Card Set Information

2011-12-14 02:40:25

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  1. Groundwater
    • water under positive pressure in the saturated zone of earth materials
    • most water enters the groundwater reservoir when infiltrated water arrives at the water table as recharge

    • our drinking water
    • its almost everywhere
    • 0.6% of the 3% of water
    • more than lakes, seas, atmosphere, river
    • 2,000,000 cubic meters
    • lakes have 30,000 to put it into perspective

    • Groundwater is water stored in the saturated zone.
    • Groundwater completely fills all the soil or rock pores.
    • The top of this zone is called the water table.
    • Underground water stored in aquifers.
    • Groundwater is created by rain that soaks into the ground and flows down until it collects above an impervious zone
  2. Hydraulic Conductivity
    the rate of flow of water in gallons per day through a cross section of one square foot under a unit hydraulic gradient, at the prevailing temperature

    • the ease of the water moving throughout the material (K)
    • in rock, you have to account for the difficulty of the water to move throughout the pores
    • don’t need to do this for above ground water movement
    • how is the property of the rock itself helping or hurting the movement of the water?

    assumes that water is the fluid moving through a soil or rock type and represents the ease with which water can move through the medium
  3. Hydraulic Gradient
    fancy word for slope

    the decrease in the hydraulic head per unit distance in the direction of flow

    • Determined by hydraulic head:
    • Elevation above a standard reference (mean sea level) at which the water stands in a well at a specific point in aquifer – represents gravitational potential energy


    • Elevation of h2 is 1000
    • Elavation of h1 is 950
    • Space in-between is 1000
    • HG = ?
    • 1000-950/1000
    • HG = 5% or 0.05

    • Steep HG = faster movement
    • Flat HG = slower movement
  4. Aquifer
    A layer of consolidated or unconsolidated rock that is able to transmit and store enough water for extraction

    • Geologic formation that is porous and permeable
    • Lots of pores, water can move through it

    Store, transmit, and yield significant amounts of water to springs and wells

    • Saturated alluvium makes a good aquifer
    • Lavas make good aquifers
    • Igneous and metamorphic rocks make poor aquifers
  5. Permeability
    • the acceptance or resistance of a liquid into a solid
    • in our case, water into a rock
    • low permeability passes slowly (clay)
    • high permeability passes quickly (sand)
  6. Unconsolidated Sediments
    Loose granular deposit of natural earth materials in which particles are not cemented together

    • Soils, beach sands, sand and gravel in stream channels,
    • glacial deposits

    • When saturated, they can be aquifers
    • Alluvium is the most important worldwide
  7. Specific Yield
    • Ratio of aquifer volume holding gravity water to total volume
    • useable ground water

    volume of stored ground water released (taken up) per unit serface area per unit decline (increase) of water table.
  8. Karst
    • refers to a type of terrain, usually formed on carbonate rock (limestone and dolomite) where groundwater has solutionally-enlarged openings to form a subsurface drainage system
    • in karst aquifers, turbulently flowing underground streams have velocities approaching those of surface streams

    there are 5 types of sinkholes and karst is involved in 4 of them...
  9. Runoff
    • The movement of water to a channelized stream after it has reached the ground as P
    • Movement can be above or below the surface. So, this can include overland flow, shallow sub-surface flow, and groundwater flow.
    • Once the water reaches a channel, it is called streamflow, river flow, or discharge

    Water that is not absorbed by soil and drains off the land into bodies of water, either in surface or subsurface flows

    Contributors to runoff

    • Overland flow
    • Subsurface flow (throughflow, interflow) – through the unsaturated zone
    • Groundwater flow (through the saturated zone)

