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Lec (1): Hydrologeologic cycle concepts

EVAPOTRANSPIRATION

Evapotranspiration
is the combined net effect of two processes: evaporation and
transpiration. Evapotranspiration uses a larger portion of
precipitation than the other processes associated with the hydrologic
cycle.

Evaporation
is the process of returning moisture to the atmosphere. Water on any
surface, especially the surfaces of mudholes, ponds, streams, rivers,
lakes, and oceans, is warmed by the sun's heat until it reaches the
point at which water turns into the vapor, or gaseous, form. The water
vapor then rises into the atmosphere.

Transpiration
is the process by which plants return moisture to the air. Plants take
up water through their roots and then lose some of the water through
pores in their leaves. As hot air passes over the surface of the
leaves, the moisture absorbs the heat and evaporates into the air.

CONDENSATION

Condensation is the cooling of water vapor until it becomes a liquid. As the dew point
is reached, water vapor forms tiny visible water droplets. When these
droplets form in the sky and other atmospheric conditions are present,
clouds will form. As the droplets collide, they merge and form larger
droplets and eventually, precipitation will occur.

PRECIPITATION

Precipitation
is moisture that falls from the atmosphere as rain, snow, sleet, or
hail. Precipitation varies in amount, intensity, and form by season and
geographic location. These factors impact whether water will flow into
streams or infiltrate
into the ground. In most parts of the world, records are kept of snow
and rainfall. This allows scientists to determine average rainfalls for
a location as well as classify rainstorms based on duration, intensity
and average return period. This information is crucial for crop
management as well as the engineering design of water control
structures and flood control.

INFILTRATION

Infiltration
is the entry of water into the soil surface. Infiltration constitutes
the sole source of water to sustain the growth of vegetation and it
helps to sustain the ground water supply to wells, springs and streams.
The rate of infiltration is influenced by the physical characteristics
of the soil, soil cover (i.e. plants), water content of the soil, soil
temperature and rainfall intensity. The terms infiltration and percolation are often used interchangeably.

PERCOLATION

Percolation
is the downward movement of water through soil and rock. Percolation
occurs beneath the root zone. Ground water percolates through the soil
much as water fills a sponge, and moves from space to space along
fractures in rock, through sand and gravel, or through channels in
formations such as cavernous limestone. The terms infiltration and percolation are often used interchangeably.

RUNOFF

Runoff
is the movement of water, usually from precipitation, across the
earth's surface towards stream channels, lakes, oceans, or depressions
or lowpoints in the earth's surface. The characteristics that affect
the rate of runoff include rainfall duration and intensity as well as
the ground's slope, soil type and ground cover.

هذه المحاضرات لطلاب الفرقتين الثالثة والرابعة جيولوجيا جميع الشعب

Hydrogeology Lecture (2)

Water table, aquifers and aquitards, potentiometric surface
-
Today: will continue with chapter 4 (aquifer characteristics), look at
the water table, aquifers and aquitards, potentiometric surface, cone
of depression, confined and unconfined aquifers

I) Water Table:

A) Background:

- A misunderstood concept among the general public:

- People (well owners...) often refer to underground "lakes" and "rivers"

- As geologists: most of us have a better understanding of the water table

- Groundwater is stored in pores and fractures

- These voids are usually very small

B) 3 zones of water distribution in the subsurface:

- Zones are defined by differences between fluid pressure and atmospheric pressure

1) Saturated (phreatic) zone:

- Fluid pressure > overlying atmospheric pressure

- Caused by the weight of the overlying water

- As top of saturated zone is approached:

fluid pressure decreases

- At the water table: fluid pressure = atmospheric pressure

Def: Water Table: The undulating surface at which pore water pressure = atmospheric pressure

- Map of the water table = a potentiometric surface map

2) Capillary fringe:

- A transition zone

- Still has abundant hygroscopic water (water in pores)

- Can be very thick

ex: up to 40' thick near the Nevada test site

- This zone is an important consideration with contaminant flow:

- Contaminants may be soluble in pore water even if pores aren't saturated

3) Unsaturated (vadose) zone:

- Hygroscopic pressure is less than atmospheric pressure

- Capillary water is minimal

- Water is "bound" water: contained in clay mineral structure or in organic compounds

II) Rules of the water table:

A) In the absence of ground-water flow, the water table will be flat

B) A sloping water table indicates that groundwater is flowing

C) Groundwater discharge zones are in topographic low spots

D) The water table has the same general shape as the surface topography

E) Groundwater generally flows from topographical high spots toward topographical low spots

Note: last two rules primarily apply to humid regions

III) Aquifers:

A) Definitions

Confining layer: A geologic layer with little intrinsic permeability (k_i < 10^-2cm²)

- Permeability value is arbitrary, BUT:

- Does not transmit significant amounts of water

- Below the water table, all units contain groundwater

- Rates of water storage and transmittance are relative

-
A problem: comparisons are relative; one person's aquifer (silty sand
in an otherwise clay-rich area) may be another person's aquitard (silty
sand in a gravelly area)

- Brings us to some more definite terms:

Aquifuge (confining layer): is essentially impermeable

Aquitard (leaky confining layer): can transmit small amounts of water

B) Types of aquifers:

1) Unconfined aquifers (water table aquifers)

- Are close to the land surface

- Have continuous permeable layers from land surface to the base of the aquifer

- Recharge is by seepage from land surface OR by baseflow (lateral groundwater movement)

2) Confined aquifers:

- Are overlain by a confining layer

- Amount to a non-renewable resource

- Water may be 1,000’s, 100,000's or millions of years old

a) Formation of confining aquifers:

- Form in several different geologic settings:

i) alternating units deposited on a regional dip

ii) facies changes

iii) upwarp created by intrusions

b) Methods of recharge:

- 2 possible types of recharge:

i) Outcrop area:

- May be far away

ii) Slow leakage from overlying leaky confining layer

- Recharge is very slow

c) Artesian wells

- A special case in some confined aquifers

- Water in confined aquifers is under pressure

- Creates a potentiometric surface that lies above the upper confining layer
Def: Potentiometric surface: the height to which water will rise in a well

-
Artesian aquifer: pressure in a confined aquifer (represented by
potentiometric surface) is higher than the bed surface, water in a well
bore will rise above the bed

- Flowing Artesian well: Potentiometric surface is higher than the land surface

d) Pumping in confined aquifers:

- Pumping lowers the aquifer surface in a cone of depression

- Cone of depression represents a pressure boundary

- Position above the upper confining bed is not really related to water levels (potentiometric surface) in upper bed

3) Perched aquifers:

- Are unusual

- Are small

- Occur when a confining layer prevents groundwater from percolating through the unsaturated zone

IV) Potentiometric surface maps

- Are two-dimensional representations of a three-dimensional surface (the water table)

- Are similar to contour maps

- Are useful for identifying groundwater divides:

Definition: Groundwater divide: a “high” on the contour map that restricts groundwater flow:

A drop of water on the divide will split, go in either (both?) directions

Separates groundwater basins, is important when considering contaminant flow.

- A minor difference between contour and topo maps:

Potentiometric surface lines can divide or converge

A) Constructing a potentiometric surface map:

- Has many similarities to constructing a contour map:

- Use a topo map as a base map

- Topo map influences interpolations when contouring, since groundwater mimics topography in unconfined aquifers

- In confined aquifers: groundwater potentiometric surface does not necessarily mimic topography

- Groundwater v's uphill at gaining streams

- Groundwater v's downhill at loosing streams

B) Measurements must meet certain criteria to be included on a potentiometric surface map:

1) must be made within a short time interval

2) must be made from the same aquifer

3) must be referenced to a common datum (normally sea level)

4) water must be in static state (not responding to pumping

Note: the topo map influences interpolations when contouring, since groundwater mimics topography in unconfined aquifers

Prepared by: Dr. Fathy Ahmed