Return flow is surface and subsurface water that leaves the field following application of

irrigation Irrigation (also referred to as watering of plants) is the practice of applying controlled amounts of water to land to help grow crops, landscape plants, and lawns. Irrigation has been a key aspect of agriculture for over 5,000 years and has bee ...

water. While irrigation return flows are point sources, in the United States they are expressly exempted from discharge permit requirements under the Clean Water Act.United States. Clean Water Act, section 502(14), . Return flows generally return to the irrigation centre after a period of about three to four weeks; due to this, the farmers usually need to pour bleach into the water to clean it of any

organisms An organism is any living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have been pr ...

that have entered the stream. If this is not taken care of,

diseases A disease is a particular abnormal condition that adversely affects the structure or function of all or part of an organism and is not immediately due to any external injury. Diseases are often known to be medical conditions that are asso ...

such as

typhoid Typhoid fever, also known simply as typhoid, is a disease caused by ''Salmonella enterica'' serotype Typhi bacteria, also called ''Salmonella'' Typhi. Symptoms vary from mild to severe, and usually begin six to 30 days after exposure. Often ther ...

cholera Cholera () is an infection of the small intestine by some Strain (biology), strains of the Bacteria, bacterium ''Vibrio cholerae''. Symptoms may range from none, to mild, to severe. The classic symptom is large amounts of watery diarrhea last ...

could enter the irrigation and pose a risk of epidemic

disease A disease is a particular abnormal condition that adversely affects the structure or function (biology), function of all or part of an organism and is not immediately due to any external injury. Diseases are often known to be medical condi ...

to surrounding towns and cities. The return flows in irrigation is nearly 50% of the water supplied in silty clay

soil type A soil type is a taxonomic unit in soil science. All soils that share a certain set of well-defined properties form a distinctive soil type. Soil type is a technical term of soil classification, the science that deals with the systematic categ ...

in tropical countries. The

salinity Salinity () is the saltiness or amount of salt (chemistry), salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensio ...

of the return flow water increases with decrease in % of return flow quantity. Rest of the water supplied to irrigation evaporates to

atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

due to

evapotranspiration Evapotranspiration (ET) refers to the combined processes which move water from the Earth's surface (open water and ice surfaces, bare soil and vegetation) into the Atmosphere of Earth, atmosphere. It covers both water evaporation (movement of w ...

. When ground water is extracted for irrigation and other uses, most of the return flows seep back into the ground instead of joining the nearby surface stream. When ground water is used in excess of recharge from rainfall/

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, rain and snow mixed ("sleet" in Commonwe ...

, the quality of groundwater deteriorates over a period of time and becomes unfit for irrigation use.

References

* United States Department of Agriculture Water law Water pollution {{ecology-stub