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Earth 123 Final Exam Review

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Department
Earth Sciences
Course
EARTH 123
Professor
Richard Amos
Semester
Fall

Description
Introduction to Hydrology Memorize: change in S= P + Gin + Qsw – (Gout + E + Qsw out) Hydrology: the occurrence, circulation and distribution of water of the Earth and its atmosphere Levee forms when… 1. During the dry summer months there is a reduction in the levels of precipitation, so the river level drops 2. There is then a decrease in velocity results in materials falling to the bottom of the river bed, due to the inability of the river to transport it 3. During the winter, the water level rises due to an increase in precipitation, this lifts the previously deposited material from the bed and carries it 4. As the river spills out over the floodplain, and deposits the sediment and eventually the river level drops back down 5. Repeated events forms large layers of sediment called levees and provide protection from floods Effects of levee construction – flood protection, accumulation of sediment in river channel, sediment supply to flood plain will be stopped, river level increase so ground level remains at the same elevation Floodwalls: concrete and steel vertical barriers Surge barriers: gate and floodwalls to prevent a flood surge in a populated area Diavik Diamond Mine -> kimberlite pipes were discovered in ’94-95, significant diamond recovery in 1997, diamond production begins in 2003, produce 9 million carats annually; the Northwest Territories produced 13% of the world’s diamonds in 2008, GDP tripled from 1991 to $4.5B in 2007 and unemployment dropped from 13.7 to 5.4% -significant challenges of the mine include its remote location, Kimberlite is under Lac des Gras and its climate is challenging to work in Dyke: a long mass of igneous rock that cuts across the structure of adjacent rock or an embankment of earth and rock built to prevent floods Waste rock disposal – sulphide mineral oxidation creates the potential for acid mine drainage -pit dewatering is done to dry an area for excavation of minerals Global and Regional Hydrology Hydrology & human activities -> drinking water, agriculture, industrial, floods, habitats, water quality, fisheries, navigation, recreation -oceans lose more water by evaporation than they gain by precipitation (opposite on land) but excess water on land returns to the oceans -oceanic fluxes are 79% of global precipitation and 88% of global evapotranspiration Great Lakes Basin – largest freshwater system on Earth, 23,000km of water, less than 1% of volume drains per year Grand River Basin – drains an area of 7,000km , 300km in length, average flow of 55m /sec 3 -Kitchener/Waterloo depends almost entirely on groundwater Laurel Creek Watershed – 75km , 0.2m /sec, urban and rural land use, highly complex geology, hydrological and ecological studies Precipitation Precipitation: any form of moisture originating in the atmosphere that is transferred to the Earth’s surface and exists as water in a condensed form -all water moving in the land-bound portion of the hydrological cycle is derived from precipitation -types of precipitation are based on its physical form or genesis (formation origin); liquid (rainfall, drizzle, dew) or solid (snow, hail, frost) Genesis 1. Convergence – movements along warm or cold fronts 2. Orographic rainfall – uplifting of air masses along mountain ranges 3. Convectional rainfall – air masses rise due to heating of the ground surface during the day Atmospheric moisture – water contained in the atmosphere in vapour, liquid or solid form -evaporation from the oceans is nearly 90%, the rest is from freshwater bodies and zones of lush vegetation -movement of air masses are due to non-uniform heating of the Earth and rotation of the planet from west to east forming a westerly wind system Atmospheric Circulation Cells 1. Tropical cells – warm equator air rises and moves poleward, it cools and descends to a return flow South and continued flow North (30N) (Hadley cell) – cause of low-latitude deserts (Sahara) 2. Middle cell – circulates air between the tropical and polar cells (Ferrell cell) – rising moist air 3. Polar cells – air rises at 60N and flows towards the pole, it cools and flows back near the ground surface Cloud Formation 1. Evaporation or transpiration releases water into the air 2. The warm, moist air rises up through the atmosphere and begins to cool adiabatically (without loss of heat) Adiabatic lapse rate – decrease in temperature with elevation in the atmosphere: -1C/100m for dry air, -0.5C/100m for wet air (lower because of energy released from changing phase from vapor to liquid) and the environmental lapse rate is -0.65C/100m PV=nRT (as P decreases with elevation, V increases and T decreases due to decreased kinetic energy from the volume increase) -once rising air mass intersects with the environmental lapse rate they are at the same buoyancy 3. The warm moist air is not at the saturation point in the warm, lower regions of the atmosphere but as it rises and cools it approaches saturation (dew point) RH=vapor pressure of water vapor/saturated vapor pressure (function of temperature) 4. At saturation water will begin to condense (condensation nucleus or seed to form around, can be dust or salt) 5. Small droplets increase in size as further condensation and collisions occur in the turbulent air 6. The small droplets are soon large enough to fall due to gravity, however