Geology
446
Hydrology-hydrogeology
Part I of the lecture notes.
Lecture 1. Significance of Water and its Study.
Course overview.
Attendance.
Hand out syllabus.
Policies.
Text books and lecture notes.
Significance of Water.
Water is such a vital and ubiquitous resource that its
study and management is the principal focus of hundreds of thousands of
scientists and engineers around the world and hundreds of billions of dollars
in costs every year.
Water is essential to all life on earth.
The earth is the only planet that appears to have
significant quantities of water, the earth is also the only planet that appears
to have life (Mars may have had life when it was wetter).
More than 70% of the earths surface is covered by
water or ice more than 70% of the human body is water.
Significance to You:
Death within 3 days without water, less in
Desert.
Average use in USA 180 Gal. per person per day.
A perpetual resource, but distribution &
degradation is a problem.
Most serious natural resource challenge in
Southwestern U.S.
The Environment for life.
A saline solution
of water bathed the environment in which life evolved, as species have evolved
they have emerged from the water, but without inputs of water they can only
survive for hours or days.
Water based fluids bathe
the cells in the bodies of all living things, this solution is quite similar to
that in the oceans from which cellular life evolved.
Definition of Hydrology:
Hydrology is
the study of water and its behavior in the environment. There are several
branches of hydrology:
Surface water hydrology:
Surface
water hydrology: The study of water on the surface of the earth. (not
including the Oceans).
Groundwater hydrology:
The study of water in the subsurface, closely related
to study of the geology of unconsolidated deposits and soil physics the
study of water in soils..
Water Resources Management:
combining hydrology with planning, law, economics,
aquatic biology, chemistry, engineering and often forestry this discipline
seeks to manage water resources to minimize flooding, insure water supply,
maintain or improve water quality, etc.
Related disciplines include:
Hydraulics:
The study of movement and control of water, generally by means of dams, canals,
pipelines, etc.
Limnology:
The study of Lakes.
Meteorology: Study of weather including precipitation
and atmospheric processes.
Climatologic:
Study of the long term climate of earth.
Water, water everywhere
With increasing demands on a finite supply of clean
water to supply agriculture, industrial and municipal uses, sustain natural environments
and help generate electricity and disperse wastes, the importance of the study
of water continues to increase.
LECTURE 2. The Properties of Water, The
Hydrologic Cycle, & Evaporation..
Distribution of Water.
97.3 % in oceans
90% of fresh remainder is ice caps (Antarctic &
Greenland).
Less than .3% in lakes, streams and groundwater
aquifers: supports all terrestrial life.
Only .001% in atmosphere, but vapor & clouds have
huge effect on climate.
Distribution of Surface Water in USA:
Abundant in southeast (except some parts of Florida).
and Northeast (Except major urban areas and coastal
areas).
Less abundant in Midwest.
Rare in West, except Pacific Northwest.
Texas is marginal, except deep east Texas.
I. Properties of Water.
Water consists of two hydrogen and one oxygen
molecules strongly bonded together.
Because of the nature of the molecular and atomic
structure of water, water has a separation of charge (also called polarity).
This separation of charge has several important effects:
Strange attraction:
Water molecules attract
each other quite strongly for a liquid, this in turn cause surface tension that
helps explain why water striders can walk on water.
More importantly this is
why water tends to remain in (and to rise) in soil pores (capillarity). If
water had lower surface tension all water would drain from the pores in soil
and plants would not exist on land.
The Universal solvent:
Water has a negatively charged oxygen end and a
positively charged hydrogen end hence many compounds such as salts (NaCl,
CaS04, etc) will rapidly dissolve in water. The ability of water to dissolve
most substances.
Houston, we have a solution
Gold and platinum are
resistant but all other minerals are soluble which accounts for the salinity of
the oceans. It also accounts for the dissolved solids and substances in
freshwater.
Since plants and single
celled animals take in nutrients in a water solution, if water did not dissolve
most other substances it would not support plants and cellular organisms and
hence life on earth would not exist.
