Controlling Pond and Dam Evaporation

Low rainfall, long sunny days, high water evaporation rates, sandy soils and flat landscapes all contribute to a lack of abundant runoff and surface water resources in many countries around the globe.  In addition, the underground water resources in many areas are too saline for human and animal consumption, surface water flows are limited, and there is no connection to mains water system.  As a result, a substantial proportion of farms and industries are reliant on pond and water storage dams to provide water agriculture, industrial and domestic use.

With all pond and dams, there are 2 issues that must be addressed. First, how to more effectively capture water so that losses are minimized and rainfall provide enough water to last through droughts? The second issues is how to prevent loss through evaporation once the water is harvested, as evaporation will slowly eliminate any improvements that have been made collecting water. Graded Catchments’ or ‘Lined Catchments’ are strategies that can be used to increase run-off and enable more reliable capture of rainfall into the dam.

While your rain harvesting into a pond, you might as well bring your greywater into it to be cleaned and used as well, rather than piping it to the sewer or septic system. Your sink, shower and laundry water can all be put to use. Make sure to use non-toxic soaps, shampoos and detergents.

Water quality in the dam or pond should also remain a priority. A third issue not always addressed, but which is critical to water management, is algae growth. Algae can have toxic or lethal consequences if not properly controlled or eliminated. Information on how to control algae is available at www.pond-algae-control.com .

How much evaporation occurs in dryer areas?

The average annual evaporation from open water ponds varies from more than 1.6m inland to below 1.3m on the coast and highlands.  This can be approximately 20% of the volume of an average stock dam.

Evaporation loss can be minimised

1. Appropriate dam design to minimise the surface area
2.  Management strategies such as:
   2.1    Windbreaks
   2.2    Evaporation control covers:
   1. Floating covers (solid or modular)
   2. Suspended shade structures
   3. Chemical monolayers
3. Underground storage.

Options for Controlling Evaporation

Appropriate Dam Design

Evaporation will be reduced if the ratio of the surface area to the volume is reduced.  The surface area can be minimized by building the pond as deep and as narrow as is practical.  This is highly effective but can be costly if the dam is already constructed.  However, hard soils, shallow water table and the presence of saline underground water table in many parts of the world limits the depth to which a dam can be dug. Soil composition will also impact the angle of the pond slopes, or how verticals the walls can be.

A second option to consider is to split the dam into multiple cells so that the water storage is formed of independent areas of water that can be managed to minimize surface areas.  Instead of building one large pond, it would be beneficial to construct two or more equal sized cells of water connected to each other.  After drawing water from the first cell and, the remaining balance is transferred to the other cell(s) thus reducing the surface area of the remaining volume.

Pond depth and multiple cell management can also help minimize the cost of the cover. To further reduce evaporation loss, a cover can be installed over one of the two the cells.  This reduces the initial cost of covering the entire storage area and provides the greatest reduction in evaporation loss when the water is needed most and comprises less than half of the dam capacity.