they can evaporate and break up as they fall (drops are usually 0.1-3mm) Snowfall Precipitation -high in the atmosphere where temperature are below freezing and water droplets exist as crystals -formation of ice crystals requires ice nuclei which are less common than condensation nuclei -once the ice crystals begin to form it will grow by condensation faster than water droplets that can exist in temperatures of -35C Bergeron-Findeisen Process – precipitation may form in a mixed cloud (both ice crystals and liquid water drops); based on the saturation vapor pressure with respect to ice is lower than supercooled water at the same temperature; if both types of particles are present and total water content is high enough, the ice crystals would grow by sublimation at the expense of the liquid drops who will lose mass by evaporation; ice crystals will fall as snow or if large enough as hail or sleet -if the air temperature below the cloud remains below freezing, the precipitation will fall as snow, otherwise it will fall as rain Precipitation Processes 1. Frontal Convergence Fronts: zones separating large air masses with significantly different physical properties -when a cold, dense front moves past a warm front, the warm moist air is forced upwards, resulting in large scale uplifting; cold fronts are steeper and produce more intense rain -warm front will rise over the cooler air, providing lift for cloud formation and precipitation; the slope isn’t as steep and would produce less intense rain over longer periods -predominant precipitation and thermal transfer mechanism in mid-latitudes (30 to 60) 2. Convection -uneven heating of the ground surface will cause an air mass to become more buoyant than the surrounding air and it will rise -as the air rises it cools and condensation begins -the upward movement draws more air in below to feed the cloud -very high volume and short duration thunder storm events often accompanied by lightning, thunder and hail, tend to be local and randomly occurring events -most active in summer months due to the need for strong heating; common in Prairie regions on hot, humid days 3. Orographic Lifting -warm, moist air is forced upwards along a topographic barrier such as a mountain range -lateral winds push the air mass up the mountains and with increasing elevation, cooling begins -tend to be gentle precipitation events lasting a considerable time -downwind of the mountain range (leeward side) the air is dry and precipitation is low (rain shadow) Variation in Precipitation -distribution of precipitation varies in both space and time due to circulation patterns in the atmosphere and local factors A. Variability over the land surface controlled by… -regional and local topography -distance from a moisture surface -global and local atmospheric circulation patterns B. Annual variation -temperature (amount of precipitation) -prevailing wind directions C. Variation during a single rainfall event -local availability of moist air -duration and interaction of lifting mechanisms Main Characteristics of Snow -generally snowfall makes up a small portion of the total annual precipitation ex. Waterloo has annual rain of 765cm and snow only 160cm A. Hydrologist is primarily interested in… -lateral distribution of the snow cover -amount accumulated before melting occurs -how quickly the melting occurs -not as interested in the duration and intensity of snowfall (unlike rainfall) B. The occurrence of snowfall is directly related to the existence of temperatures below freezing (latitude and altitude) C. Snowfall events tend to be more uniform than rainfall events D. Snowfall remains in solid form until melting occurs and is therefore not participating in the hydrological cycle immediately E. Measurement of precipitation in the form of snow is difficult because it is hard to capture a representative sample in a gauge due to redistribution on ground surface and variation in density Factors Influencing Snow Accumulation 1. Mountainous terrains - high percentage of precipitation is snow but the accumulation is highly variable 2. Low topography or prairie terrains -accumulation is uniform but local variations in topography and vegetative cover can have a significant effect on distribution ex. snow fences and buildings can cause drift formation -eroded gullies or coniferous forests can act as local traps where snow will accumulate -in deciduous forests where trees have no leaves in the winter, snow can appear to accumulate in a rather uniform pattern but snowfall measurements made there are often inaccurate Snow Density -density can vary considerably and is directly proportional to the water equivalent of a given thickness of snow Density = depth of water equivalent of snow pack/depth of snow cover -density can change as a result of: heat exchange with air and ground, wind, pressure of overlying snow, presence and movement of liquid water -new snow density is about 0.06 while wind packed snow can be 0.3 Characteristics of Precipitation Data -widely distributed (limited data points) -variable quality -variable representativeness -interpretation of data can differ Uses of Rainfall Data -moisture available for agriculture activities -total rainfall in drainage basin for design of control structures like dams, reservoirs, flood control -water balance calculations for stream flow, recharge to groundwater Techniques of Measuring Rainfall 1. Non-recording Gauges -determines total rainfall over a certain time period which may range between a day and several