    Factors Affecting Runoff

    • Climate
    • Characteristics of the drainage basin
  10. Streamflow
    water moving through a channel
  11. Cross Section Area
    width of channel times average depth
  12. Antecedent Precipitation
    An index of how much moisture is stored within a drainage basin at the beginning of a new storm
  13. Stage
    fancy name for water depth (water surface elevation)

    whats being measured at gaging stations
  14. Manning's n
    way to calculate streamflow

    used to estimate V in channels without measurements

    Whats on top of the equation is making velocity faster

    Whats on bottom is making velocity slower
  15. How is groundwater different from soil water?
    GW is water under positive pressure in the saturated zone of earth materials and is used for our drinking water; this water is located under the water table

    SW is the unsaturated zone of the soil. this water is located above the water table and is only wet for a period of time
  16. Why is GW important?
    • It’s the water we drink
    • It grows the food we eat: groundwater is a major source of irrigation
    • It enables our recreation: groundwater contributes to the flow of our lakes and rivers.

    • According to 2005 United States Geological Survey (USGS) figures, groundwater provides an estimated
    • 22% of all freshwater withdrawals
    • 37% of agricultural use (mostly for irrigation)
    • 37% of the public water supply withdrawals
    • 51% of all drinking water for the total population
    • 99% of drinking water for the rural population
  17. What are recharge areas and what are discharge areas?
    Recharge (gain water)

    • Infiltration of precipitation through the soil
    • Streams flowing above the water table – losing or influent streams

    Discharge (lose water)

    • Evapotranspiration through plants
    • Surface seeps and springs
    • Streams flowing at or near the water table
    • Most streams in humid areas
    • Natural discharge from beds or banks into streams – gaining or effluent streams
  18. What factors affect recharge and discharge?
    • Climate
    • More infiltration in humid vs. arid regions
    • In US, about 10% of average annual P goes to GW
    • Geologic conditions
    • Recharge will occur where gravity can bring water down to the water table
    • Depth to water table is usually good indicator of how effective recharge is
    • Humid regions – water table is near surface
    • Arid regions – much deeper – up to 1000 ft.
  19. What are springs?
    • Opening in the ground with water flowing over the surface
    • Negligible amount

    • Types of springs
    • Contact
    • Fracture
    • Fault
    • Solution

    • Direct discharge in a stream bed or banks
    • Most natural discharge this way
  20. What are unconsolidated sediments?
    What is alluvium?
    Are unconsolidated sediments good aquifers?
    Why or why not?
    • Loose granular deposit of natural earth materials in which particles are not cemented together
    • Soils, beach sands, sand and gravel in stream channels,glacial deposits
    • When saturated, they can be aquifers
    • Alluvium is the most important worldwide
    • Alluvium is sediment that is deposited by running water; gravel, sand, silt, and clay
    • over time it will accumulate and get buried
    • with the different types of particle joining together, there is usually lots of pore space for storing water...making it a good aquifer
  21. What are consolidated sediments?
    Are consolidated sediments good aquifers
    Why or why not
    • Major landscape features sometimes over wide areas
    • Composition similar to unconsolidated sediments but materials now cemented together to form sedimentary rocks
    • Sand – sandstone
    • Silt – siltstone
    • Clay – shale

    • Sandstone is most important worldwide
    • 25% of all sedimentary rock
    • Porous and permeable uniform
    • Wide areal extent means large storage capacity
  22. What is limestone?
    Is limestone a good aquifer?
    Why or why not?
    • Sedimentary rock made up of calcium carbonate (CaCO3)
    • Formed by precipitation from natural waters or lime-secreting animals
    • Snail absorbs the calcium carbonate in the water Makes shell
    • Dies
    • Shell sinks to the bottom of ocean
    • Shell stays, snail dissolves
    • Shell gets formed together with another rock through pressure
    • When compacted they form dense hard rocks with low porosity
    • Sometimes pressure and heat change them to marble
    • So, why is limestone important for GW?
    • It is soluble in water
    • Dissolves the limestone, creating cracks and waterways for water to move through the material; doesn’t really move that much through the pores, just through the cracks
    • Good for water storage
    • Bad cause all containments go directly through as well, no seeping and filtering to make it…less bad…
    • Rain seeping through cracks carries CO2 which is slightly acidic and dissolves CaCO3
    • Over a long period, a lot gets dissolved and even caves can form, but most of the time just space to hold water is formed
  23. What is karst?
    How is it formed?
    Is karst a good source of groundwater?
    Why or why not?
    What are some problems associated with karst?
    • refers to a type of terrain, usually formed on carbonate rock (limestone and dolomite) where groundwater has solutionally-enlarged openings to form a subsurface drainage system
    • A mild carbonic acid produced from carbon dioxide in the atmosphere, particularly the soil atmosphere, is primarily responsible for the solvent power of groundwater on carbonate rocks.

