Shade Balls for Evaporation Reduction
HDPE shade balls float in a self-arranging layer to cut evaporation ~66–75% in the field, famously at the LA Reservoir — with water-chemistry trade-offs.
What it is
Shade balls are hollow or partly water-filled HDPE spheres — typically around 10 cm (4 in) across — that float in a single self-arranging layer across a water surface. Tipped in by the thousand, they spread out and cover the water without any anchoring or assembly. The best-known deployment is the Los Angeles Reservoir, which received roughly 96 million balls in 2017.
How it works
A floating layer of opaque spheres shades the water and physically caps most of the surface, attacking the radiation and surface-exposure terms of the evaporation balance (see what is evaporation). Less sunlight reaches the water, so it stays cooler, and the covered fraction is shielded from direct exchange with the air.
How well it works
Reported field performance is ~66–75% reduction at ~91% coverage. The ceiling is geometric: even when packed tightly, round balls cannot fully seal a flat surface — gaps remain between adjacent spheres, and those open channels keep evaporating. That is why shade balls land below the near-full-coverage figures claimed for gap-free tile and panel systems; the review by Yao et al. (2021) places floating covers of this kind firmly in this mid-range.
Shade balls were chosen at the LA Reservoir as much for water quality as for evaporation: blocking sunlight suppresses algae and limits the formation of certain disinfection by-products.
Trade-offs
- Coverage ceiling and removal. The ~91% packing limit caps performance, and the balls are awkward to walk on and must be netted or scooped to access the water beneath.
- Volume and lifecycle. Covering a large reservoir takes an enormous number of spheres to manufacture, transport and eventually recycle.
- Water chemistry and ecology. Introducing a vast HDPE surface area can affect water chemistry and aquatic ecology, which should be assessed per site and water use.
Where it fits
Shade balls suit large, open, irregularly shaped municipal reservoirs, especially where algae and water-quality control are priorities. Where a higher, gap-free reduction is the goal, compare interlocking tiles and hybrid panels in modular floating covers, or the near-total seal of geomembrane covers. See the full methods comparison for the side-by-side view.