Page 307 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 9 Water resources 271
concepts review
key learning
■ Describe the origin of Earth’s waters, report the quan- tity of water that exists today, and list the locations of Earth’s freshwater supply.
Water molecules came from within Earth over a period of billions of years in the outgassing process. Thus began endless cycling of water through the hydrologic system of evaporation–condensation–precipitation. Water covers about 71% of Earth. Approximately 97% of it is salty seawa- ter, and the remaining 3% is freshwater—most of it frozen.
The present volume of water on Earth is estimated at 1.36 billion km3, an amount achieved roughly 2 billion years ago. This overall steady-state equilibrium might seem in conflict with the many changes in sea level that have occurred over Earth’s history, but it is not. Eustasy refers to worldwide changes in sea level and relates to changes in volume of water in the oceans. The amount of water stored in glaciers and ice sheets explains these changes as glacio-eustatic factors. At present, sea level is rising because of increases in the temperature of the oceans and the record melting of glacial ice.
outgassing (p. 243) eustasy (p. 243)
1. Approximately where and when did Earth’s water originate?
2. If the quantity of water on Earth has been quite constant in volume for at least 2 billion years, how can sea level have fluctuated? Explain.
3. Describe the locations of Earth’s water, both oceanic and fresh. What is the largest repository of fresh- water at this time? In what ways is this distribution of water significant to modern society?
4. Why would climate change be a concern, given this distribution of water?
5. Why might you describe Earth as the water planet? Explain.
■ Illustrate the hydrologic cycle with a simple sketch, and label it with definitions for each water pathway.
The hydrologic cycle is a model of Earth’s water sys- tem, which has operated for billions of years from the lower atmosphere to several kilometres beneath Earth’s surface. Evaporation is the net movement of free water molecules away from a wet surface into air. Transpira- tion is the movement of water through plants and back into the atmosphere; it is a cooling mechanism for plants. Evaporation and transpiration are combined into one term—evapotranspiration.
Interception occurs when precipitation strikes veg- etation or other ground cover. Water soaks into the sub- surface through infiltration, or penetration of the soil surface. Water may puddle on the surface or flow across the surface toward stream channels. This overland flow, also called surface runoff, may become streamflow as it moves into channels on the surface.
Surface water becomes groundwater when it permeates soil or rock through vertical downward movement called
percolation. The volume of subsurface water stored in the soil that is accessible to plant roots is contained in the soil- moisture zone. Groundwater is the largest potential fresh- water source in the hydrologic cycle and is tied to surface supplies. The portion of streamflow that discharges natu- rally at the surface from groundwater is the base flow.
hydrologic cycle (p. 244) evaporation (p. 245) transpiration (p. 245) evapotranspiration (p. 245) interception (p. 245) infiltration (p. 245) overland flow (p. 246) surface runoff (p. 246) percolation (p. 246) soil-moisture zone (p. 246) base flow (p. 246)
6. Sketch and explain a simplified model of the com- plex flows of water on Earth—the hydrologic cycle.
7. What are the possible routes that a raindrop may take on its way to and into the soil surface?
8. Compare precipitation and evaporation volumes from the ocean with those over land. Describe advec- tion flows of moisture and the countering flows of surface and subsurface runoff.
■ Construct the water-budget equation, define each of the components, and explain its use.
A water budget can be established for any area of Earth’s surface by measuring the precipitation input and the output of various water demands in the area considered. If demands are met and extra water remains, a surplus occurs. If demand exceeds supply, a deficit, or water shortage results. Understanding both the supply of the water resource and the natural demands on the resource is essential to sustainable human interaction with the hydrologic cycle. Precipitation (P) is the moisture sup- ply to Earth’s surface, arriving as rain, sleet, snow, and hail and measured with the rain gauge. The ultimate de- mand for moisture is potential evapotranspiration (PE), the amount of water that would evaporate and transpire under optimum moisture conditions (adequate precipita- tion and adequate soil moisture). If we subtract the defi- cit from the PE, we determine actual evapotranspiration, or AE. Evapotranspiration is measured with an evapora- tion pan (evaporimeter) or the more elaborate lysimeter.
The volume of water stored in the soil that is ac- cessible to plant roots is the soil-moisture storage (ST). This is the “savings account” of water that receives deposits and provides withdrawals as water-balance conditions change. In soil, hygroscopic water is inacces- sible to plants because it is a molecule-thin layer that is tightly bound to each soil particle by hydrogen bonding. As available water is utilized, soil reaches the wilting point (all that remains is unextractable water). Capillary water is generally accessible to plant roots because it is held in the soil by surface tension and hydrogen bonding between water and soil. Almost all capillary water is available water in soil-moisture storage. After water drains from the larger pore spaces, the available water remaining for plants is termed field capacity, or