Weathering:
Water also dissolves rocks and is the single most
important factor in all erosion on earth and hence the nature of the landscape
(i.e. it is frequently flat and covered with soil) not composed of bare craggy
rocks and meteorite craters.
Density:
Water also has the very unusual property that its
density does not decrease continuously with decreasing temperature. Rather at 4
degrees C it reaches maximum density.
Why Margaritas are possible:
This means that ice is less dense than cold water and
therefore ice floats. That makes frozen Margaritas possible, but it also allows
life to exist in many northern lakes and oceans that would otherwise freeze
from the bottom up and hence solidly.
Potholes explained:
Since ice expands when it freezes (density decreases)
liquid water that enters cracks in rocks or pavements will expand and exert
tremendous force rending rocks and causing both erosion and potholes.
Boiling and Freezing points:
Pure water boils at 100 degrees C (212 F) and freezes
at zero (32 F) (the standard scales for temperature being based on its
behavior). Water is also used as standard for mass in metric system (1 liter or
cubic decimeter at 25 C weighs 1 kilogram).
Altered States:
Three states are: Ice
(Solid), Liquid & Vapor.
Water is present in all
three on earth, partly because some vaporization occurs at all temperatures
above freezing. Therefore liquid and vapor forms of water are ubiquitous
(particularly in the summer in East Texas). Ice may not be as obviously
present but clouds are generally
composed of ice particles even in warm areas.
Heat capacity:
Water has a great capacity to store heat without a
change in state. This capacity has a huge effect on global weather particularly
temperature. Those areas with little water vapor undergo greater temperature
seasonally & diurnally swings as do moist areas. Thus the hottest and
coldest areas tend to be dry and far from the moderating influences of oceans.
A watched pot never boils
This heat capacity explains why it takes a long time
to bring a pot of water to boil.
The heat capacity of water also means that water can
be used in heat pumps and is essential to power production which uses the heat
capacity of water in cooling towers to condense steam used in turning turbines.
Other properties:
Viscosity: water is not
as viscous (syrupy) as molasses or motor oil or chocolate syrup but is more
viscous that gasoline. At higher temperatures water is less viscous.
Compressibility: water
in a liquid or solid state is almost incompressible, but in a vapor state steam
may be greatly compressed. This makes hydroelectric power plants possible.
Electrical conductivity:
water itself
does not conduct electricity (it is an insulator) however the dissolved ions
(like CA, Mg, Na, Cl, etc) in water do have the ability to conduct electricity.
A shocking experience:
All natural water has some ions, so water ends up
being a pretty good conductor of electricity and therefore using a hair dryer
in the bathtub or going wading in a lighting storm is not a great idea.
II. The Hydrologic Cycle.
Distribution:
97.3 % of all water on earth is in the oceans and 90%
of the remainder is locked up in glaciers and ice caps largely in the Antarctic
and Greenland.
However, the small proportion in lakes streams and
groundwater aquifers is essential to all terrestrial life.
Driving Forces:
Since gravity drives water on land to return to the
oceans, And heat from the sun evaporates water in the Oceans which returns to
land as precipitation, a process is necessary to maintain availability of water
in both systems and hence sustain life on terra firma and in the seas..
The hydrologic cycle:
The hydrologic
cycle refers to the constant process of evaporation of water from the oceans
(and to a lesser extent from lakes, forests and soil), its assent into the
atmosphere, its condensation into clouds, its movement on the wind and its
deposition as rain or snow. The rain and snow then return to the ocean via
rivers and groundwater flow.
Rates:
The process of return of precipitated moisture to the
ocean can be rapid (i.e. rainfall on the ocean itself), moderately rapid (due
to stream flow), or less rapid (infiltration into the soil and recharge of
groundwater aquifers). The cycle has no beginning or end.
Vaporization.
When water is converted from a liquid to a vapor form
(usually from evaporation from the oceans) the energy that evaporated the water
is neither created nor destroyed but is converted into a latent heart of evaporation.