    • in karst aquifers, turbulently flowing underground streams have velocities approaching those of surface streams
    • The nature of the groundwater flow system causes karst areas to be extremely vulnerable to groundwater contamination
    • Other serious hydrogeologic problems include sinkhole flooding and sinkhole collapse.
    • there are 5 types of sinkholes and karst is involved in 4 of them...
  24. How do sinkholes form?
    joint intersections

    in karst areas covered by soils or other unconsolidated materials

    sudden mass movements of karst bedrock due to sudden drops in the water table

    streams sinking through deposits of alluvium on the surface of the landscape into the underlying soluble karst bedrock
  25. What are crystalline rocks?
    Are they good aquifers?
    Why or why not?
    • Igneous and metamorphic rocks
    • Hard, dense rocks
    • Low porosity and permeability
    • Generally poor aquifers

    • GW is mainly found in fractures
    • Wells can be drilled into the fracture systems Sometimes the only game in town
  26. What are volcanic rocks?
    Are they good aquifers?
    Why or why not?
    • Lavas make good aquifers
    • They are widespread in the world
    • Have many (especially vertical) fractures and porous zones
  27. Compare unconfined and confined aquifers.

    Be able to draw a figure showing them.
    • Unconfined
    • Beneath a free water table
    • Water table is defined as the surface of groundwater at which water pressure is atmospheric
    • Unconfined has contact with atmosphere through pores


    • Beneath an impermeable formation that seals the aquifer from above
    • Confined is under pressure greater than the atmosphere
    • Has a water table defined by the level at which water stands in wells that penetrate the water body just far enough to hold standing water.


    • In wells tapping a confined aquifer, the water will rise in the well when it is first encountered during drilling and will stand at a level above the top of the aquifer.
    • Water in a confined aquifer is under pressure because aquifer in the recharge area is at a higher elevation than it is at the well location.
    • Artesian Wells are the result
    • No pumping needed
    • Keep draining water will result in less pressure

  28. What are specific yield and specific retention?
    How are they related to porosity?
    Which is more important in studying GW?
    • Ratio of aquifer volume holding capillary water to total volume is called specific retention
    • Ratio of aquifer volume holding gravity water to total volume is called specific yield
    • Specific retention + specific yield = porosity (total open space to total volume)

    GW hydrologists are interested mainly in specific yield not so much in specific retention
  29. Discuss storage in a confined aquifer.

    What happens in the aquifer when pressure changes are noted in a well?
    • Storage in confined aquifers
    • The aquifer responds to pressure changes by expanding or contracting slightly
    • This produces an increase or decrease in porosity Water also responds to pressure changes

    When water is removed

    • Well levels fall
    • Pressure in the aquifer falls
    • Water expands slightly
    • Aquifer contracts slightly

    When water is added

    • Well levels rise
    • Pressure in the aquifer rises
    • Water contracts slightly
    • Aquifer expands slightly

    Because the change in aquifer porosity and pressure is very small, storage coefficient is much smaller than for an unconfined aquifer

    Storage Coefficient (SC) is not determined by gravity

    Water is yielded by

    • Compaction of the aquifer as pressure is reduced
    • Expansion of the water that is pumped out
    • SC is proportional to porosity, thickness, and compressibility of the aquifer
  30. Describe groundwater movement in an aquifer.

