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Usually, permitted point sources of water pollution, such as wastewater treatment plants, have high discharge treatment costs, whereas nonpoint sources of water pollution, such as agriculture, have low costs of pollution reduction. Therefore, it is generally assumed that most trades would take place between point sources and nonpoint sources. [54]
Trans-boundary water pollution occurs when pollution in one country’s waters spreads and damages another country’s environment or water supply. Trans-boundary pollution can travel through rivers or ocean currents. [19] This means that this type of pollution is not contained within the boundaries of a nation.
Thus the water discharge of a tap (faucet) can be measured with a measuring jug and a stopwatch. Here the discharge might be 1 litre per 15 seconds, equivalent to 67 ml/second or 4 litres/minute. This is an average measure. For measuring the discharge of a river we need a different method and the most common is the 'area-velocity' method.
A practical definition of water pollution is: "Water pollution is the addition of substances or energy forms that directly or indirectly alter the nature of the water body in such a manner that negatively affects its legitimate uses." [1]: 6 Water is typically referred to as polluted when it is impaired by anthropogenic contaminants.
Water flowing in channels comes from surface runoff from adjacent hillslopes, from groundwater flow out of the ground, and from water discharged from pipes. The discharge of water flowing in a channel is measured using stream gauges or can be estimated by the Manning equation. The record of flow over time is called a hydrograph.
Discharge regime, [1] flow regime, or hydrological regime (commonly termed river regime, but that term is also used for other measurements) is the long-term pattern of annual changes to a stream's discharge at a particular point. Hence, it shows how the discharge of a stream at that point is expected to change over the year. [2]
Aristotle correctly hypothesized that the sun played a role in the Earth's hydraulic cycle in his book Meteorology, writing "By it [the sun's] agency the finest and sweetest water is everyday carried up and is dissolved into vapor and rises to the upper regions, where it is condensed again by the cold and so returns to the earth.", and believed ...
reduced discharge into the sea, which may have various consequences like coastal erosion (e.g. in Ghana [7]) and saltwater intrusion in delta's and estuaries (e.g. in Egypt, see Aswan dam). Current water withdrawal from the river Nile for irrigation is so high that, despite its size, the river does not reach the sea in dry periods. [6]