Condensation:
When the water condenses
this heat is released.
The vapor may condense
as a liquid or a solid (ice). These particles of liquid (or ice) may coalesce
and form tiny droplets. These droplets form clouds that are blown about by the
prevailing winds. When the size and number of these droplets exceeds the
ability of a buoyant air mass to suspend them, they fall as rain or snow
(precipitation).
Precipitation:
Various processes can cause precipitation. If warm
moist air moves over land with cold air at the surface this may trigger
precipitation. Also if clouds are forced up and over a mountain range by
prevailing winds the cooling of the air mass as it rises can cause condensation
then precipitation. This is termed an orographic effect.
Run-off and variations.
Once precipitation falls
it may either run-off into streams and rivers, or it may land on vegetation
(interception) and then evaporate or be captured in puddles which usually
evaporate again (depression storage) or soak into the ground (infiltration).
Snowfall may also be
converted back to water vapor directly in a process called sublimation.
Recharge and variations
The water that soaks into the ground (infiltrates) may
be rapidly discharged into streams (interflow), it may remain in the soil as
soil moisture and gradually evaporate, or it may be taken up by plant roots and
be respired through plant leaves (a process called transpiration) or it may
percolate down to the water table and recharge groundwater aquifers.
Groundwater flow:
Groundwater in turn may maintain the flow in creeks
and springs (the base flow) or it may remain for thousands of years as fossil
water in aquifers in closed basins. It
may even bubble-up from the sea floor.
Home again, home again
The water on the land all eventually returns to the
oceans most of it within a few weeks of falling as precipitation.
Process of Evaporation:
Evaporation requires an input of energy (solar) to
convert liquid water into vapor. Evaporation removes heat from the liquid water
and carries that energy away with the water vapor as latent heat of
vaporization.
When water vapor condenses this latent heat is
released at actual heat.
Bonding:
A water molecule in the
liquid phase is tied to thousands of other water molecules by hydrogen bonding.
In the vapor phase, the
molecules are separated so no hydrogen bonding takes place.
Thus water vapor
occupies 42,000 as much space at standard temperature and pressure (STP) as
liquid water.
Water vapor is 60% as
dense as the atmosphere so it will rise.
Example: Why Gore-Tex Keeps You Dry.
]A given volume of air can
only hold so much water vapor at any given temperature. The proportion of water
vapor saturation of the air adjusted for temperature is called relative
humidity.
Influence of temperature:
At low temperatures the water molecules are less
active and hence hydrogen bonding will cause condensation if much water vapor is present. At higher
temperatures much more water can remain in the vapor phase.
That explains why it is more muggy around here when it
is hotter.
Local conditions:
Because of the many water bodies and the extensive
vegetation here, humidity is generally above 50% except when cold air with very
little moisture moves in from the arctic (or Amarillo).
Frozen deserts or is that desserts?
The arctic and Antarctic despite the accumulation of
snow and ice, are arid (they receive less than 10 inches of precipitation per
year).
In arid areas,
humidity as low as 4-10% are common.
Urban heat island:
Urban areas (even in deserts) generate their own
weather, hence the humidity in Las
Vegas has increased from about 5% to about 15% over the last 50 years.
Hot but not humid:
Of course if there is no source for water vapor such
as in desert areas it doesnt matter
how hot it gets, the humidity will not increase.
Moderating influence of moisture:
Since evaporating water uses up energy and since water
vapor blocks solar energy (both as a vapor and as clouds) it is not surprising
that it gets hotter in dry areas than areas with lots of water.
Different strokes:
However, I personally prefer a temperature of 118 with
10% humidity in Las Vegas to 106 in
Saint Louis with 90% humidity.
Why the Dead Sea exists:
Saline water will evaporate more slowly than fresh
water.
Evaporation will increase the salinity of water bodies
such as saline lakes (the dead sea) and even portions of the oceans, like the
Red Sea.
Cold waters run deep
Water only evaporates from the surface of water bodies
and circulation of colder, deeper water to the surface helps reduce
evaporation.
