听力原文：[Professor] Well, today I thought we'd talk about hydrologic cycle. As we all know, water is essential for life, and in parts of the word, it's a precious commodity. Water continuously circulates from the ocean to the atmosphere, to the land, and back to the ocean, providing us with a renewable supply of purified water. This complex cycle, known as the hydrologic cycle, balances the amounts of water in the ocean, in the atmosphere, and on the land. We get our understanding of how the cycle operates from research in climatology and hydrology. So... who can tell me what climatology is? [Student A] It's the study of climate... and... uh... the causes and effects of different climates. [Professor] That's right. And what is hydrology? [Student B] Well, 'hydro' means 'water', so it's something to do with water... like the study of water? [Professor] Yes. The prefix 'hydro' does refer to water. The hydrologic cycle is the water cycle. And hydrology is the study of water...the distribution and effect of the water on the earth's surface and in the soil and layers of rock. Think of climatology as the atmosphere phase and hydrology as the land phase of the water cycle. Climatologist study the role of solar energy in the cycle. They're mainly concerned with the atmospheric phase of the cycle, how solar energy drives the cycle through the...uh...processes of evaporation, atmospheric circulation and precipitation. Water is continuously absorbed into the atmosphere as vapor, evaporation, and returned to the earth as rain, hail, or snow, precipitation. The amount of water evaporating from oceans exceeds precipitation over oceans, and the excess water vapor is moved by wind to the land. The land phase of the cycle is the concern of hydrologists. Hydrologists study the vast quantities of water in the land phase of the cycle, how water moves over and through the land, and how it's stored on or within the earth. Over land surfaces, of the precipitation that falls over land, small amounts evaporate while still in the air and... oh... reenter the atmosphere directly. The rest of it reaches the surface of the land. The water that falls to earth is stored on the surface in lakes, or it penetrates the surface, or it runs off over the surface and flows in rivers to the ocean. Some of the water is stored temporarily in the upper soil layers and used later by trees and plants. When it rains... yes? [Student A] I was... um... I wondered if that makes trees and plants part of the hydrologic cycle. I mean, they take in water, and the water moves through them, and then later on... um... the water evaporates from their leaves. [Professor] I am glad you mentioned that. Plants do play an important role in the land phase of the cycle and are therefore part of the cycle. Trees and plants circulate and store water, they draw it up through their roots and return it to the atmosphere through their leaves during cvapotranspiration. When it rains, if the soil is already saturated, water will seep downward through the upper soil layers and possibly reach the water table. When it reaches the water table, it passes into groundwater storage. Most of the groundwater later returns to the surface, either as springs or as stream flow, supplying water to plants. Eventually, all of the water falling on land makes its way back to the ocean. The movement of water from land to the ocean is called runoff. Runoff and groundwater together balance the amount of water that moves from the ocean to the land. Every molecule of water in the natural system eventually circulates through the hydrologic cycle. Tremendous quantities of water are cycled annually. And as living organisms, plants and animals as well, are also part of the cycle, since water is a large part of the mass of most organisms. Living organisms store and use water, since water is the...uh...solvent for most biological reaction. Narrator Now get ready to answer some question