2.3 The Water Cycle
2.4 Runoff and Groundwater
2.5 The Water Cycle and Climate Change
2_Hydrological-Cycle
Hydrological Cycle
Overview
Title image "The Natural Water Cycle" by Howard Perlman and John Evans of the United States Geological Survey is in the Public Domain.
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Introduction
Learning Objectives
Illustrate the hydrological cycle and discuss its significant to plant growth and development.
Explain the role of precipitation & condensation in water cycle.
Describe how transpiration from plants effect water cycle.
Distinguish between runoff and ground water.
Key Terms
condensation - the process by which water vapor in the air is changed into liquid water
evaporation - the process by which water changes from a liquid to a gas or vapor
evapotranspiration - the sum of evaporation from the land surface plus transpiration from plants
ground water - the water beneath earth's surface in soil pore spaces and in the fractures of rock formations
hydrologic cycle - the continuous circulation of water from land and sea to the atmosphere and back again
precipitation - the process by which water droplets fall to earth as rain, hail, or snow
runoff - the flow of water downhill across saturated or impervious surfaces above ground
transpiration - the process by which water is taken up by plants and released into the atmosphere
Significance of Water to Plants
Plants need water to support cell structure, for metabolic functions, to carry nutrients, and for photosynthesis. The majority of volume in a plant cell is water; it typically comprises 80 to 90 percent of the plant’s total weight. Soil is the main water source for land plants, and it can be an abundant source of water, even if it appears dry. Plant roots absorb water from the soil through root hairs and transport it up to the leaves through the xylem. As water vapor is lost from the leaves, more water is drawn up from the roots through the plant to the leaves (Figure 4.2.1).
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The Water Cycle
The water cycle has no starting point, but since most of Earth's water exists in the oceans, that is a good place to begin. The hydrologic cycle (or water cycle) is the continuous circulation of water from land and sea to the atmosphere and back again. (See Figure 4.2.2 for a visual representation of the basic aspects of the water cycle.) The sun, which drives the water cycle, heats water in the oceans. Some of it changes from water to gas or vapor and is added to the air through the process of evaporation. A relatively smaller amount of moisture is added as ice and snow sublimate directly from the solid state into vapor. Just as humans release water vapor when they breathe so do plants—although the term "transpire" is more appropriate than "breathe." Transpiration is the process by which water is taken up by plants and released into the atmosphere. Studies have revealed that transpiration accounts for about 10 percent of the moisture in the atmosphere, with oceans, seas, and other bodies of water (lakes, rivers, streams) providing nearly 90 percent and a tiny amount coming from sublimation (ice changing into water vapor without first becoming liquid). Evapotranspiration is the sum of evaporation from the land surface plus transpiration from plants.
Rising air currents take the vapor created by transpiration and evapotranspiration up into the atmosphere. Condensation is the process by which water vapor in the air is changed into liquid water. The vapor rises into the air where cooler temperatures cause it to condense into clouds. Clouds regulate the flow of radiant energy into and out of Earth's climate system. They influence the Earth's climate by reflecting incoming solar radiation (heat) back to space and outgoing radiation (terrestrial) from the Earth's surface. Often at night, clouds act as a "blanket," keeping a portion of the day's heat next to the surface. Changing cloud patterns modify the Earth's energy balance, and, in turn, temperatures on the Earth's surface. Condensation is the opposite of evaporation.
Air currents move clouds around the globe. Cloud particles collide, grow, and fall out of the sky during precipitation, the process by which water droplets fall to earth as rain, hail, or snow. Precipitation that falls as snow can accumulate as ice caps and glaciers, which can store frozen water for thousands of years. Snow-packs in warmer climates often thaw and melt when spring arrives, and the melted water flows overland as snowmelt. Most precipitation falls back into the oceans or onto land, where due to gravity, the precipitation moves over the ground as surface flow or runoff. Figure 4.2.3 gives a more complete picture of the water cycle.
Runoff and Groundwater
A portion of runoff enters rivers in valleys, with streamflow moving water towards the oceans. Runoff, and groundwater seepage, accumulate and are stored as freshwater in lakes. Not all runoff flows into rivers, though. Much of it soaks into the ground as infiltration. Some of the water infiltrates into the ground and replenishes aquifers, which store huge amounts of freshwater for long periods of time. Some infiltration stays close to the land surface and can seep back into surface-water bodies (and the ocean) as groundwater discharge, and some groundwater finds openings in the land surface and emerges as freshwater springs. Yet, more groundwater is absorbed by plant roots to end up as evapotranspiration from the leaves. Over time, though, all of this water keeps moving and some of it reenters the ocean, where the water cycle "ends."
Large amounts of water are stored in the ground as groundwater as seen in Figure 4.2.4. The water is still moving, possibly very slowly, and it is still part of the water cycle. Most of the water in the ground comes from precipitation that infiltrates downward from the land surface. The upper layer of the soil is the unsaturated zone, where water is present in varying amounts that change over time but does not saturate the soil. Below this layer is the saturated zone, where all of the pores, cracks, and spaces between rock particles are saturated with water. The term groundwater is used to describe this area. Another term for groundwater is "aquifer," although this term is usually used to describe water-bearing formations capable of yielding enough water to supply peoples' uses. Aquifers are a huge storehouse of Earth's usable fresh water. People all over the world depend on the groundwater in aquifers in their daily lives for domestic, industrial and agricultural purposes.
The top of the surface where groundwater occurs is called the water table. Figure 4.2.5 displays how the ground below the water table is saturated with water (the saturated zone). Aquifers are replenished by the seepage of precipitation that falls on the land, but there are many geologic, meteorologic, topographic, and human factors that determine the extent and rate to which aquifers are refilled with water. The characteristics of groundwater recharge vary all over the world. Rocks have different porosity and permeability characteristics, which means that water does not move around the same way in all rocks.
The Water Cycle and Climate Change
Water vapor is a gas that contributes to the greenhouse effect. The water, or hydrologic, cycle describes the pilgrimage of water as water molecules make their way from the Earth’s surface to the atmosphere and back again, in some cases to below the surface. This gigantic system, powered by energy from the Sun, is a continuous exchange of moisture between the oceans, the atmosphere, and the land. As external factors like increasing carbon dioxide warm the atmosphere, the amount of water vapor in the atmosphere will increase. This will then slowly increase the greenhouse effect, reducing the amount of heat able to escape from Earth. The atmosphere warms further, enabling more water vapor to be held in the atmosphere. Water vapor that remains in the atmosphere will eventually condense and form clouds. Clouds can add to the greenhouse effect by trapping heat in the atmosphere. The process shows how global warming causes the hydrologic cycle to accelerate.
Dig Deeper
USGS: The Fundamentals of the Water Cycle
USGS: Condensation and the Water Cycle
USGS: Freshwater (Lakes and Rivers) and the Water Cycle
USGS: Evapotranspiration and the Water Cycle
USGS: Surface Runoff and the Water Cycle
USGS: Groundwater Flow and the Water Cycle
USGS: Groundwater Storage and the Water Cycle
USGS: Precipitation and the Water Cycle
Attributions
"Biology 2e: Chapter 31: Section 1" by Mary Ann Clark, Matthew Douglas, and Jung Choi is licensed under CC BY 4.0.
"A Multi-Phased Journey" by the United States National Atmospheric Space Administration is in the Public Domain.