Hence, deep lakes have less evaporation per unit of
surface area and far less per unit of volume than shallow lakes.
Examples:
Lake Mead near Las Vegas
(which is the largest reservoir in North America) has surface water temperatures
in the 70's while 200 feet down the temperature is in the low 50's.... Canyon
Lake is another example.
Temperatures in large
lakes can lag air temperatures so evaporation may be maximal in the winter in
lakes like Lake Superior.
Role of wind:
Wind also plays an important role in evaporation with
windy hot and dry areas having the highest evaporation rates. For Lake Mead
estimated evaporation is 800,000 acre feet per year (or 260 billion gallons).
More water than the whole Dallas area is likely to use in a year.
Significance:
Evaporation and other losses from reservoirs can be
substantial. Lake Mead losses about 6% of all the water flowing in the Colorado
River (a River draining 12% of the U.S. and supplying water to 16 million
people), the other 61 major dams on the Colorado and its tributaries cause
losses of about 15% of all the water in the river.
Applications of evaporation:
Thin Films: Can cut evaporation to near zero, but get
blown away by wind, so they do not work well in practice.
Sand Storage Dams: do work well in areas with
infrequent violent storms and general aridity where normal dams do not work
well.
Evaporation ponds:
can reduce
volumes of liquid wastes from industrial and agricultural sources but have
serious drawbacks.
Salt ponds: are major source of sea salt and certain
chemicals.
Evaporative cooling:
is necessary for electric generation since steam cycle
requires condensation which in turn requires cooling water loop at all power
plants except geothermal, wind and solar.
Evaporation ponds:
So called evaporation ponds were often unlined in the
past, thus much waste water infiltrated causing groundwater contamination:
Example:
Mission Olive Ponds.
Summary:
Water makes all life on earth possible both by its
presence and by its many unique properties.
All water in earth is involved in different portions
and moving at different rates in the hydrologic cycle
Lecture 3.
Condensation, Precipitation &
Interception.
Its out there
Only .001% of all water on earth is in the atmosphere,
but water vapor and water in clouds have a huge effect on climate.
Condensed water often precipitates in many forms and
has many impacts after it falls to ground.
Condensation:
Condensation is the conversion of a vapor (a gaseous
phase) into a liquid (possibly subsequently into a solid).
Condensation Process.
The
Process:
In condensation the heat
energy that was absorbed during evaporation is released. The condensation
process therefore releases heat.
The heat released during condensation can cause
violent convective storms (thunderstorms, hurricanes) to form particularly when
condensation occurs rapidly. The process of evaporation and condensation drives
much of the weather on the planet.
For water vapor to condense two factors
must be present
- 100% relative humidity
and
- A surface upon which the water can condense.
Condensation nuclei:
Water condenses as tiny droplets averaging .0004 in
diameter. Theses droplets are formed around far smaller particles
called condensation nuclei.
Condensation nuclei are tiny particles of sea salt,
volcanic gases, air pollution or dust.
Sources of nuclei:
Volcanoes can affect climate via this mechanism.
Also dust storms may affect climate by a similar
mechanism
The dew point:
The temperature at which (given the prevailing
humidity) water vapor present in the atmosphere can condense.
Besides sufficient water vapor and sufficiently low
temperature, most condensation in the
atmosphere requires condensation nuclei.
From dust storms to rain storms...
Dust storms may affect climate by creating
condensation nuclei. Thus droughts that generates dust storms such as those of the dust bowl may sow the seeds of
their own demise.
Formation of clouds:
Viewed close-up condensed water is either invisible or
forms mist or fog. When trillions of condensed water droplets are present light
is diffused and refracted and hence clouds appear white or with a heavy burden of water they actually
block light, appearing dark.
Uplifting experience:
The water droplets held in clouds would fall to earth
except for winds and convection (heating) of the clouds caused by the
condensation process which causes the air and the suspended water droplets in
the clouds to rise.
