The water cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above, and below the surface of the Earth. This fundamental process involves four main stages: water changes from liquid to gas through evaporation, then transitions back to liquid during condensation, falls to the surface as precipitation, and finally gathers in bodies of water as collection. Creating a working model for a science project offers a practical way to observe these phases in a contained environment. This guide provides detailed instructions for constructing a self-contained, closed-system model to demonstrate the endless circulation of water.
Selecting Your Model Type and Materials
Before beginning construction, it is helpful to choose the model type best suited for your goals. Two common, accessible models are the simple zip-top bag model, which is hung on a window to show a flat, two-dimensional cycle, and the more robust closed-system jar or terrarium model. The jar model provides a three-dimensional, long-term demonstration that better simulates the Earth’s surface and atmosphere.
To build this model, you will need a large, clear glass jar with a tight-fitting lid, or a clear plastic container. The base layer will require small pebbles or gravel for drainage, followed by a layer of sand, and then potting mix or soil. You will also need a small amount of water to initiate the cycle and, optionally, a small, slow-growing plant or moss to introduce transpiration, which is water released from leaves.
Step-by-Step Assembly Instructions
The construction process begins with creating the landscape base layer inside your chosen clear container. Start by placing a layer of pebbles or gravel at the bottom of the jar. This layer provides a space for collected water to drain and prevents the soil from becoming saturated. Next, add a thin layer of sand over the gravel to act as a barrier.
The potting mix or soil should be added on top of the sand. The soil layer should be deep enough to support any small plant you choose to include, typically around three to four inches. If you are including a plant, carefully create a small space in the soil and place the plant inside, gently tamping the soil around the roots.
After the landscape is complete, you must add the water that will sustain the entire cycle. Pour water slowly down the side of the jar until the soil is visibly moist, but stop before a large pool of standing water forms on the surface. The goal is to saturate the soil and have a small reservoir of water that mimics a lake or ocean at the base. Secure the lid tightly onto the jar to create a sealed environment, which is necessary to trap the water vapor.
Once sealed, the model must be placed in a location where it can receive a consistent, indirect heat source, such as a sunny windowsill or near a lamp. Avoid placing it in direct, intense sunlight, as this can overheat the contents and harm any plant life inside. The sealed jar is now ready for the natural processes of the water cycle to begin, driven by the energy from the sun or light source.
Observing and Explaining the Processes
Once the sealed model is placed in the warm location, the heat energy activates the process of evaporation. The water in the soil and the small reservoir heats up, causing the liquid water to change into water vapor that rises within the container. If you included a plant, it contributes to the water vapor through transpiration, where moisture is released from its leaves.
As the warm water vapor rises, it encounters the cooler surface of the jar’s lid or the upper sides of the container. This difference in temperature causes the vapor to cool and change back into liquid water droplets, a process known as condensation. These droplets cling to the inside surface of the jar, visually representing the formation of clouds in the atmosphere.
When enough water droplets collect on the lid, they merge into larger, heavier drops that the surface tension can no longer hold. These drops then fall back down to the soil and landscape below, simulating precipitation, which is the return of water to the Earth’s surface. This falling water then soaks into the soil or flows back into the bottom reservoir, completing the final stage of collection and runoff.