18.4. Methods for Reducing Reservoir Evaporation

Due to the high magnitude of losses, several engineering and environmental strategies have been developed to minimize reservoir evaporation.

18.4.1 Surface Cover Methods

a) Floating Covers (Monomolecular Films)

• Chemicals like hexadecanol, octadecanol, or cetyl alcohol are sprayed over the surface.
• Form a one-molecule-thick film that resists vapor escape.
• Advantages:
  • Easy to apply
  • Reduces evaporation by 20–40%
• Limitations:
  • Effectiveness reduced by wind and water turbulence
  • Requires frequent reapplication
  • Environmental toxicity concerns

b) Floating Plastic Modules

• Plastic spheres or tiles that float and interlock to cover surface.
• Reduce exposure to air and sunlight.
• Durable but costly for large-scale application.

In this chunk, we learn about two main surface cover methods used to reduce evaporation from reservoirs. The first method involves using monomolecular films, which are very thin coatings of chemicals that create a barrier on the water surface. These coatings can significantly decrease evaporation by 20-40%, making them effective but also requiring frequent reapplication, especially in windy conditions. The second method involves floating plastic modules that cover the surface of the water with interlocking spheres or tiles. While this method also reduces evaporation, it is more expensive to implement on a large scale.

Imagine you have a small swimming pool that loses water because of evaporation. If you place a thin sheet of plastic over the water's surface, this is similar to the monomolecular film strategy—it's easier to apply and can help keep the water from evaporating too quickly. However, if it's gusty outside, the wind can blow the plastic away, just like wind can challenge these evaporation-reducing methods.

18.4.2 Wind Breaks and Shelterbelts

• Vegetative or structural barriers around reservoirs reduce wind velocity, thus minimizing evaporation.
• Trees like Eucalyptus or Casuarina are planted in strategic alignment.
• Limitations:
  • Require land space and long-term maintenance
  • Might increase transpiration losses if water-intensive species are used.

This chunk describes how wind breaks and shelterbelts—specifically lined rows of trees—can help decrease evaporation from reservoirs. By slowing down the wind that blows across the water's surface, these trees reduce the amount of water lost to evaporation. However, planting and maintaining these trees requires a certain amount of land and ongoing care. Additionally, if the species of trees used are thirsty, they may take up water from the reservoir, which can counteract some of the benefits of reduced evaporation.

Think of this as creating a wind shield. If you're standing by a pool on a windy day, you'll notice the water splashing and evaporating more rapidly. If you put an umbrella up (like a windbreak), the wind is diverted and the water stays calmer, resulting in less evaporation. However, if you choose a giant umbrella that drinks up a lot of water from your pool, it defeats the purpose, which is why choosing the right type of trees is important.

18.4.3 Deepening of Reservoirs

• Reduces surface-area-to-volume ratio, lowering exposure.
• Involves desilting or modifying design contours.
• Technically effective but often cost-prohibitive.

Here, we learn about the method of deepening reservoirs to reduce evaporation. By making a reservoir deeper, the ratio of its surface area to its volume decreases. This lower ratio means there is less surface area exposed to the air, which reduces the evaporation rate. However, this solution can entail significant costs and engineering challenges, so it's not always the most feasible option.

Imagine a shallow pond where the sun warms up the entire surface quickly. If you were to dig a hole in the middle of it, making it deeper, you'd notice that the middle part stays cooler since less of the surface area is exposed to the sun. However, not everyone has the resources or ability to dig such a hole, which is similar to the challenges faced when deepening a reservoir.

18.4.4 Use of Underground Reservoirs (Subsurface Storage)

• Artificial recharge structures like percolation tanks, infiltration galleries, and subsurface dams.
• Store water below the surface, preventing direct evaporation.
• Useful in arid zones with permeable soil profiles.

This section introduces underground reservoirs, a strategy designed to store water beneath the ground where it is shielded from evaporation. These can include structures such as percolation tanks, which allow water to seep into the earth, and subsurface dams that capture and hold water underground. This method is particularly advantageous in regions with dry climates where surface water evaporation rates are high, and the soil allows for adequate water absorption.

Think of an underground reservoir like a sponge cake. When you pour syrup onto it, it soaks it all up beneath the surface, keeping it from evaporating into the air. In places that are hot and dry—like a dessert—it makes sense to keep water stored down where it won't be lost to the heat, just like hiding leftover cake in a cool, dark place to enjoy later.

18.4.5 Operational Adjustments

• Water level management: Maintaining lower water levels during high evaporation seasons.
• Zoning of reservoir use: Limit evaporation-exposed zones to non-critical uses.
• Seasonal scheduling of storage and releases based on evaporation forecasts.

This chunk covers operational adjustments that can be made to manage water resources effectively. For instance, during hot seasons, reservoir managers can keep water levels lower to increase the amount of surface area that is not exposed to sunlight. This tactic can minimize evaporation. Additionally, certain areas of the reservoir can be designated for non-critical uses during high evaporation periods, ensuring that essential water needs are prioritized. Adjusting the schedule for when water is stored or released based on weather forecasts can also help reduce evaporation losses.

Think of it like managing your water bottle on a hot day. If you know it’s going to be super hot, you might decide to drink less water during the day to save for later. You might also avoid spilling any water on the ground because you want to use every drop. Similarly, managing reservoir levels and uses can help conserve water and minimize losses due to evaporation.

18.4.6 Use of Shade Covers and Floating PV Panels

• Use of solar panels on floating structures:
  • Dual benefit: evaporation control + power generation
  • Increasingly adopted in India and other countries
• Shading covers also help reduce algal growth and water warming.

In this section, we learn about using floating solar panels as a dual-purpose solution for reducing evaporation and generating energy. By covering the surface of a reservoir with these panels, the sunlight that would heat the water and cause evaporation is blocked. This not only conserves water but also produces renewable energy. Additionally, these covers can lead to cooler water temperatures, which can decrease algal blooms caused by excessive sunlight.

Imagine a picnic blanket laid over your food to keep the sunlight from spoiling it too quickly. In this case, the blanket acts to protect the food as well as provide a nice place for you to eat. Similarly, floating solar panels act as a protective layer for water while also providing energy—it's like getting two benefits from one solution!

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