Page 308 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

272 part II The Water, Weather, and Climate Systems
storage capacity. When soil is saturated after a precipi- tation event, surplus water in the soil becomes grav- itational water and percolates to groundwater. Soil-moisture utilization removes soil water, whereas soil-moisture recharge is the rate at which needed mois- ture enters the soil. The texture and the structure of the soil dictate available pore spaces, or porosity. The soil’s permeability is the degree to which water can flow through it. Permeability depends on particle sizes and the shape and packing of soil grains.
Unsatisfied PE is a deficit. If PE is satisfied and the soil is full of moisture, then additional water input be- comes a surplus. Drought can occur in at least four forms: meteorological drought, agricultural drought, hydrologic drought, and/or socioeconomic drought.
water budget (p. 246)
surplus (p. 247)
deficit (p. 247)
precipitation (p. 247)
rain gauge (p. 247)
potential evapotranspiration (p. 247) actual evapotranspiration (p. 247) soil-moisture storage (p. 248) hygroscopic water (p. 248)
wilting point (p. 248)
capillary water (p. 248)
field capacity (p. 249) gravitational water (p. 249) soil-moisture utilization (p. 249) soil-moisture recharge (p. 250) permeability (p. 250)
drought (p. 251)
9. What are the components of the water-balance equation? Construct the equation, and place each term’s definition below its abbreviation in the equation.
10. Explain how to derive actual evapotranspiration (AE) in the water-balance equation.
11. What is potential evapotranspiration (PE)? How do we go about estimating this potential rate? What fac- tors did Thornthwaite use to determine this value?
12. Explain the difference between soil-moisture utiliza- tion and soil-moisture recharge. Include discussion of capillary water and the field capacity and wilting point concepts.
13. In the case of silt loam soil from Figure 9.9, roughly what is the available water capacity? How is this value derived?
14. Use the water-balance equation to explain the chang- ing relation of P to PE in the annual water-balance chart for Hamilton, Ontario.
15. How does the water-budget concept help your un- derstanding of the hydrologic cycle, water resources, and soil moisture for a specific location? Give a specific example.
16. Describe the four definitions of drought.
■ Discuss water storage in lakes and wetlands, and describe several large water projects involving hydro- electric power production.
Surface water is transferred in canals and pipelines for redistribution over space and stored in reservoirs for re- distribution over time to meet water demand. Hydroelec- tric power, or hydropower, provides 20% of the world’s electricity, and many large projects have changed river systems and affected human populations.
Lakes and wetlands are important freshwater storage areas. A wetland is an area that is permanently or sea- sonally saturated with water and that is characterised by vegetation adapted to gleysolic soils (soils saturated for a long enough period to develop anaerobic, or “oxygen- free,” conditions).
hydropower (p. 254) wetland (p. 256)
17. What changes occur along rivers as a result of the construction of large hydropower facilities?
18. Define wetland, and discuss the distribution of lakes and wetlands on Earth.
■ Describe the nature of groundwater, and define the elements of the groundwater environment.
Groundwater lies beneath the surface beyond the soil- moisture root zone, and its replenishment is tied to sur- face surpluses. Excess surface water moves through the zone of aeration, where soil and rock are less than satu- rated. Eventually, the water reaches the zone of satura- tion, where the pores are completely filled with water. The upper limit of the water that collects in the zone of saturation is the water table, forming the contact surface between the zones of saturation and aeration.
The permeability of subsurface rocks depends on whether they conduct water readily (higher permeability) or tend to obstruct its flow (lower permeability). They can even be impermeable. An aquifer is a rock layer that is perme- able to groundwater flow in usable amounts. An unconfined aquifer has a permeable layer on top and an impermeable one beneath. A confined aquifer is bounded above and below by impermeable layers of rock or unconsolidated ma- terial. An aquiclude is a solid, impermeable layer that forms such a boundary, while an aquitard has low permeability but cannot conduct water in usable amounts.
Water in a confined aquifer is under the pressure of its own weight, creating a pressure level to which the water can rise on its own. This potentiometric surface can be above ground level. Groundwater confined under pressure is artesian water; it may rise in wells and even flow out at the surface without pumping if the head of the well is below the potentiometric surface.
As water is pumped from a well, the surrounding water table within an unconfined aquifer will experience draw- down, or become lower, if the rate of pumping exceeds the horizontal flow of water in the aquifer around the well. This excessive pumping causes a cone of depression. Aqui- fers frequently are pumped beyond their flow and recharge capacities, a condition known as groundwater mining.
In many areas, especially where groundwater levels are declining, desalination is becoming an increas- ingly important method for meeting water demands. Desalination of seawater and saline groundwater involves the removal of organics, debris, and salinity through