Science of Water : Concepts and Applications

All about Water 17
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Type of plant: Plants transpire water at different rates. Some plants, which grow in arid
regions, such as cacti and succulents, conserve precious water by transpiring less water
than other plants.
√ Interesting Point: It may surprise you that ice can also vaporize without melting fi rst; however,
this sublimation process is slower than vaporization of liquid water.
Evaporation rates are measured with evaporation pans. These evaporation pans provide data that
indicate the atmospheric evaporative demand of an area and can be used to estimate (1) the rates
of evaporation from ponds, lakes, and reservoirs, and (2) evapotranspiration rates. It is important
to note that several factors affect the rate of pan evaporation. These factors include the type of pan,
type of pan environment, method of operating the pan, exchange of heat between pan and ground,
solar radiation, air temperature, wind, and temperature of the water surface (Jones, 1992).
Precipitation includes all forms by which atmospheric moisture descends to the Earth—
rain, snow, sleet, and hail. Before precipitation can occur, the water that enters the atmosphere
by vaporization must fi rst condense into liquid (clouds and rain) or solid (snow, sleet, and hail)
before it can fall. This vaporization process absorbs energy. This energy is released in the form
of heat when the water vapor condenses. You can best understand this phenomenon when you
compare it to what occurs when water that evaporates from your skin absorbs heat, making you
feel cold.
√ Note: The annual evaporation from ocean and land areas is the same as the annual
precipitation.
Run-off is the fl ow back to the oceans of the precipitation that falls on land. This journey to the
oceans is not always unobstructed—fl ow back may be intercepted by vegetation (from which it later
evaporates), a portion is held in depressions, and a portion infi ltrates into the ground. A part of the
infi ltrated water is taken up by plant life and returned to the atmosphere through evapotranspiration,
while the remainder either moves through the ground or is held by capillary action. Eventually,
water drips, seeps, and fl ows its way back into the oceans.
Assuming that the water in the oceans and ice caps and glaciers is fairly constant when averaged
over a period of years, the water balance of the Earth’s surface can be expressed by the following
relationship: Water lost = Water gained (Turk and Turk, 1988).
Q AND Q FACTORS
While potable water practitioners must have a clear and complete understanding of the natural water
cycle, they must, as previously mentioned, also factor in two major considerations (Quantity and
Quality—the Q and Q factors): (1) providing a “quality” potable water supply—one that is clean,
wholesome, and safe to drink; and (2) fi nding a water supply available in adequate “quantities” to
meet the anticipated demand.
√ Important Point: Two central facts important to our discussion of freshwater supplies
are: (1) water is very much a local or regional resource, and (2) problems of its shortage or
pollution are equally local problems. Human activities affect the quantity of water available
at a locale at any time by changing either the total volume that exists there, or aspects
of quality that restrict or devalue it for a particular use. Thus, the total human impact on
water supplies is the sum of the separate human impacts on the various drainage basins
and groundwater aquifers. In the global system, the central, critical fact about water is the
natural variation in its availability (Meyer, 1996). Simply put—not all lands are watered
equally.
To meet the Q and Q requirements of a potential water supply, the potable water “ practitioner”
(whether the design engineer, community planner, plant manager, plant administrator, plant