Many water reservoirs and canals can benefit from covers to reduce evaporation and provide shade to minimize unwanted algae blooms to improve water quality and reduce maintenance.  Considering only these benefits, such covers are cost-beneficial for only a few reservoirs or canals.  However, subsidizing placement and maintenance of reflective covers that reduce evaporation and reduce bio growth will also reflect large amounts of solar energy out to space where that energy would otherwise be absorbed.  Such a subsidy will make it cost-beneficial to put such covers on many more reservoirs and canals.  The subsidies can most effectively be marshalled by selling reflection cooling credits.  

For example, the Egyptian government has been studying the feasibility of covering 2000 square mile Lake Nasser to reduce evaporation.  Covering 60% of the lake, which has a reflectivity potential of 186 to 193 watts per square meter,[1] for a reflectivity gain from about .1 to .85 would reflect about 435,000 megawatts.^[2]  This would be worth encouraging with a subsidy from reflection cooling credits. Covering reservoirs with floating solar photovoltaic panels may be a better use for many reservoirs near urban areas or hydropower dams,^[3] but even if all these opportunities are pursued, there will still be large reservoirs and canals where a reflective cover is the best option.

[1]  https://www.reflectiveearth.org/map?&latitude=22.458775409072004&longitude=31.76027956554867&zoom=6.977216106481924&bearing=0&pitch=0

[2] 5,250 km2 = 5.2 x 10^9 m2 x .75 = 3.9 x 10^9 m2 x 186 w/m2 x .6 = 725 x 10^9 =435,000 megawatts

[3] https://www.nature.com/articles/d41586-022-01525-1

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