Fog drip:
In arid areas near cold
coastal currents: Namibia, Chile, California (Baja and southern) condensation
of fog can account from 30% to 100% of input of water into the ecosystem.
Plants such as the Welwitchia, beetles and even eucalyptus need this input to
survive. During 9 year drought in the Santa Barbara area, cool foggy summers
prevented a far worse disaster.
Precipitation
The process whereby condensed liquids or solids
suspended in the atmosphere fall (precipitate) usually striking the surface.
Critical size:
The droplets
suspended in clouds fall to earth when they have reached such a size that their
mass exceeds the ability of wind or rising air to keep them aloft.
Typically they fall as .004 inch droplets (100 times
the typical size of water droplets suspended in clouds).
Replenishment:
Water can fall out of a
single cloud for only a short period. Replenishment of the moisture is required
for prolonged intense precipitation.
This requires a source
of water vapor and intense solar energy to evaporate it and hence more intense
storms are likely to be near oceans and in tropical or sub-tropical areas.
The intense storms in
the equatorial areas such as the Congo or Costa Rica.
Forms
of Liquid Precipitation:
Drizzle A fine mist less than .1 inch per hour.
Rain: droplets larger than drizzle (.02-.2 inch in
diameter).
Vega: is rain that evaporates after falling from a
cloud but before hitting the ground.
Intensity
Rain can be light .1 inch per hour or less, moderate
.1-.3 inches per hour and heavy .3 or more inches per hour. Torrential rain
exceeds 1 inch per hour (4 inch per hour in S.B. probably more in Costa Rica).
Oddities:
Vega: is rain that evaporates after falling from a
cloud but before hitting the ground.
Raining fish or frogs
possible due to water spouts.
Watermelon snow
photosynthetic bacteria in melting
snow.
Forms of solid precipitation:
Sleet: is formed by
small pellets of ice this caused by rain drops freezing as they fall.
Snow: is precipitation
of ice crystals and requires colder temperatures than sleet.
Hail: is ice that is associated with
thunderstorms. The large size of hail is due to a process of uplift and
aggregation of falling rain that freezes in the clouds and is repeatedly
uplifted (Human Hailstones??!).
ICE STORM:
In Jan. 1997 here, a warm moist front from the Gulf
slid over much colder surface air fell
as rain that froze on the ground. In Dallas it fell as sleet In the Woodlands
and east, it was worse, but in Galveston
there was no problem.
Hail:
Ice that is associated with thunderstorms. The large
size of hail is due to a process of uplift and aggregation of falling rain that
freezes in the clouds and is repeatedly uplifted adding new coats.
(Human
Hailstones??!).
Areas with strong thunder-storms like N. Texas have
heavy hail (2 lbs. or more)...
Snow:
Snow: Falling ice
crystals, each is unique. There is a whole field of snow hydrology, mostly
concerned with estimating moisture content.
10 inches of snow
correspond to about1 inch of liquid water. ,The dryer and colder, the dryer
the snow. Powder common in Utah and Nevada, 81 feet of snow one year in
Tahoe...
Sublimation:
Very little fallen snow
evaporates although a substantial portion can sublimate. When the snow melts
gradually much infiltrates into the soil recharging shallow groundwater and
maintaining flow in streams.
However, unusually warm weather, especially
if accompanied by rain, can cause rapid melting and flooding.
Albedo:
Snow has a very high
albedo and hence clean snow reflects light cooling the environment, however
dirty snow absorbs light and melts faster.
Snow can have a wide
range of water equivalents. Fresh now averages about 10% water equivalent. In
dry areas snow has a lower water equivalent so snow in Utah or Nevada is more A powdery than snow in Ohio. Snow may be compacted over
time increasing its water equivalent.
Implications of snow behavior:
Since many areas in the west depend on spring run-off
from melting snow to water crops, generate hydroelectricity and support urban
demand estimation of available water supply is important.
Weather processes:
There are three process that cause condensation and
induce precipitation:
Cyclonic
Convective
Orographic.