months -about 3.5-5 inches in diameter and a foot high -contains a collection funnel that forces the precipitation into a collecting bottle -the bottle is periodically emptied into a graduated cylinder to determine the amount of rainfall over a given amount of time 2. Recording Gauges Tipping-bucket gauge – 2 balanced buckets tip back and forth as they are consecutively filled by the same amount of rain; each time the bucket tips a mark is made on a moving chart so that the record consists of a number of steps each representing a given bucket volume; volumes less than the total in the bucket are not recorded since tipping does not occur Optical disdrometer – video or laser sensor, characterizes raindrops Problems and Limitations of Rain Gauges -mechanical failure and observed error -wind effects (turbulence, local variations and wind speed) -orientation of the gauge -evaporation from gauges (can use a funnel bottle to reduce vapor flow, thin film of oil to prevent water exposure and using highly reflective material) -gauge placement – gauge should be in the horizontal place, sheltered from the wind and separated from nearby objects by at least 4x the height of the object and can expect 5-15% error at best Field Measurements of Snowfall 1. Snow gauge – 5 inch diameter, hollow metal cylinder open at one end, held 2m above the snow surface; often shielded to minimize wind, snow is captured and melted to measure its water equivalent 2. Recording snow gauge – must be emptied regularly, tipping buckets must be heated and may be supplied with antifreeze 3. Snow depth measurements – using a metered stick, the depth of the snow pack can be measured and converted to a water equivalent by choosing an average density (often 0.1); a network of graduated stakes can permit remote measurement with a telescope from a forest fire tower or helicopter 4. Water equivalent – cores of the snow pack are taken, depth measured and the core is weighed; an equivalent depth of water can be calculated and used to estimate water available for run-off; most accurate but also the most labour intensive National rain gauge networks – broad scale, used for design and planning on a regional scale, location depends on access, observer availability and economic factors Mountain networks – wide variety of physical settings on the mountain, often access is limited, requires careful design Watershed networks – gauge density will depend on the nature of rainfall, topography and required accuracy (long duration and large area precipitation is less variable); very high density of gauges is required 1/3 square mile for a hyetograph; for mean daily areal rainfall a much lower density would be required; designed to provide data for evaluating the water balance in a drainage basin Analysis of Precipitation Data Analyzed in terms of – frequency of event repetition, extreme ends of hydrological events (max and min), regression, physical relationships Analysis techniques involve – interpolating regional data to estimate precipitation at non-monitored site, adjusting long-term data, estimation of average precipitation over an area, observation of the frequency of precipitation, variation in areal distribution of precipitation Interpolation to Estimate Missing Data Method 1 – if a gauge in a network fails, get data from nearby gauges are available for the specific event and historical data from all gauges are available the missing data can be interpolated by P4=(N 43)[P 1N +1P /N2+ 2 /N 3 w3ere P are recorded data and N are long-term normal Method 2 – sketching a simple isohyetal map with precipitation data from neighbouring gauges Method 3 – double mass analysis – can plot data on a graph and adjust any inconsistencies K=adjustment factor=slope after change/slope before change Estimation of Average Precipitation Over an Area Method 1 – arithmetic mean – best for flat areas with relatively uniform precipitation (take the average of known precipitation values) Method 2 – thiessen polygen method –gauge points are connected and perpendicular bisectors are drawn, the area for each section is calculated and multiplied by the precipitation for that region Method 3 – isohyetal method – rain gauge data is contoured, area is evaluated as a percentage of total area and is assigned the average of confining isohyetals Evapotranspiration -processes by which water in liquid or solid phase at or near the Earth’s land surface becomes atmospheric water vapour -evaporation from rivers, lakes, bare soil; transpiration from plants; sublimation from ice and snow Evapotranspiration=evaporation from water, soil, vegetation + water used by plants during growth and in transpiration -the difference between precipitation and evapotranspiration is available for use and management Principal sources – oceans, terrestrial water, vegetative surfaces (interception), soil surfaces, snow cover (sublimation) -estimation of evaporative loss is critical for irrigation and reservoirs -the net exchange of water molecules at the surface will determine the rate of evaporation and rate of condensation -latent heat is 590cals/gm of water at 100C (required heat to change liquid water to vapour) Evaporation from a Free Surface -evaporating water body cools and overlying air becomes saturated -the rate and amount of evaporation depends on temperature and turbulence in the overlying air -for evaporation to proceed a source of energy and a vapour pressure deficit in the overlying air must be present If vapor pressure of surface > vapor pressure of air