Glossary/water dictionary

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ecological impact

The total impact of an environmental change, either natural or man-made, on the ecology of the area.^

economic value of unit of irrigation water

The value of a crop raised by a unit of irrigation water if run continuously throughout the life of the crop.^

effective rainfall

(1) In general, rain that produces runoff. (2) In irrigation practice, that portion of the total precipitation which is retained by the soil so that it is available for use for crop production. (3) In geohydrology, that part of the total precipitation that reaches the groundwater.^

effective rooting depth

Soil depth from which the crop extracts most of the water needed for evapotranspiration (also called 'design rooting depth').^

effective water holding capacity

The amount of water held in the soil after the excess gravitational water has drained away and after the rate of downward movement of water has materially decreased, preferably given as a percentage by volume and not by weight. ^


Water that flows from a sewage treatment plant after it has been treated.^

environmental assessment

A formal process to predict the environmental consequences of human development activities and to plan appropriate measures to eliminate or reduce adverse effects and augment positive effects (also, environmental impact assessment (EIA)).^

environmental impact

A change in effect on an environmental resource or value resulting from human activities including project development, often called an 'effect'.^

environmental monitoring

Observation of effects of development projects on environmental resources and values.^


The process by which a material is worn away by a stream of liquid (water) or air, often due to the presence of abrasive particles in the stream.^

erosion control

The application of necessary measures to control accelerated erosion of land surfaces by vegetation or artificial structures, such as terraces, dams or bunds.^


A place where fresh and salt water mix, such as a bay, salt marsh, or where a river enters an ocean.^


The process of a water body becoming anaerobic, i.e. without oxygen. Human activities that add nutrients to a water body can accelerate this process.^

evaluation of alternatives

The searching out of comparative advantages and disadvantages of alternatives in an attempt to find the one that ‘fits’ best. The criterion of fit is sometimes called measure of effectiveness. During the planning process, the original goals and objectives are frequently modified and refined. Thus, the measure of effectiveness also needs to be altered. As the planner approaches the design stage, evaluation becomes more precise.^


The process of liquid water becoming water vapour, including vapourisation from water surfaces, land surfaces, and snow fields, but not from leaf surfaces. ^


The sum of evaporation and transpiration.^

exchangeable sodium percentage (ESP)

The degree of saturation of the soil exchange complex with sodium. It may be calculated by the formula: ESP = exchangeable sodium (meq/100 g soil)/cation exchange capacity (mec/100 g soil) x 100.^

expert system

Software that applies the knowledge, advice and rules defined by experts in a particular field to a user’s data to help solve a problem.^