All depend on the cooling of moist air as it rises but
the process that cause this are different.:
Cyclonic:
Can be due to warm air
sliding over cold air or cold air sliding under warm air.
Examples: In an ice
storm (like those in Huntsville in 84 & 98, warm moist air from the
Gulf slides over much colder air at the
surface and falls as rain that froze when on ground.
Farther North it falls
as sleet or even snow and causes few problems.
Farther south no
problems only rain.
Hurricanes:
Over tropical oceans this process can produce huge
convective storms called hurricanes, cyclones or typhoons depending on what
ocean is involved.
A hurricane by any other name...
Over tropical oceans this process can produce huge
convective storms called hurricanes, cyclones or typhoons depending on what
ocean is involved. Because more heat and more moisture is involved the power of
these storms in proportionately greater.
Convection:
The process of heat
transfer. Convection begins when the sun heats moist air causing it to rise. As
it rises, condensation takes place releasing heat energy this drives the
process of further heating and up-lift and condensation. This heat engine
runs as long as heat from the sun is present and a source of moist air is
available.
Hence thunderstorms are
likely in the afternoon but less likely in the early morning. (Tornado watch in
Amarillo).
Lightning:
The process of ice
falling through a cloud causes a separation of charge between the top and
bottom of the cloud and this potential energy difference is released in the
form of a stoke of lighting.
The sound produced by
the rapids expansion of air induced by the heating of the air as the lighting
passes through it causes thunder. Frequently thunderstorms are accompanied by
hail.
Wotans hammer...
Ice falling through a
cloud causes a separation of charge (voltage) between the top and bottom of the
cloud and between the cloud and ground, lighting restores this voltage
difference.
Thunder is the sound
produced by the rapid expansion of air induced by the heating due to the 40,000
degree temp. of lighting.
Convection & Tornadoes.
Convective storms derive their energy from the release
of heat as a result of condensation.
This energy can have devastating impact as anyone from
Jarrel, Waco, Wichita Falls or from any of the hundreds of communities that
have experienced powerful tornadoes can attest.
Orographic precipitation:
Due to the presence of
mountains.
Mountains force moisture laded air masses to
rise, causing condensation and precipitation. The wettest places on earth are
on the windward side. Mountains produce the driest areas as well on leeward
side, an area called the rain shadow and the greatest contrasts in
precipitation over short distances.
Examples include:
Cascades. Hawaii and the Mt Whitney/ Death Valley area.
Orographic precipitation:
Mountains force moisture-laded air masses to rise,
causing condensation and precipitation. The wettest places on earth are on the
windward side of mountains. Mountains produce the driest areas as well on their
leeward side (the rain shadow) and the greatest contrasts in precipitation over
short distances.
The grass is greener on the other side of
the mountain...
Examples: Cascades,Olympic pen/Yakima.
Hawaii Kona/drylands.
Mt. Whitney/ Death Valley area.
The higher the mountains and the stronger the
prevailing storm patterns, the stronger will be the rain-shadow effect.
Local color:
Local factors
such as mountains (orography), jungles (rain forest), large lakes (lake effect snows) and movement of warm and cold
air masses (fronts) driven by the jet stream cause local variations in
precipitation.
Its wetter, if not better, in the
tropics...
Water can fall out of a single cloud for only a short
period. Replenishment of the moisture is required for prolonged, intense
precipitation. This requires a source of water vapor and intense solar energy
to evaporate it. Intense storms are likely to be near oceans and in tropical
areas such as the Congo or Costa Rica.
Double whammy:
Intense weather if two
or more factors exist.
Cheripungi, India:
strong orography, convective and cyclonic.
Gulf coast: strong
convective and cyclonic.
Tornado alley: very
strong cyclonic and moderate convective (hail & tornados).
Olympic peninsula, Kona
Coast: cyclonic and orographic.
Desert Mountains: Strong
orographic and convective flash floods.
Drip, Drip, Drip...
Fog drip: in arid areas
near cold coastal currents: Namibia, Chile, California: condensation of fog can
account from 30% to 100% of input of water. Welwitchia, beetles and even
eucalyptus need this input.
