The effect of evaporation on water quality
A specific form of ‘pollution’ can occur when water quality deteriorates as a
result of evaporation. When a part of a water flow evaporates, the concentrations
of chemicals in the remaining water flow will increase (because when water
evaporates the chemicals in the water stay behind). Consider, as an example, the
case of high salt concentrations in drainage water from irrigated fields. When there is continued irrigation with little drainage compared to the volume of water
evaporating, the salts naturally contained in the irrigation water accumulate in
the soil (since the water evaporates, not the salt). As a result, also the drainage
water will have a relatively high salt content. One may call this ‘pollution’. But
obviously it is another sort of pollution than when humans add chemicals to
the water, because in this case there is no addition of chemicals by humans, but
the naturally present chemicals get concentrated by water evaporation. We can
generalize this case to all cases where ‘water is taken out of the system through
evaporation’. It also happens, for instance, in artificial reservoirs where water
evaporates and chemicals accumulate