Introduction to the Hydrosphere and Earth's Water Systems
Entry Ticket and Initial Concepts
The study of Earth's systems begins with a focus on Lesson 3, regarding the Hydrosphere, as outlined in the science textbook on pages 198 through 207. To begin the lesson, students were presented with an Entry Ticket activity lasting 5 minutes. This activity posed two primary questions: First, identifying the percentage of fresh water out of all the water on Earth. Second, the students were asked to think of specific examples where water cycles through the four major spheres: the Biosphere, the Geosphere, the Atmosphere, and the Hydrosphere.
The Water Cycle and Its Fundamental Processes
The water cycle is defined as the continuous, never-ending process by which water moves from the surface of the Earth to the atmosphere and subsequently back again. This movement is not isolated, as water transitions through the geosphere, the biosphere, the hydrosphere, and the atmosphere. This entire system is powered and driven by two main forces: energy from the sun and the force of gravity. A critical principle of this system is that the total amount of water cycling remains fairly constant because the rates of evaporation and precipitation are globally balanced.
A primary biological process in this cycle is transpiration. In this process, plants draw in or absorb liquid water from the soil through their root systems. This water eventually moves through the plant and is released into the air from the plant’s leaves in the form of water vapor.
Following evaporation or transpiration, water undergoes condensation. This is the process where water vapor is converted back into liquid water. As water vapor rises higher into the atmosphere, it encounters colder air, causing it to condense into liquid droplets or solid ice crystals. These droplets and crystals collect around solid particles floating in the air, which eventually leads to the formation of clouds.
Precipitation occurs when the continued condensation causes water droplets and ice crystals to grow larger and heavier. When they reach a certain weight, the force of gravity causes them to fall back to the Earth's surface. Precipitation can take various forms depending on atmospheric conditions.
Global Distribution of Earth's Water
The distribution of water on the planet is highly uneven between salt and fresh varieties. Of all Earth’s water, salt water accounts for approximately , while fresh water accounts for only . Within that small of fresh water, the distribution is further divided. Approximately two-thirds, specifically , is frozen in the form of glaciers and massive ice sheets located near the North and South poles. Another one-third, specifically , is stored beneath the surface of the Earth as groundwater. Freshwater lakes and rivers represent an extremely small fraction, barely reaching of all available fresh water.
When precipitation falls, the majority of it goes directly back into the oceans. The water that falls over land surfaces follows several paths: it may evaporate again, seep into the ground to become underground water, or move across the surface. Only a small portion of precipitation on land becomes runoff, which refers to water that runs off the surface into rivers, streams, and lakes.
Surface Water Systems: Rivers, Watersheds, and Lakes
Surface water encompasses all water found on the exterior surface of the Earth, including oceans, rivers, lakes, and ponds. Rivers specifically originate from a source, which can be an underground stream, runoff from rainfall, or the melting of snow and ice. Gravity dictates that these small streams flow downhill, eventually joining other streams to form a larger river that flows into a sea or ocean. Small streams and rivers that feed into a larger primary river are known as tributaries. Together, a main river and all of its contributing tributaries constitute a river system.
The specific land area that supplies water to a given river system is known as a watershed. As smaller rivers merge into larger systems, the areas they drain are incorporated into the larger river's watershed. These watersheds are separated from one another by a ridge of land known as a divide. In contrast to the moving water of rivers, ponds and lakes consist of mostly still water that collects in low-lying areas of land.
Groundwater and Underground Storage
Water that exists beneath the surface is known as groundwater. Certain layers of rock have the capacity to hold water; these are called aquifers. Humans can access this water by digging a well, which is a man-made hole sunk into the earth deep enough to reach below the water table, the level where the ground is saturated with water. This provides a consistent supply of water for human use.
Physical Properties of Ocean Environments
The oceans are a unique segment of the hydrosphere characterized by specific levels of salinity, temperature, and depth. Salinity refers to the total amount of dissolved salts in a water sample. Salinity levels are not uniform; they decrease near the surface when fresh water is added by rain, snow, or melting ice. Conversely, salinity increases through evaporation or through the formation of ice near the North and South poles, which leaves salt behind in the remaining liquid. High salinity results in ocean water having a higher density than fresh water and a lower freezing point of .
The temperature of the ocean is influenced by the sun, as the broad surface of the water absorbs thermal energy. Surface temperatures fluctuate based on location and season; for example, surface temperatures near the equator often reach , but these temperatures drop as one moves toward the poles. Furthermore, depth plays a significant role in ocean characteristics. The average depth of the ocean is approximately . As depth increases and one moves further down from the surface, the water temperature consistently decreases.