the hydrologic cycle
The hydrologic cycle explains how water is constantly replenished in our environment. Through a continuous cycle of distillation and circulation, water evaporates off the oceans, falls in the form of rain or snow, percolates into the ground and returns to the ocean through rivers and lakes (Hough 26). In the process, water transports sediment and dissolved materials and over time alters the earth's surface by erosion and deposition (Dunne and Leopold 4).

In urban environments, "the profusion of paved streets, sidewalks, and parking lots, and the storm sewers that drain them short-circuit the hydrologic cycle" (Spirn 1984, 13). Water that is intercepted by impervious surfaces - areas that are paved or roofed - instead of being infiltrated by soil or plants, becomes runoff that travels along other paved surfaces picking up debris and pollutants on its way to underground storm sewers that often empty into our waterways untreated.

Thus, urbanization not only thwarts the hydrologic cycle by diminishing evaporation, transpiration and infiltration, it also contributes to polluting the sources of our water supply (Hough 31).

Over the past century, the disposal of excess surface water in urban areas has depended on extensive centralized infrastructure like storm sewers and piped drainage. That this vast infrastructure for disposing and providing water is located underground and out of sight has resulted in people's disassociation of the importance of water as an essential life-giving force. It has also made people unaware of how urbanization challenges the integrity of the hydrologic cycle in terms of water quality and the incidence of flooding and erosion in cities.

"The hydrologic cycle, the nutrient cycle and the food chain are essential to human life; they sustain us, and they link us to the environment in which we live and to the other organisms, both human and non-human that share our habitat. Yet to most people these cycles are abstractions, something read about in textbooks, then quickly forgotten. The urban landscape affords abundant opportunities to celebrate these cycles, to make legible and tangible the connections they forge." (Spirn 1988)