Lesson 6 Introduction

Agua es vida (Water is life)—that statement adorned the logo of a water-well-drilling company I worked with many years ago in Puerto Rico, and to me, the phrase highlights the importance of water on Earth. Remember that 78% of a human baby's body is composed of water, as is 55–60% of an adult's body; obviously, water is a truly critical component of sustaining life on Earth.

A portion of the water cycle includes the flow of water across Earth's surface, as shown here from the Little River, Great Smoky Mountains, North Carolina. Flowing water not only sustains life, among other processes you will study in this lesson, but can also be visually and aesthetically pleasing.

Credit: Michael Collier. Image source: Earth Science World Image Bank

Water covers ~70% of Earth's surface. Most of this is in the oceans, a realm of Earth not covered in this course. However, the global ocean is more than peripheral to the Critical Zone. Ultimately, all of the water that bathes the Critical Zone was evaporated from the ocean surface, transferred to the continents as vapor via the atmosphere, and deposited on the land surface as liquid or solid precipitation, depending on regional climate. In this unit, we will learn about water's role in the Critical Zone. We will accomplish this in Lesson 6 by first considering in some detail the so-called water cycle. Because of the role of the atmosphere, we will revisit some aspects of atmospheric processes. This should serve to demonstrate the intimate link between the various "spheres" (our last unit with this one) that overlap in the Critical Zone!

The water cycle involves much more than the transfer of water from the ocean to the land surface. Once precipitated, water can flow across the land surface, infiltrate into the subsurface as soil pore water or groundwater (remember that the base of the Critical Zone is defined as the depth to which groundwater freely circulates), or be evaporated or transpired by plants (the top of the Critical Zone!) back to the atmosphere. In this lesson, we will consider the many processes involved in the flow of water through the Critical Zone.

Water is known as the universal solvent—rarely is it found in a pure state because many substances will readily dissolve in it. Thus, we will include brief considerations of water chemistry and quality (we will return to this topic again when considering landforms and biota) and the role human society has played in degrading this invaluable resource. In our landform unit, you will also learn about the role water plays in transforming and sculpting the land surface, while in the biota unit we will explore the many interactions between water and life in the Critical Zone, thus further linking the Critical Zone "spheres."

Finally, because of the somewhat mysterious nature (out of sight, out of mind) of groundwater, you will study various aspects of groundwater flow in Lesson 7, and complete the water unit by reading about links between water and Critical Zone science. Enjoy.