    What factors affect groundwater movement?
    • GW moves slowly
    • Surface water measured in ft or m per second
    • GW fraction of an inch or a few mm to a few ft or m per day

    Factors affecting

    • Slope
    • Steep HG = faster movement
    • Flat HG = slower movement

    Properties of the media (rock type, porosity, permeability)

    Viscosity of the water
  31. What is the difference between permeability and porosity?

    • the acceptance or resistance of a liquid into a solid
    • in our case, water into a rock
    • low permeability passes slowly (clay)
    • high permeability passes quickly (sand)
    • this shows how much water can pass through a soil; not how much a soil can carry


    • the percentage of pore space in a soil
    • this shows how much water a soil can carry; not whether it can pass through easily or not
  32. What is Darcy’s Law?
    What are its components?
    What is it used for?
    describes the flow of a fluid through a porous medium

    Used to find

    • horizontal hydro gradiant between wells
    • velocity of water between wells

    • Q = KA [(hA - hB) / L]
    • or
    • Q = KA (dh/dl)

    • where
    • Q=volume of water flow in ft3/day
    • K=hydraulic conductivity in ft/day
    • A=cross-sectional area in ft2
    • dh/dl=hydraulic gradient
  33. Darcy's Law EX)
    Two wells are located 100 feet apart in a sand aquifer with a hydraulic conductivity of 0.04 feet per day. The head of well 1 is 96 feet and the head of well 2 is 99 feet.

    What is the horizontal hydraulic gradient between the wells?

    • [(head 2 - head 1) / L
    • [(99-96) / (100)] = 3/100 or 0.03

    What is the velocity of water between the two wells?

    • V= (K) (dh / dl)
    • V= (0.04 ft/d) (0.03)=0.0012 ft/d

    Velocity is a speed…so its distance divided by time…milesperhour
  34. Why do we need to study stream response to inputs of water?
    • Water supply
    • Flood prediction
    • Water quality
  35. What are the main types of overland flow?
    Explain each.
    Hortonian Overland Flow

    • overland flow that results from saturation from above; including that which occurs on impermeable surfaces
    • Horton himself postulated that overland flow due to saturation from above would occur from virtually an entire upland watershed

    Saturation Overland Flow

    overland flow that occurs due to saturation from below; it consists of direct water input to the saturated area plus the return flow
  36. How does climate affect runoff?
    What aspects of climate are important?
    • Overall moisture supply – total P
    • Form of moisture supply – Precipitation (P)
    • rain or snow
    • Day to day weather patterns determine P
    • Amount
    • Timing of delivery
    • Frequency
    • Duration
    • Intensity
    • Distribution
    • Dependability over time (droughts vs. wet periods)

    • A lot of P usually means a lot of runoff
    • “Floods are caused by too much rain.”

    Timing of P is important also

    • Steady rains
    • often Soil stays wet and water runs off easily

    • Rains after long periods of no rain
    • Soil and vegetation will absorb and hold more water

    • Antecedent precipitation
    • An index of how much moisture is stored within a drainage basin at the beginning of a new storm

    Temperature is also important

    Rain becomes snow

    • Snow falling on unfrozen ground will insulate and prevent freezing of the soil
    • When snowmelt begins, water will infiltrate slowing overland runoff

    Snow falling on frozen ground will run off more quickly

    Rain falling on snow in spring will often cause floods – sudden melting
  37. What physical (natural) characteristics of a basin affect runoff?
    Be able to explain them.
    • Elevation
    • Mainly related to temperature
    • Change from rain to snow
    • Reduced ET due to lack of vegetation
    • Most runoff in spring – runoff light in winter
    • Orientation of the basin
    • Affects snow accumulation and snowmelt
    • Affects timing of streamflow due to relation to storm tracks – P begins and ends at different times
    • Affects type and amounts of P due to relation to prevailing wind – orographic effects


    • Total relief of basin – steep means faster runoff and time to peak in streams
    • Steep means streams have more energy to do work and damage (erosion)


    • Affects ET, interception, and infiltration
    • Affects erosion and amount of runoff
    • Soil type

