaporation from an open storage. Evaporation of a free water surface is the result of complex processes affected by incident solar radiation, wind speed, air temperature and humidity, and the energy stored in the water body, especially surface water temperature.
A water balance approach provides a practical approach for estimating aggregate losses due to evaporation. For periods when there is no inflow, outflow or rainfall and for small incremental time steps when surface area is constant, the net change in evaporation and seepage can be determined by measuring changes in water depth.
The accuracy of this method depends greatly on the accuracy of the equipment used to measure the change in water depth. Monitoring systems and data analysis techniques have recently been commercialised by Aquatech Consulting as the Irrimate Seepage and Evaporation Meter. This uses an accurate pressure sensitive transducer to measure changes in water level every 15 minutes. Rainfall, wind velocity and water temperature are also logged for use in the analysis which are achieved using software developed by the CRC for Irrigation Futures through the National Centre for Engineering in Agriculture.
There are a wide range of products available for controlling evaporation loss. These systems include:
• Continuous floating covers.
• Modular covers.
• Shade structures.
• Chemical covers
Continuous floating covers
Continuous floating plastic covers act as an impermeable barrier that floats on the water surface and can achieve above 90% evaporation savings for full cover of the dam. Most of these products have a high capital cost and replacement life varies (typically between 10 and 20 years). The structural integrity of the product under windy conditions and fluctuating water levels is important. Water quality can be impacted by reduced dissolved oxygen, light penetration and change in water temperature. This can have a positive impact on reducing algal growth. Significant difficulties can be encountered with installation on large storages above 5ha. In some cases these covers can be deployed as a series of large rafts covering up to 1ha.
Modular floating covers come in a range of sizes typically up to 3m2 in area and act in a similar manner to floating covers. However they do not have the structural challenges of a continuous sheet. Modular floating covers can also be deployed to cover only a portion of the storage, for example that portion always holding water.
Modules can be free-floating or connected together to form a larger raft. Modules are typically made from a plastic material and can generally provide up to 90% savings for 100% area covered. The actual area covered will depend on the number, shape and size of the module and storage characteristics.
Generally these systems have a very high capital cost (in excess of US$17/m2). Repair and replacement of modules is possible and water quality impacts will depend on the relative area covered, oxygen transfer and changes to water temperature.
Shade structures in general are suspended above the water surface using cables creating a web-like structure with shade cloth fitted between the cables. The shade cloth can come in a range of UV ratings (to describe the amount of UV blocked by the shade cloth). Evaporation savings of 70 to 80% have been demonstrated in trials.
Floating shade cloth modules or rafts have recently been marketed. Most of these products have a relatively high capital cost. In general shade structures are not as effective in reducing evaporation as floating covers. They allow free flow of oxygen to the water, although wind velocity and wave action will be reduced, which impacts dissolved oxygen levels. Algae may be reduced owing to less light penetration.
Chemical covers have been promoted as a low cost method to reduce evaporation losses. Some products are true monolayers (ie a single molecule thick) while others are multiple layers with different water saving characteristics and water quality impacts. These products are generally biodegradable and there is a need to reapply frequently (between three and ten days). Water savings have been shown to be highly variable, from less than 10% to up to 50% and are impacted by prevailing wind, temperature and water quality.
True monolayers are applied at very low application rates and rely on the self spreading ability of the chemical. Advantages of these products are the low capital cost and choice to apply only when needed. Monolayers offer much potential for affecting evaporation savings but inconsistent evaporation saving performance has limited their adoption in Australia. It has been recognised that further research is needed and the Cooperative Research Centre for Irrigation Futures (CRC-IF) is currently working on such a project.
The key objectives of the project are to:
• Develop standardised methods for evaporation and seepage monitoring of storage dams based on depth sensing technologies, analysis procedures and use of meteorological based evaporation estimates.
• Improve understanding of the monolayer product performance and factors affecting this performance.
• Develop improved monolayer products.
• Develop monolayer application, monitoring and control systems and recommendations for best management practice.
The cost benefit of evaporation control is a key driver in investment in the technologies described above. The potential cost of installing and operating an evaporation control product per unit of water saved ($/ML) will be a function of:
• Installation and maintenance costs which are very dependent on site situation and installation issues.
• Annual and seasonal evaporation losses from storages at the location.
• Efficiency of the evaporation mitigation technology.
• Storage operating conditions.
This needs to be compared with the value of water to the landholder in terms of increased crop production, the cost of water to be purchased or the potential to trade water surplus. A ready reckoner has been developed by the CRC for Irrigation Futures to help undertake such an economic analysis. For more details log onto http://www.readyreckoner.ncea.biz
Erik J Schmidt, CRC for Irrigation Futures, National Centre for Engineering in Agriculture, University of Southern Queensland, Australia