then evaporation occurs If vapor pressure of surface < vapor pressure of air then condensation occurs E=K E ae se a -evaporation from a surface body will continue until the air becomes saturated and the rate at which molecules are leaving and entering the water body are equal -best conditions for evaporation will be with warm (holds more moisture than cool air), dry air and strong winds to carry the evaporated water away and keep the vapor pressure deficit Factors Affecting Evaporation Meteorological Factors 1. Solar radiation – most important factor; governed by amount and intensity 2. Temperature – dictates the saturation vapor pressure and thus the gradient between water and air 3. Humidity – rate of evaporation is directly proportional to the difference between the actual humidity and the saturated humidity; the capacity of the air to hold the evaporated moisture decreases as temperature decreases -actual vapor pressure may remain constant but the relative humidity will change drastically as the temperature changes RH= actual vapor pressure/saturated vapor pressure 4. Wind – air turbulence will mix the moist air close to the surface water body with the drier, upper layers -evaporation rate is directly related to the wind speed, where wind is the mechanism for removing evaporated water -however if the wind removes the water vapor at a rate higher than the max evaporation rate, there is no increase in evaporation The evaporation decreased in China due to -> increased temperature and decreased radiation, wind speed and vapor deficit Geographical Factors 1. Water quality – evaporation decreases proportionally to increasing salt content because the dissolved ions in water tend to decrease vapor pressure at a given temperature -water clarity may affect the amount of solar radiation that can penetrate the water (albedo) and have a small, indirect effect 2. Water body depth – deep lakes take a long time to warm up and require much more solar energy for evaporation; the deep water body then slowly releases the stored energy in the cooler months and permits evaporation to occur in the absence of sufficient solar radiation – highest evaporation in winter 3. Size and shape of water surface – the rate of evaporation from a surface water body will decrease with increasing size -air moving across the water surface will pick up moisture due to evaporation; the air will have a low water content at the wind-ward side but will become wetter as it proceeds across the lake (evaporation rate will decrease) -if relative humidity of incoming air is high, the size difference effect on evaporation rate will be low (air is already moist), this effect can be significant in evaporation pan studies -amount of evaporation from a lake depends on the relationship between the prevailing wind direction and the orientation of the long axis of the lake -air moving parallel to the long axis will pick up less overall moisture because of the lengthy contact time with the water, then will air moving along the short axis of the lake Evaporation from Soils -atmosphere controlled (radiation, wind, humidity) and a soil controlled stage -evaporation is dependent on the availability of water and the rate at which water can be conducted to the surface 1. Soil moisture content – available water, as moisture content decreases so does evaporation rate 2. Water table depth – max evaporation rate will occur when the water table is at the ground surface; fine grained soils have a large capillary rise 3. Soil color – dark soils will absorb more solar radiation than light soils and will generally display high evaporation rates (more energy storage) 4. Vegetative cover – plants tend to shade ground surface, block the wind reducing its velocity in contact with soil or increase relative humidity through transpiration (evaporation rate decreases) Evaporation from snow -> low evaporation because it reflects solar energy, tends to melt when sufficient energy is available, at low temperatures the saturated vapor pressure is low and the gradient is small Methods of Estimating Evaporation -based on determining the vapor pressure gradient and the energy available 1. Energy Balance Approach – estimates the amount of incoming radiation that will be available for evaporation; radiation expended = incoming radiation – reflected radiation R-R ER A H +EH + A + HB C R=total incoming radiation R =direction reflection (albedo) E RA=return radiation as longwave H Eenergy available for evaporation H Aheating of overlying air H =heating of soil or water body B H Cplant growth -using a blackened thermocouple that can measure incident radiation coming from the atmosphere and the surface, the net radiation available for evaporation can be determined -estimates of H ,AH aBd H areCgenerally only 5-10% of total radiation 2. Water balance approach Evaporation = inflow – outflow – change in storage in a surface water body E=I+P-O-Og-change in S I=surface and groundwater input P=precipitation input O= surface outflow Og= groundwater outflow Change in S = change in water level -all measured errors in each component must be added together for the total error in the evaporation estimate -groundwater is the most difficult to estimate but is useful when -> surface inflow and outflow are low, measurements are made over a long period of time, lake bed has low permeability -if evaporation is only a small component of the water budget this method can lead to large errors 3. Empirical Methods -useful if one of the data required for the energy or