Dry season

chemical or form of radiation to which an organism is exposed or which it ingests (its exposure dose).

2. The amount of an absorbed substance that actually reaches specific tissues and is available to interact with metabolic processes, for example, to elicit a toxic response (the target dose).

dose equivalent (HT) The dose of ionizing radiation absorbed by an organism multiplied by a factor that takes into account the difference in the biological effects of different kinds of radioactive particle having the same energy. It is expressed in sieverts (Sv). 1 sievert is the radiation dose delivered in one hour at a distance of one centimeter from a point source of 1 mg of radium enclosed in platinum 0.5 mm thick. For beta-particles, gamma-rays, and x-rays this factor is 1, but for alpha-particles it is 20. See also dosage.

downstream drift The drifting downstream of inorganic and organic material and organisms as a result of the flow of water. It affects especially bottom-dwelling invertebrates such as the larvae of mayflies, caddisflies, and midges in streams and rivers. For some of these species downstream drift is part of a regular cycle of migration, the larvae drifting downstream and the adults flying back upstream to lay their eggs. It allows for the rapid repopulation of parts of the stream after floods or pollution have removed the original inhabitants, and also gives immature insects the chance to avoid overcrowding. Downstream drift is most marked at night, and during spring and summer.

drainage The movement of water from land, either naturally or artificially, as it flows over the surface and through rocks and soil under the influence of gravity, eventually reaching the sea, an inland lake, or underground reservoir.

drainage basin See catchment area.

dredging The removal of mud or other sediments to deepen channels or keep waterways and harbors clear. The sediments removed by dredging may be used for land reclamation. Dredging can have deleterious effects on the environment, releasing toxic chemicals and heavy metals from bottom muds, and stirring up silt that kills aquatic life and benthos.

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