Summary of the Chapter

Hydrology is the science that studies the Earth's water molecules and their movement through the hydrologic cycle. The Earth and its various abiotic and biotic systems are greatly influence by water. Water is essential for life and plays an important role in atmospheric and lithospheric processes.

The hydrologic cycle is used to model the storage and movement of water molecules between the biosphere, atmosphere, lithosphere and hydrosphere. Water is stored in the following reservoirs: atmosphere, oceans, lakes, rivers, glaciers, soils, snowfields, and groundwater. It moves from one reservoir to another by processes like: evaporation, condensation, precipitation, deposition, runoff, infiltration, sublimation, transpiration, and groundwater flow.

Water molecules are stored in the atmosphere in all three states of matter. Water vapor in the atmosphere is commonly referred to as humidity. If liquid and solid forms of water can overcome atmospheric updrafts they can fall to the Earth's surface as precipitation. The formation of ice crystals and water droplets occurs when the atmosphere is cooled to a temperature that causes condensation or deposition. Four processes that can cause atmospheric cooling are: orographic uplift; convectional uplift; air mass convergence; and radiative energy loss.

Precipitation can be defined as any aqueous deposit, in liquid or solid form, that develops in a saturated atmospheric environment and generally falls from clouds. A number of different precipitation types have been classified by meteorologists including rain, freezing rain, snow, ice pellets, snow pellets, and hail. Fog represents the saturation of air near the ground surface. Classification of fog types is accomplished by the identification of the mechanism that caused the air to become saturated.

The distribution of precipitation on the Earth's surface is generally controlled by the absence or presence of mechanisms that lift air masses to cause saturation. It is also controlled by the amount of water vapor held in the air, which is a function of air temperature. A figure is presented that illustrates global precipitation patterns.

In certain locations on the Earth, acid pollutants from the atmosphere are being deposited in dry and wet forms to the Earth’s surface. Scientists generally call this process acid deposition. If the deposit is wet it can also be called acid precipitation. Normally, rain is slightly acidic. Acid precipitation, however, can have a pH as low as 2.3. The formation of acid deposition in the atmosphere is explained in detail. The process of lake acidification by acidic pollutants is also discussed.

Evaporation and transpiration are the two processes that move water from the Earth’s surface to its atmosphere. Evaporation is movement of free water to the atmosphere as a gas. It requires large amounts of energy. Transpiration is the movement of water through a plant to the atmosphere. Scientists use the term evapotranspiration to describe both processes. In general, the following four factors control the amount of water entering the atmosphere via these two processes: energy availability; the humidity gradient away from the evaporating surface; the wind speed immediately above the surface; and water availability. Agricultural scientists sometimes refer to two types of evapotranspiration: Actual Evapotranspiration and Potential Evapotranspiration. The growth of crops is a function of water supply. If crops experience drought, yields are reduced. Irrigation can supply crops with supplemental water. By determining both actual evapotranspiration and potential evapotranspiration a farmer can calculate the irrigation water needs of their crops.

The distribution of precipitation falling on the ground surface can be modified by the presence of vegetation. Vegetation in general, changes this distribution because of the fact that it intercepts some the falling rain. How much is intercepted is a function of the branching structure and leaf density of the vegetation. Some of the water that is intercepted never makes it to the ground surface. Instead, it evaporates from the vegetation surface directly back to the atmosphere. A portion of the intercepted water can travel from the leaves to the branches and then flow down to the ground via the plant’s stem. This phenomenon is called stemflow. Another portion of the precipitation may flow along the edge of the plant canopy to cause canopy drip. Both of the processes described above can increase the concentration of the water added to the soil at the base of the stem and around the edge of the plant’s canopy. Rain that falls through the vegetation, without being intercepted, is called throughfall.