During 9-year drought in Southern California,
cool foggy summers prevented a far worse disaster.
Wettest & Driest...
Cherrapunji in Assam
(India): strong orography, convective and cyclonic factors.
Kona Coast & Mt.
Waialeale on Kauai also.
Atacama Desert in Chile,
no measurable rainfall in several hundred years: Strong rain shadow (Andes,
strong negative effect), cold coastal current (Humbolt, no convection),
Mid-latitude = no cyclonic activity.
Namib also...
Interception:
The process
whereby vegetation (generally the forest canopy) intercepts rain or snow before
it reaches the ground. This moisture then evaporates from the leaves.
Interception can account for a loss of 10-20% of all precipitation.
Benefits & detriments of
interception:
Interception reduces available water but it also
greatly reduces the impact of raindrops on bare soil which would cause both
erosion and much more rapid run-off.
This more rapid run-off from denuded areas in turn
causes flooding.
Example:
Burned chaparral shows
impact of vegetation loss on flooding...
After wildfires, floods
and debris flows can occur in streams and rivers, in southern California in
particular.
If a heavy storm event
occurs within a couple of years of a wild fire a flood is a good possibility.To
reduce this risk, reseeding, and catchment basins are used. Historical studies
by Keller indicate that there is a strong relationship between debris flows and
fires.
Precipitation Patterns
Regional & Global patterns are due to several
influences:
Latitude
Global circulation patterns
Proximity to oceans
Orographic effects
Seasonal Variations: many areas have strong seasonal
variations in rainfall.
Seasonality:
Changes in wind patterns
can cause a strong rainy season such as in India and southeast Asia and
northern Australia where summer monsoons are critical to the survival of more
than 1 billion people.
Weaker seasonal patterns
occur elsewhere, such as winter rains and snow in the far west and summer
thunderstorms in the Midwest.
Other areas of the U.S. have a more uniform
distribution of rainfall.
Long Term Variation in Rainfall.
In many areas (particularly in the western U.S.) a
series of wet years alternate with a series of dry years but the lengths of
these series can vary usually not exceeding 5 in a row.
California has one of the most variable and intensely
studied long term precipitation patterns.
Stability vs. vulnerability:
However its
very dependability in these areas makes these areas vulnerable, due to lack of
alternative sources and storage capacity.
Weather Modification:
Silver Iodine has been used to create nucleation
centers to induce condensation and formation of water droplets that will
fall-out down wind as rain, Method is often called cloud seeding..
How Used:
Silver iodide smoke
released from planes, towers or rockets is used in the western U.S. to increase
local precipitation in the watersheds of power dams and areas dependent of
precipitation for water supply.
Radar and radiosondes
are used to determine appropriateness of a storm system.
Where it Can Work.
Silver iodide only works to wring moisture out of
clouds with sufficient moisture and low (but not frigid) temperatures.
Possible 5%-10% increase on local precipitation.
Examples:
Project Skywater.
PGE.
Cuba.
Santa Barbara
County.
Advantages and Disadvantages of Cloud
Seeding:
Can increase
precipitation with proper planning and help generate additional hydroelectric
power or more water in a water supply reservoir.
May decease
precipitation from warm storms
May be robbing Peter to
pay Paul.
Could cause local
flooding.
The sad story of the
Harris Brothers....
HYDROLOGY LEC #4 Infiltration, Soil
Moisture, Recharge & Transpiration.
Soil: A critical part of hydrology.
Soil properties and processes control things like
run-off of rainfall and hence flooding
Also recharge of groundwater and in many cases its
flow as well
Soil can be eroded by water and soil moisture supports
all terrestrial plants, so hydrology is critical to soil science and agronomy
as well
Composition of soils:
Soils are composed of various layers with differing
properties. However in hydrology we are interested in the composition of soil
with respect to the behavior of water in it.
Horizons:
Most soils have horizons (layers) with different
properties with depth, generally less organic matter and more consolidation at
depth.
