Windbreaks to Reduce Evaporation

What it is

A windbreak is a barrier — a row of trees and shrubs, a hedge, or a built fence or wall — placed around or upwind of a water body to slow the wind passing over its surface. Unlike most methods on this site, it does not cover the water at all; it works purely by reducing wind speed.

How it works

Wind is one of the six dominant evaporation factors (see what is evaporation). It strips away the thin, humid boundary layer that forms just above the surface and replaces it with drier air, keeping the vapour-pressure deficit — and the loss — high. By cutting wind speed near the surface, a windbreak lets a more humid boundary layer persist, which slows evaporation.

How well it works

The effect is real but modest: roughly ~5–20% in small systems. Two things limit it:

• Fetch. A windbreak sheltering one edge protects only a limited downwind distance — generally a few times the barrier height — before the wind recovers its speed. On a large reservoir, most of the surface lies well beyond that shelter, so the percentage reduction shrinks as the water body grows.
• Geometry. Performance depends on the barrier’s height, porosity and orientation relative to the prevailing wind. A solid wall can create turbulence on its lee side; a semi-porous barrier often shelters a longer distance more smoothly.

Because of these limits, windbreaks are best understood as a partial, small-system measure rather than a primary solution.

Trade-offs

• Living vs structural. Tree and shrub windbreaks are inexpensive and add shade, habitat and dust control, but take years to establish and transpire, using water themselves. Built fences or walls work immediately but cost more and add no co-benefits.
• Maintenance. Vegetative breaks need watering and pruning; structures need inspection.

Where it fits

Windbreaks suit small, wind-exposed ponds and farm dams, and they pair well with other approaches — for example combining a windbreak with a surface cover, or with the operational measures in storage management. For larger bodies, a surface-covering method delivers a far larger reduction; see the methods comparison. Living windbreaks overlap with natural shading and carry the same transpiration caveat.