UNITED STATES—During a drought, there really is no way to use less water in a garden pond. Aquatic plants can not survive without adequate water; not to mention fish! Pumps that circulate water must stay submerged to operate properly, and to not get damaged by operating without water. Some degree of water needs to be added regularly to compensate for evaporation.
Water does not evaporate from below the surface of the water. Therefore, depth is irrelevant to water loss. The area of the surface of the water is more important. Sunlight and wind accelerate evaporation. So do fountains or pumps that broadcast water through the air for circulation. Aquatic plants that stand above the surface of the water lose water to evaporation from foliar surfaces.
Yet, most of us who enjoy gardening cannot resist growing aquatic plants if a pond is available. Not only do they provide distinctive foliage and bloom, they also provide shelter for goldfish or minnows that control mosquitoes. They keep water clearer by competing with algae.
Submerged aquatic plants, like anacharis, do everything in the water. Some do not even need soil to root into. Because they do not come above the surface of the water, they do not affect evaporation. None of the floating plants, like water hyacinth, water lettuce and duck weed, have any use for soil either, although some of the leafiest sorts can accelerate evaporation.
Water lilies and lotus are emergent aquatic plants, which naturally live in mud on the bottom of ponds, but develop foliage and flowers that emerge and float on the surface. Although they have the potential to affect evaporation, most are more likely to inhibit evaporation by keeping water shaded. In garden ponds, they must be potted, and should be under at least a foot of water.
Relative to other aquatic plants, bog plants such as cattails, aquatic cannnas, and blue or yellow flag iris consume the most water from saturated soil at or just below the surface of a pond. They produce the most foliage that stands well above the water. Like water lilies and lotus, bog plants need to be potted, but with the tops of their soil barely below the surface of the water.
Pots should be low and wide, and obviously do not need drainage holes. Unnecessary holes only spill a bit of soil, and allow roots to escape and grab onto the bottom of a pond. Good old fashioned soil (yes, dirt from the garden) is fine. Good quality potting soil merely floats away.
Highlight: Water Lily
The fragrant flowers of water lily are unreal! They either float on the surface of the water, or hover just above. Abundant pointed petals radiate outward from central tufts of pronounced stamens. Most flowers are soft hues of yellow, pink, blue, lavender, peachy orange or white. Some are brighter yellow or pink, or richer shades of red or purple. Some open in the morning. Others open in the evening.
The rubbery leaves that float on the surface of the water are nearly circular. Some are symmetrically cleft to the center, like Pac-Man or a pizza with a slice taken out. The thick rhizomes that the foliage and flowers emerge from stay buried in pots or mud under the ponds that they live in. Rhizomes can be divided to propagate, but take a year or so to recover and bloom.