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The Hydrologic Cycle

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Author: Janice VanCleave

The Movement of Water from Place to Place

There is a continuous interchange of water between the oceans, land, plants, and clouds known as the hydrologic cycle, or water cycle. Most of this water is provided by the oceans.

In this project, you will model the sources of water in the hydrologic cycle, including transpiration (water from plants). You will also demonstrate whether dissolved pollutants in water are transported by the hydrologic cycle.

Getting Started

Purpose: To model the hydrologic cycle.

Materials

  • 1/2 cup (125 ml) of warm tap water
  • 2-quart (2-liter) transparent heat-proof glass bowl
  • Plastic food wrap
  • Ice cube
  • Resealable plastic sandwich bag

Procedure

The Movement of Water from Place to Place

  1. Pour the warm water into the bowl.
  2. Loosely cover the top of the bowl with plastic wrap so that about 2 inches (10 cm) of wrap extends past the edges of the bowl.
  3. Put the ice cube in the bag and seal the bag.
  4. Place the bag of ice in the center of the plastic wrap that covers the bowl.
  5. Gently push the ice down about 1 inch (2.5 cm) so that the plastic wrap sags in the center. Then seal the plastic wrap by pressing its edge against the sides of the bowl (see Figure 34.1).
  6. Observe the surface of the plastic wrap directly under the ice cube every 15 minutes for 1 hour or until the ice melts.

Results

At first, the underside of the plastic wrap becomes cloudy and water droplets form under the ice. Over time, the drops under the ice get larger and most of the plastic wrap looks clear. Some of these drops fall back into the water in the bowl.

Why?

The three common forms of matter, called phases of matter, are solid, liquid, and gas. The change from one form or phase of matter to another is called a phase change. Phase changes are generally the result of gain or loss of energy by a substance in the form of heat. The phase change from liquid to gas by a gain of heat is called evaporation and the phase change from gas to a liquid by a loss of heat is called condensation. In this experiment, the liquid water in the bowl evaporates, forming water vapor (gaseous phase of a substance, such as water that normally exists in the liquid phase). The energy needed for this phase change came from the warm water, but in nature, with phase changes such as evaporation of water from the surface of any body of water, the energy generally comes from the Sun. Water vapor in the bowl rises and comes in contact with the cool surface of the plastic wrap, where it loses energy and condenses, forming water droplets. In Earth's atmosphere (blanket of gases surrounding a celestial body), as water vapor rises it cools and condenses on tiny suspended particles. The tiny drops of water form clouds (visible mass of water drops that float in Earth's atmosphere), which are moved from one place to another by winds. Eventually the water in clouds falls as precipitation (liquid and solid phases of water that fall from the atmosphere). This movement of water from one place to another via changes in phase is called hydrologic cycle, or water cycle.

The primary source of water in the hydrologic cycle is the oceans, which contain about 97% of the Earth's water supply. The remaining water comes from water evaporated from other sources such as lakes, rivers, and moist soil, and from water given off by plants in a process called transpiration (the process by which plants lose water through their leaves). About 75% of the precipitation falls back into the oceans, and much of the remaining 25% falls on the land.

Several things happen to the precipitation that reaches land. Some of it evaporates. In cold areas, snow and ice can remain on the ground for short or long periods before eventually melting. Rainwater soaks into the ground or runs along the surface in rivers and streams. Some collects in lakes and other bodies of standing water, but most finds its way back to the oceans. Rainwater that is unable to soak into the ground and that moves over its surface is called runoff. This moving water generally causes erosion (process by which materials are broken into smaller parts and moved) of the surface it flows over.

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