The hydrologic cycle and groundwater (chapter 11)
(Note: not yet revised for Fall 1999)
In discussing the material in this chapter, you should take your focus
from these notes rather than try to cover every topic in the chapter. We
begin with:
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Basic aspects of the global water balance: cycling of water between the
three major compartments (ocean, atmosphere, and continents) and comparison
of the annual average volumes of precipitation, evaporation, and runoff
that are exchanged among these compartments.
Water reaching the land surface may infiltrate into the soil, and
some of that water may percolate down into the saturated zone to become
groundwater. But most groundwater will eventually re-emerge as runoff in
a nearby stream channel.
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Major components of the hydrologic cycle:
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precipitation
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interception, infiltration and percolation
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storage of moisture in the soil
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evapotranspiration
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shallow subsurface flow (throughflow) in the unsaturated zone
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groundwater flow
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overland flow (including sheetflow and rill flow)
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streamflow
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Spatial distribution of precipitation, evapotranspiration, and runoff (e.g.
p. 294)
Next, in discussing groundwater we need to start with the basic understanding
that soil and rock are porous media, and that the ability to transmit water
is in large measure dependent on the nature of the pore space: the total
amount of pore space as well as the size distribution and spatial arrangement
of the pores. Different kinds of geologic materials may be radically different
with respect to their ability for storing or transmitting water. A rock
layer that is capable of storing and transmitting water in usable amounts
can be described as an aquifer, whereas a rock layer that acts as a barrier
to flow is characterized as an aquiclude. In nature the permeability (also
known as "hydraulic conductivity") of rock layers is extremely variable.
Some rock layers have low enough permeability that they behave as aquicludes
and they also may act as confining layers, which in turn leads us to characterize
underlying aquifers as confined aquifers.
In discussing groundwater, you should be familiar with the following
features:
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porosity and permeability (also known as hydraulic conductivity) of rock
or soil; causes of natural variation in permeability (p.296-298)
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unsaturated and saturated zones and the water table (p.298-299)
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interaction between groundwater and surface water; influent and effluent
streams (p.299)
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aquifers and aquicludes; confined and unconfined aquifers, piezometric
(also known as potentiometric) surface, and artesian wells (p.300)
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recharge and discharge zones; perched water tables and springs (p.300-301)
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wells and drawdown due to pumping; cone of depression; depletion of aquifers
(e.g. Ogallala aquifer), land subsidence, and saltwater intrusion (p.301-303,
306)
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concept of hydraulic gradient and its influence on flow rate and direction;
Darcy's Law (p.303-304)
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Landforms created by solution: karst topography (p.305-308)
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closed depressions, sinkholes, caverns and speleothems (stalactites and
stalagmites)
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Hydrothermal waters and hot springs (p.311-312)