Unit 40


Unit Overview

This unit examines the movement of water on and within the lithosphere, which is one component of the hydrologic cycle. The main sections are as follows:

  • Water at the surface
  • Water flow in streams
  • Water beneath the surface

Water is probably the most critical resource on our planet. Water cycles through the Earth's spheres via the hydrologic cycle. Before precipitation can interact with and possibly enter the lithosphere it is subject to interception by vegetation. Once reaching the surface, water infiltrates the soil, is stored in underground reservoirs, or flows over the land as runoff. Infiltration is controlled by the physical characteristics of the soil, existing soil moisture, quality and quantity of vegetative cover, slope of the surface, and type of rainfall. The overland flow (i.e., runoff) ceases when water enters a stream. The term "stream" designates any water-conveying channel in the landscape, ranging from a large river to a small creek. Gradients, velocities, and discharge characterize streams and each of these variables has values that vary both among and within streams.

Besides being present in streams, fresh water on the planet can exist in ice caps, glaciers, lakes, and as groundwater. Water existing within the lithosphere is known as groundwater. Associated with groundwater are two subterranean zones, the zone of aeration (i.e., vadose zone) and the zone of saturation (i.e., phreatic zone). Water enters the vadose zone prior to entering the phreatic zone. Soil has a field capacity (i.e., maximum amount of water that soil can possibly contain without being saturated) within the vadose zone. The water table marks the top of the phreatic zone, which is a zone permanently saturated with water.

Lithospheric materials below the water table include aquifers (conducive to water flow) and aquicludes (prohibit water flow).

Unit Objectives

  • To discuss the various paths water may take on and within the surface of the lithosphere
  • To introduce fundamental aspects of river flow
  • To outline basic concepts related to groundwater hydrology

Glossary of Key Terms

Aquiclude Impermeable rock layer that resists the infiltration of groundwater; consists of tightly packed or interlocking particles, such as in shale.
Aquifer A porous and permeable rock layer that can at least be partially saturated with groundwater.
Artesian well One that flows under its own natural pressure to the surface; usually associated with a confined aquifer that is recharged from a remote location where that aquifer reaches the surface.
Discharge The volume of water passing a given cross-section of a river channel within a given amount of time; measured as average water velocity multiplied by the cross-sectional area.
Field capacity The ability of a soil to hold water against the downward pull of gravity; also the maximum amount of water a soil can contain before becoming waterlogged.
Gradient (slope) The slope of a river channel as measured by the difference in elevation between two points along the stream course.
Groundwater Water contained within the lithosphere; this water hidden below the ground accounts for about 25 percent of the world's fresh water.
Hydrograph A graph of a river's discharge over time.
Infiltration The flow of water into the Earth's surface through the pores and larger openings in the soil mass.
Interception The blocking of rainwater from reaching the ground by vegetation; raindrops land on leaves and other plant parts and evaporate before they can penetrate the soil below.
Perched water table A separate local water table that forms at a higher elevation than the nearby main water table; caused by the effects of a local aquiclude.
Runoff The removal - as overland flow via the network of streams and rivers - of the surplus precipitation at the land surface that does not infiltrate the soil or accumulate on the ground through surface detention.
Slope See gradient (slope).
Spring A surface stream of flowing water that emerges through the ground.
Velocity The rate of speed at which water moves in at a river channel; this rate varies within the stream.
Water table The top of the (phreatic) zone of saturation; does not lie horizontally but follows the general profile of the land surface above.
Zone of aeration The upper of the two subterranean zones that contains groundwater; lies above the water table and is normally unsaturated, except during heavy rainfall (also known as the vadose zone).
Zone of saturation The lower of the two subterranean zones that contains groundwater; lies below the water table and is also known as the phreatic zone).

Unit Outline

  • Water at the surface
    • Interception occurs when water falls on vegetation and evaporates before it reaches the soil
    • Impermeable surfaces do not allow water to pass through them, permeable surfaces do
    • Infiltration is the flow of water into the surface through pores in the soil
      • infiltration rate depends upon
        • characteristics of the soil
        • how moist the soil is
        • type and amount of overlying vegetation
        • slope of the surface
        • type of rainfall (intensity)
      • water accumulates in any hollows on surface (known as surface detention)
      • runoff (overland flow) occurs when there is more rainfall than the soil can hold
  • Water flow in rivers (Fig. 40.4)
    • A river's gradient (slope) is the difference in elevation between two points along its course
      • when the gradient is high, flow is more turbulent
    • Movement is not uniform in a river channel, water at the center moves faster where resistance is lowest
    • The discharge of a river is the amount of water flowing in a given cross section of it
      • calculated by multiplying average velocity by cross-sectional area
    • River flow
      • eddies have almost as much backward as forward movment
      • discharge of a river varies with season, year, etc.
      • hydro graph is a graph of a river's discharge over time
      • rivers contain about 0.03% of total fresh water supply
  • Water beneath the surface (Fig. 40.7 and Fig. 40.8)
    • Groundwater is water underneath the Earth's surface
      • about 25% of total fresh water supply is groundwater
      • zone of aeration (vadose zone) is the upper zone
      • zone of saturation (phreatic zone) is the lower zone
    • Soil moisture in the zone of aeration
      • rainwater enters soil by infiltration, and is called soil moisture
      • water continues to move downward by percolation
      • water can move upward by capillary action
      • moisture can also move by evaporation, water vapor movement, and recondensation
      • field capacity is the maximum amount of water that can be held by soil without it becoming saturated
      • hygroscopic water is a thin film of moisture that clings to soil particles, and is unavailable to plant roots
      • the wilting point is the lower limit to soil moisture, below which plants will be damaged
      • the total soil storage capacity is the product of average depth of roots and the water storage per cm for that type of soil
    • Groundwater in the zone of saturation (Fig. 40.8)
      • the water table is the upper surface of the phreatic zone
      • aquifers are porous, permeable layers that can be saturated
        • sandstone
        • limestone
      • aquicludes (aquitards) are zones of impermeable rock layers
        • mudstone
        • shale
      • unconfined aquifers get water from local infiltration
      • confined aquifers lie in-between aquicludes, and get water from more distant sources by gravity flow
  • Wells and springs (Figs. 40.9 & 40.10)
    • a traditional well is a hole that penetrates the water table
    • artesian wells are dug into confined aquifers that are constantly recharged by a distant water source
    • a cone of depression may form when water is drawn out of a well faster than it is replaced, causing the local water table around it to drop
      • drawdown is the amount that the water table drops when a cone of depression form
    • springs are surface streams that emerge above the surface, commonly when an aquiclude halts the percolation of water, and is forced upwards
      • a perched water table often forms higher than the regular water table to feed the spring
    • hot springs usually occur where the spring lies over a buried magma chamber
      • geothermal steam can be used as an energy source
    • mineral springs are a form of hot springs, used for medicinal or recreational purposes

Review Questions

  1. How does an artesian well differ from a traditional well?
  2. What are the conditions necessary for spring formation? Use Fig. 40.10 as a reference.
  3. How does water velocity vary within a river channel? Refer to Fig. 40.4.