Ephemeral pools are typically depressions which temporarily hold water on poorly drained soils. These pools require the appropriate contours to hold water long enough to balance losses to infiltration or evaporation and impervious soils. These are also easily drained, often accidentally, when drainage is “improved”. They vary greatly, both in size and the duration of the hydroperiod, and can be important for birds, amphibian populations and specialised species such as fairy shrimps. Toad tadpoles, for example, prefer ponds in open locations with warm water as this allows them to develop quickly.

One of the more spectacular examples of ephemeral pools in Wales are the dune slacks found on some of the coastal dune systems: these can flood to a depth of 2m or more during wet winters but be devoid of standing water during the summer months. Elsewhere in Europe, particularly in south-west Spain and Mediterranean regions, the ephemeral standing waters can cover large areas, e.g. the coastal marshes and pools of Doñana National Park. The length of the hydroperiod here is of critical importance for both breeding and wintering birds.