    • Affects infiltration and permeability
    • Geology

    • Tectonics created the basin and its topography
    • Rock type affects steepness and types of stream channels (drainage pattern) through control of erosion
    • Rock type affects storage time:
    • Porous and permeable rocks allow infiltration to GW – slowing runoff and feeding streams and springs over longer periods
    • Impermeable rocks force a lot of and fast runoff
    • Flashy vs. even flow regimes
  38. What kinds of human modifications of a basin can affect runoff?
    Be able to explain them.
    • Deforestation
    • Diversion of natural stream channels
    • Channelization
    • Dams
    • Urbanization – buildings, roads, parking lots, shopping centers; square mile after square mile of impermeable layers
    • Agriculture - changes soil conditions, vegetation, erosion; whatever isn’t urbanized, is agriculturized
  39. Compare and contrast runoff from an urban area and a natural forest.
    time inbetween peak rainfall and peak discharge will depend on elevation, topo, etc

    • urban is fast peak, quick drop off
    • forrest is slow peak, slow drop off
  40. What is a hydrograph?
    What does it indicate?
    Be able to drawn and label a basic hydrograph.
    • A continuous record of streamflow over some period of time
    • A hydrograph is a graph that shows changes in discharge of a river or stream at a given point over time. The time scale may be in minutes, hours, days, months, years or decades
    • A graph of river flow during a given time period, often seasonal or annual. A hydrograph typically shows mean (average) daily stream flows, in cubic feet per second

  41. How can discharge be measured in the field?

    What information do we need and how can we get it?

    For small streams, we can use

    • A measuring tape to find cross sectional area
    • A current meter to measure velocity
    • Measure the area of a cross section of flowing water along a line across the stream at right angles to the direction of the flow, then measure the velocity of the flowing water

    • For bigger rivers
    • Boats or bridges can be used
    • Cableways are built to take readings
  42. Why is velocity difficult to measure?
    What does velocity depend on?
    How does it vary?
    Why Difficult

    • It varies over time depending on rainfall and ground water recharge
    • It varies within the channel cross section so we need to find some kind of average
    • Water moves fastest in the middle of the river…less friction
    • Water moves slowest near the top, sides, bottom…more friction


    • Water moves slower at the surface than just below
    • Water moves much slower near the bottom and sides of the channel due to friction
    • Peak velocity is about 20% of the depth below the surface
  43. Explain what a stage-discharge curve is

    How it is constructed

    What it is used for
    What it be

    a regression line between stage and discharge

    How it be constructed

    • Numerous measurements are made of water level (height or stage) and velocity at the different water levels and discharges are calculated using the cross sectional area
    • Enough readings of water level and discharge at different flood stages are made to draw or calculate a rating curve of stage and discharge (also called a stage-discharge curve)

    What it be used for

    After making the curve, you can predict discharge from a stage by creating this regression relationship
  44. Width of channel = 250 ft
    Mean depth = 8 ft
    Distance between cross sections = 0.5 miles
    Elevation distance between cross sections = 26.4 ft
    Manning’s n = 0.075

    What is V? What is Q?
    • V = (1.49*R^0.66*S^0.5)
    • ---------------------------------
    • n

    • R = Area
    • --------
    • WP

    R = 250*8 = 2,000 for A

    = 250+8+8 = 266 for WP

    • = 2,000
    • ----------
    • 266

    = 7.5

    R = 7.5

    • S = 0.5 miles
    • -----------------
    • 26.4 ft

    • = 2640
    • ---------
    • 26.4

    = 100

    S = 100

    • V = (1.49*R^0.66*S^0.5)
    • ---------------------------------
    • n

    • 1.49*7.5^0.66*100^0.5
    • -------------------------------
    • 0.075

    • 1.49*3.8*10
    • ----------------
    • 0.075

    V= 7.5 ft/sec

    Q = VA

    • = 7.5*2000
    • = 15,000

    Q = 15,000 ft3/sec

    • V= 7.5 ft/sec
    • Q = 15,000 ft3/sec