water balance techniques are available; based on the vapor pressure gradient E=Kf(u)*(e -o )a= constant*wind speed function*(saturated – actual vapor pressure) -only standard meteorological data are required 4. Use of evaporation pans -widespread method of directly determining evaporation -evaporation losses from a pan can differ greatly from lake Surface Pan -> 4 ft diameter, 10 inch depth, placed 6 inches above ground; pan can conduct heat in and out of the water body; water temperature will fluctuate closely with air; have larger evaporation rates; water is added regularly; air temperature, rainfall and wind velocity are commonly measured Sunken Pan -> heat losses and water leaks are hard to quantify; tend to fill with debris; variations in direct solar radiation are controlled Floating Pan -> most representative data for an open water body; water splashing is difficult to control; hard to maintain and observe Estimating evaporation from soils -> open cylinder with soil is placed into the ground, each day it is weighed and precipitation is measured; weight changes are directly related to evaporative losses and precipitation input (lysimeter) Transpiration -stomata on plant leaves open during daylight hours to permit free passage of gaseous by-products of photosynthesis and close at night -leaves absorb solar radiation and some is used to cause evaporation from the leaf surface through stomata -as evaporation proceeds, suction develops in the plants circulation and water is drawn in from the soil through roots -water used for cell growth is negligible compared to the amount transpired Climatic Influences on Transpiration 1. Solar radiation -controls the opening of stomata -max transpiration occurs during the day and in the summer and is at its min during the night and in the winter -during the day groundwater is consumed at a high rate by both evaporation and transpiration; usually exceeds the groundwater in-flow rate and the water table will drop; at night the groundwater levels recover -variations in the water table can be used as a direct measurement of ET 2. Relative Humidity -rate of transpiration off the plant is controlled by the vapor pressure gradient; the lower the relative humidity, the higher the rate of transpiration -temperature is proportional to transpiration rate since it directly affects relative humidity 3. Wind and Air Turbulence – wind will remove the transpiring vapor and maintain the vapor pressure gradient so that transpiration can continue Additional Factors Affecting Transpiration 1. Vegetational Factors a. Stomatal openings – length of time they stay open is directly related to transpiration rate; controlled by length of daylight, air temperature affecting opening and closing speed and high humidity permits longer and wider opening b. Vegetation type – very little control, nearly the same no matter what crop is planted Controlling factors Plant color – the lighter the leaf, the more solar energy is reflected Density – the thicker the vegetative cover, the more surface area however air circulation can decrease as vegetation density increases resulting in increasing humidity (decrease in vapor pressure gradient) Leaf dimensions – broad leaf plants provide greater evaporative surface area Stage of plant growth – younger plants transpire less (using energy for growth) Root type -> hydrophytes – roots in water itself, stomata lack guard cells -mesophytes – land plants -xerophytes – arid climates, shallow dense roots -phreatophytes – water thieves, roots in water table ex. salt cedar 2. Soil Factors a. Soil moisture content – availability of soil moisture to the root system will control the amount of transpiration that can occur; as soil moisture decreases, ET decreases b. Storage capacity – capacity to hold water in storage is a function of porosity; silt and clay will hold more water than sand c. Capillary tension – water is held in the unsaturated root zone under tension and soil structure and grain size will control the amount of tension that can develop (roots tension must overcome soil’s) d. Soil permeability – as soil moisture is removed from around the root system, it must be replenished by in-flowing groundwater; the higher the soil permeability, the faster the soil moisture will be replenished clay – high storage and porosity but low permeability sand – low storage and porosity but high permeability Total solar energy = evaporation of interception + transpiration + evaporation from soil + net loss ET during early growth – small plant surface (low transpiration), high surface soil area (high evaporation), shallow root system (poor access to soil moisture), no wind protection, vapor pressure gradient remains high Late growth – high leaf SA, soil is shaded (low evaporation), deep root zone, high protection from wind, transpiration is highest in summer but evaporation is its lowest -generally interested in the total moisture loss from the subsurface and we need to estimate total evapotranspiration -potential evapotranspiration represents what could occur under a given set of physical coditions if there was a continuous supply of water; the actual ET depends on water availability Estimation of Evapotranspiration 1. Energy Balance Techniques R T E SHE +PE + AH+ E R> toTal incoming solar radiation=energy used to heat soil + energy used in photosynthesis + energy used to heat air + reflected energy + evapotranspiration Enet = E AHET is used if can measure Enet, temperatures, vap
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