Infiltration is the movement of water from precipitation into the soil layer. Infiltration varies both spatially and temporally due to a number of environmental factors. After a rain, infiltration can create a condition where the soil is completely full of water. This condition is, however, only short-lived as a portion of this water quickly drains (gravitational water) via the force exerted on the water by gravity. The portion that remains is called the field capacity. In the soil, field capacity represents a film of water coating all individual soil particles to a thickness of 0.06 mm. The soil water from 0.0002 to 0.06 mm (known as capillary water) can be removed from the soil through the processes of evaporation and transpiration. Both of these processes operate at the surface. Capillary action moves water from one area in the soil to replace losses in another area (biggest losses tend to be at the surface because of plant consumption and evaporation). This movement of water by capillary action generally creates a homogeneous concentration of water throughout the soil profile. Losses of water stop when the film of water around soil particles reaches 0.0002 mm. Water held from the surface of the soil particles to 0.0002 mm is essentially immobile and can only be completely removed with high temperatures (greater than 100 degrees Celsius). Within the soil system, several different forces influence the storage of water.

Runoff is the surface flow of water to areas of lower elevation. On the microscale, runoff can be seen as a series of related events. At the global scale runoff flows from the landmasses to the oceans. The Earth’s continents experience runoff because of the imbalance between precipitation and evaporation.

Throughflow is the horizontal subsurface movement of water on continents. Rates of throughflow vary with soil type, slope gradient, and the concentration of water in the soil. Groundwater is the zone in the ground that is permanently saturated with water. The top of groundwater is known as the water table. Groundwater also flows because of gravity to surface basins of water (oceans) located at lower elevations.

The flow of water through a stream channel is commonly called streamflow or stream discharge. On many streams humans gauge streamflow because of the hazards that can result from too little or too much flow. Mechanical gauging devices record this information on a graph known as a hydrograph. In the online notes there is a representation of a hydrograph showing some of its typical features.

Oceans cover most of the Earth's surface. On average, the depth of the world's oceans is about 3.9 kilometers. However, maximum depths can be greater than 11 kilometers. The distribution of land and ocean surfaces on the Earth is not homogeneous. In the Southern Hemisphere there is 4 times more ocean than land. Ratio between land and ocean is almost equal in the Northern Hemisphere. Geographers recognize three major ocean basins: Pacific; Atlantic; and Indian.

The water found in the ocean basins is primarily a byproduct of the lithospheric solidification of rock that occurred early in the Earth's history. A second source of water is volcanic eruptions. The dissolved constituents found in the ocean come from the transport of terrestrial salts in weathered sediments by leaching and stream runoff. Seawater is a mixture of water and various salts. Chlorine, sodium, magnesium, calcium, potassium, and sulfur account for 99% of the salts in seawater. The presence of salt in seawater allows ice to float on top of it. Seawater also contains small quantities of dissolved gases including: carbon dioxide, oxygen, and nitrogen. These gases enter the ocean from the atmosphere and from a variety of organic processes. Seawater changes its density with variations in temperature, salinity, and ocean depth. Seawater is least dense when it is frozen at the ocean surface and contains no salts. Highest seawater densities occur at the ocean floor.

Atmospheric circulation drives the movement of ocean currents. Within each of the three ocean basins, the patterns of these currents are very similar. In each basin, the ocean currents form several closed circulation patterns known as gyres. A large gyre develops at the subtropics centered at about 30 degrees of latitude in the Southern and Northern Hemisphere. In the Northern Hemisphere, several smaller gyres develop with a center of rotation at 50 degrees. Similar patterns do not develop in the middle latitudes of the Southern Hemisphere. In this area, ocean currents are not bound by continental masses. Ocean currents differ from each other by direction of flow, by speed of flow, and by relative temperature.

The cyclical rise and fall of seawater is known as a tide. Tides develop because of gravitational interactions between the Earth, Sun, and moon. The timing of tidal cycles is related to the rotation of the Earth on its axis and the revolution of the moon around the Earth. Extreme tidal events, known as spring tides, occur when there is an alignment of the gravitational forces of the Sun and the moon. When the Sun and moon's forces are perpendicular to each other, moderate neap tides develop. The nature of the three major types of tides is described in detail.