Many soils have zones of elluviation (leaching) and
zones of accumulation due to water movement through soil.
Layers upon layers:
These layers are the unsaturated or vadose
zone, the capillary fridge and the groundwater zone,
the saturated zone is composed of these last two zones. See diagram...
Mineralogical properties:
Clays have
huge impact due to mineralogy. Expansive (fat clays) i.e. montmorillinite, expand to 8
times original volume when wet.
Fat clays cause huge
problems (second most costly natural hazard in U.S.).
However, lean clays (kaolin) do not
expand.
Lateritic soils: Iron and aluminum oxides. Found in tropics
have poor fertility, linked to destruction caused by loss of the rain forest.
Dirt is not simple:
Soils are principally composed of minerals that resist
weathering, these minerals include silicon dioxide, and aluminum and iron
oxides. These minerals carry a net negative charge. Hence soils have the
ability to attract positive ions (cations) like Ca, Na, Mg , K, SO4,
PO4, NO3 Etc.
Eye on Cations:
These cations are essential plant nutrients. In
excess they can cause water quality problems.
The tendency of a soil to attract these cations is
measured by its cation exchange capacity.
Example:
Goleta Water District waste-water reuse project.
Gypsum (calcium sulfate) can lower the pH of soil and
replace sodium with calcium causing flocculation of the clays and aggregation
of the soil.
Texture:
Can be sandy, silty or
clayey. Depends on size of soil particles: Sand is more than 2mm, silt 2mm-.002
mm and clay less than .002 mm.
Clay or organic matter
has most important effect on soil. Too little/too much clay produces poor tilth,
ideal soil has peds: clumps of larger particles held together by some
clay.
A mixed soil like this is called a loam.
Porosity:
Ranges from 10% to 40%. It is the percentage of holes.
If soil has too low a porosity it cannot hold water or provide air to plant
roots.
Clays have higher porosity (but smaller pores).
Ideally, half of pores should hold air while half
should hold water.
Examples of porosity:
Porosity of earth materials varies: clays can have 60%
porosities , sands typically have 40% porosity, compacted soils with an equal proportion of sand, silt
and clay can have as little as 25% pore space.
Types of porosity:
Rocks can have primary porosity and secondary
porosity consolidated rocks have less than 2% primary porosity, however if fractured they can have
higher porosities but generally less than soils.
What is in them pores
Pore space can be occupied by air or water. A typical
agricultural soil will have 40% porosity with 20% air and 20% water. This is
ideal for growing plants.
Why worry about porosity:
Measuring porosity is important to estimating the
volume of water in a soil.
Soil water
(also called soil moisture) is water that
enters the soil through infiltration and either remains in the unsaturated
(vadose) zone or percolates down to recharge groundwater.
Importance:
Soil water is important because it helps to hold soils
together, supplies water to plants, supplies dissolved nutrients to plants and
is the source from which groundwater aquifers are recharged.
Infiltration:
Infiltration occurs when water on the surface of a soil
becomes deep enough to overcome surface tension and begin to move into the soil
under the force of gravity and or soil suction.
In a soil that is not saturated, the soil particles
exert an attractive force on water called soil suction.
Down the Worm hole:
If large gaps
exist such as worm holes or cracks in the soil the water will rapidly
infiltrate.
However if the soil is bare, then the impact of rain
drops may cause small particles to block these pores and slow infiltration.
Hence, more of the rainfall will run-off often leading to erosion and flooding.
Permeability:
Is the ability of water to enter and drain through
soil:
Generally low for clays. Poor drainage can cause
flooding and starve plant root for oxygen.
Sandy soils can have too high a permeability so water
is lost easily.
Impermeability:
Soils can become impermeable due to compaction, but
also due to chemical processes.
The molecular structure of clay minerals can be
altered so that the aggregation of clays that helps permeability is reversed
and the formerly flocculated clay minerals are dispersed.
Field Capacity:
The quantity of water that a soil can hold against the
force of gravity is the field capacity of the soil.
Specific retention
Is