Management Strategies

1. Windbreaks
   Vegetation planted near ponds from a windbreak that provides some protection from evaporation.  Trees should not be planted directly on top of or adjacent to dam walls as roots may increase the risk of wall failure or even draw water out of the dam.  At best, evaporation rates will be reduced by 20%.  Be aware that trees take a long time to reach effective size and may hinder cleaning out or enlarging dams.
2. Evaporation Control Covers
   1. Solid or Continuous Floating Cover
      Floating covers are impermeable barriers that float on top of the water surface.  There are manufactured using different materials (including wax, foam and polystyrene), but polyethylene plastic has been found to be the most satisfactory and durable. Certain polyethylene cover contain air pockets (similar to bubble wrap or swimming pool cover products) which are made from a tough material designed to prevent degradation from sunlight.  The multi-layering of the floating cover enables some reflection of the heat from the sun.  To completely eliminate the transmission of light to the water underneath the cover, the reflective top layer of the cover is white and the under layers are black.
      The type of polyethylene used can be recycled and is environmentally safe.  Trials have demonstrated that a well managed cover can effectively reduce evaporation from open water storage by over 95 %.  Completely airtight covers are expensive and do not allow rainfall to enter the dam. They are also susceptible to wind, tears, snow and rain, and generally requires regular repairs over the years. Compared to modular cover, they do not last as long.
   2. Modular Covers
      Modular covers are similar to continuous plastic covers, but they consist of individual units which are not restrained. Each unit can move freely across the entire water surface.  They can be free floating or linked to each other.  Installation costs are much less for modular covers than solid cover types.  The evaporation reduction rates are affected by how tightly the modules are packed together and may be slightly lower than reductions achieved with continuous plastic floating cover types. But recent progress in modular covers, with product such as the Hexoshield®, has produced modular covers which are almost as efficient as solid covers. As each module is small in size, thousands of modules may be required to cover some water storage.  Many of those modules are made of recycles material, such as recycled HDPE (Hexprotect® AQUA) or recycled polypropylene (Hexprotect® SLIM).  The material is safe for the environment, as it is generally made of post consumer plastic food products.
      Estimates indicate that evaporation can be reduced by 90-98% using this method, but results depends widely on the module used.
      Modular covers are free floating and will migrate with wind movements to the down wind margins of water storages.  This is usually where the warmest water of the storage lies, and where the highest evaporation rates occur.  Because they are not a complete seal over the dam, they allow rainfall to enter the dam. Floating modular covers are effective and practical, especially with their growing commercial availability and more affordable than before cost. They are also very effective at eliminating algae, which will diminish water quality and could have harmful effect on humans and animal.
   3. Examples of Modular Covers
      Hexprotect® SLIM consist of a series of interlinked domes supported by a ring that penetrates the water.  When the SLIM cover 95% of the dam, evaporation can be reduced by 90+%.  They also reduce water temperature and turbidity.  The main limitation is cost.
      Hexoshield® is a another modular cover with a high draft, that packs very tightly, provide high coverage and is wind resistant. The product reduces or stops algal growth and is close to 98% effective in stopping evaporation. This type of cover is slightly more expensive than Hexprotect® SLIM.
   4. Suspended Covers
      Creating shade is an effective method to control evaporation. Shade structures are usually suspended above the water surface with the use of cables.  Shade structures reduce solar radiation and wind speed, and act as a trap for humid air between the shade structure and the water surface.  All of these factors help reduce evaporation rates by up to 75% without affecting water quality or aquatic life. As the suspended shade structure is not in contact with the water, storages can be emptied with the cover in place, and any rainfall can easily enter the dam.  Shade structures are not as effective in reducing evaporation rates compared to well managed plastic covers, but they are a practical option for small storages where support distances are short in length.The suspended shade cloth structures dry out quickly after rainfall preventing wind blown soil from collecting on the surface of the cloth: it either blows off or falls through.  This also means that the growth of weeds or algae on the cover surface is unlikely.  Shade cloth is available in a range of UV-reduction ratings.  Hail shoots or valves can be installed into the cloth to reduce potential storm damage.  High winds may damage the cloth and there can be difficulty in anchoring cables in poor quality soils.
      The main disadvantage of this product is the relatively high capital outlay on construction labor costs.  The wider the shade cloth roll, the fewer cables required and this reduces the installation cost. It is also susceptible to wind and will degrade faster than modular or even solid covers in the sun.
   5. Chemical covers
      Chemical monolayers form a one-molecule-thick oily layer on the surface of the water.  As these layers are degradable, the chemical needs to be re-applied every two to four days.  With smaller storage the chemical layer can be applied by hand from the bank as the chemical has some self-spreading ability.  A form of mechanized delivery system may be required for larger storage. Monolayer chemicals are greatly affected by wind and wave action and their effectiveness in strong winds is very low.  They are generally not as effective as physical methods in reducing evaporation.  Field trials on large storage indicate reduction in evaporation of approximately 40%.Despite only a low evaporation saving rate, the main advantage of chemical monolayers is the low initial setup cost.  The product need only be applied when it is required e.g. when the dam is full and/or during periods of high evaporation.  Chemical monolayers are particularly suited to low risk investment options for owners of agricultural storage that do not have water in them all year.  Chemical barriers are less visually intrusive than physical structures.Although chemical monolayers are ineffective for long term treatment, without continual reapplication, they may be the only cost effective solution for dams with large surface areas.

Methods at a Glance

Conclusion

Chemical treatments very temporary and can be applied seasonally when required, making them more affordable.  They have limited impact on aesthetics but are considerably less efficient than physical structures.  The exclusion of light by physical structures will suppress algal growth and reduce water temperature.  Significant reductions in evaporation can also be achieved by designing structures that are deep, relative to surface area.

The best evaporation reduction in small to medium sized dams will be achieved by floating modular system. For large dams, wind resistant must always be taken into consideration when opting for a modular covers. Not all modules are equal and opting for a non wind resistant module will lead to gaps in the cover and reduced evaporation control.