Use the terms defined in Question 6 to give a full description of the hydrologic cycle, including each of its three loops—namely, the evapotranspiration, surface runoff, and groundwater loops

Question

What is the water quality (purity) at different points in the cycle? Explain the reasons for the differences.

Answer 1

Water evaporates from surface water when it is heated, and condenses when it cools. Water is pure at this point because only water molecules evaporate. When enough water condenses, precipitation occurs. As precipitation moves through the atmosphere it may pick up water soluble pollutants but it may also be fairly pure if not many water soluble compounds are encountered. When precipitation reaches the ground, some of the water infiltrates the soil and some runs off to streams, lakes, and the oceans. This water will pick up solutes from the soil. The water that soaks into the ground may either be capillary water or gravitational water. Gravitational water will percolate through the ground and collect as groundwater in an aquifer and may be released to the surface through seeps and springs. The height of the water table will determine if springs or seeps are flowing or dry. The recharge area for an aquifer may be over a fairly large surface of land. Capillary water will be picked up by plants or evaporate from the soil. Water is transpired through plants, and evapotranspiration is the process by which water is evaporated and transpired from plants.
"There are three principal loops in the cycle: (1) In the evapotranspiration loop (consisting of green water), the water evaporates and is returned by precipitation. On land, this water, the main source for natural ecosystems and rain-fed agriculture, is held as capillary water and then returns to the atmosphere by way of evapotranspiration. (2) In the surface runoff loop (containing blue water), the water runs across the ground surface and becomes part of the surface water system. (3) In the groundwater loop (also containing blue water), the water infiltrates, percolates down to join the groundwater, and then moves through aquifers, finally exiting through seeps, springs, or wells, where it rejoins the surface water."