 

List of Key Terms

Acid Deposition, Acid Precipitation, Acid Rain, Acid Shock, Acidic, Actual Evapotranspiration, Advection Fog, Air Mass, Alkaline, Ammonia, Ammonium, Aquifer, Artesian, Artesian Well, Atmosphere,

Base Flow, Biosphere,

Canadian Shield, Canopy Drip, Capillary Action, Capillary Water, Carbon Dioxide, Condensation, Condensation Nuclei, Conduction, Confined Groundwater, Conglomerate, Coniferous, Convectional Precipitation, Convergence Precipitation, Cumulus Cloud, Cyclone,

Deposition, Deposition Nuclei, Dew, Dew Point, Dissolution, Disturbance, Diurnal Tide,

Earth's Rotation, Eddy, Element, Evaporation, Evaporation Fog, Evapotranspiration,

Field Capacity, Fog, Freezing, Freezing Rain, Friction, Frontal Fog, Frontal Lifting, Frontal Precipitation, Frost, Frost Point,

Glacier, Granite, Gravel, Gravitational Water, Gravity, Groundwater, Groundwater Flow, Gyre,

Hail, Humidity, Hydrograph, Hydrostatic Pressure, Hygroscopic Coefficient, Hygroscopic Water,

Ice Pellet, Igneous, Infiltration, Infiltration Rate, Interception,

Lake, Leachate, Leeward, Lithosphere,

Matric Force, Mid-Latitude Cyclone, Mixed Tide,

Neap Tide, Nitric Acid, Nitrogen Oxide, Nitrogen Saturation, Nonrenewable,

Ocean, Ocean Current, Organic Matter, Orographic Precipitation, Orographic Uplift, Oxidation,

Perched Water Table, pH, Potential Evapotranspiration, Precipitation,

Radiation Fog, Rain, Rainfall, Rainshadow Effect, Relative Humidity, Rill, River, River Channel, Runoff,

Sandstone, Saturation, Saturation Mixing Ratio, Secondary Pollutant, Semi-Diurnal Tide, Sleet, Snow, Snowfall, Snowfield, Snow Pellet, Soil, Soil Porosity, Specific Heat, Spring, Stemflow, Stomata, Stream, Stream Discharge, Stream Flow, Sublimation, Sulfur Dioxide, Sulfuric Acid, Supercooled, Super-Saturation, Surface Tension,

Temperature Inversion, Throughfall, Thunderstorm, Tidal Period, Tide, Trade Wind, Transpiration,

Unconfined Groundwater, Upslope Fog,

Vapor Pressure, Vortice,

Water Table, Wilting Point

 

Study Questions, Problems, and Exercises

Essay Questions

(1). What is streamflow? How can it be expressed in a mathematical model? Finally, describe the effect of an intense 1 hour storm on streamflow over 24 hours using a hydrograph.

(2). What factors control the rate of evaporation on a soil surface?

(3). Discuss the movement of water into soils. How and why does infiltration vary with time?

(4). Why does runoff occur?

(5). What forces influence the storage of water in the soil matrix?

(6). What factors remove water stored within the soil matrix?

(7). Describe the mathematical equation used to model stream discharge.

(8). How do water droplets form in clouds and fog?

(9). What is acid deposition? How does it form? What chemicals are responsible and where do they come from? How does acid deposition effect the environment?

(10). What is potential evapotranspiration and how does it differ from actual evapotranspiration. Finally, what factors control the rate at which water leaves the Earth's surface by way of evaporation and transpiration?

(11). Fully explain the global distribution patterns of precipitation.

(12). Explain how relative humidity is measured.

(13). On the map supplied draw the appropriate currents in the Atlantic and Pacific Oceans, N and S Hemisphere. Also be able to identify the following currents: Brazil, Gulf Stream, Antarctic Circumpolar (West Wind Drift), Peru, South and North Equatorial, California, Canary, Equatorial Counter, Benguela, Kuroshio, and N. Pacific.

(14). How is salinity measured? What salts make up seawater?

(15). What is the relationship between dissolved gases in the ocean and ocean temperature and salinity?

(16). Discuss how tides form. What is the difference between a Neap and Spring tide? Explain diurnal, semidurinal, and mixed tides.