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BSc Environmental Science > LEC.174 Hydrology > Flashcards

LEC.174 Hydrology Flashcards

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1
Q

Define hydrology

A

Science of distribution + movement of water

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2
Q

What % of global fatalities due to natural disasters are caused by flooding?

A

40%

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3
Q

How can selective logging disturb the environment?

A

Erosion + compaction affects drainage in slopes –> landslides –> sediment into nearby water sources which affects quality –> sediment reaches water treatment works so can damage HEP turbines –> sediment settles so channel fills –> flooding + affects shipping + kills coral

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4
Q

What is a catchment the same as?

A

Drainage basin, watershed (USA)

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5
Q

What is used to measure precipitation as a daily value?

A

Met Office Mark II rain gauge

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6
Q

Describe the features of a Met Office Mark II rain gauge which make it suitable for measuring rainfall and at what time every day is measurement taken?

A

Drop above funnel to prevent outsplash, above ground to reduce insplash, narrow neck reduces evaporation, always measured at 9am GMT

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7
Q

What is a problem with Met Office Mark II rain gauges and monthly storage rain gauges?

A

When there are floods, rain gauges aren’t accurate in measuring rain intensity

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8
Q

Describe how a siphoning-tank rain gauge works

A

Brass float moves up with increased precipitation, pen attached to brass float traces a hyetogram, siphon pulls all water out when full

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9
Q

Describe how a tipping-bucket rain gauge works (more common)

A

Bucket fills and tips so other bucket fills, reed-switch connected to a data logger

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10
Q

What is a problem with tipping-bucket rain gauges and how can this be fixed?

A

Tipping mechanism can freeze, can use a heater to insulate (more power) or a check gauge

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11
Q

What are 4 disruptions to rain gauges and how are these avoided?

A
  1. Shelter - angle between top of rain gauge and top of nearest tall object must be 30 degrees
  2. Turbulence (blows rain away from rain gauge) - Huddleston turf wall or pit with ‘anti-splash grid’ used
  3. Snowdrift (can bury rain gauge) - rain gauge set 1m above ground in UK or Nipher/Alter shields used
  4. Insects (block rain gauges)
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12
Q

What 2 things are used to calculate an areal estimation of rainfall?

A
  1. Catchment average
  2. Spatial patterns
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13
Q

What does using arithmetic mean when calculating catchment average of rainfall rely on?

A

Flat terrain and uniform distribution of rain gauges

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14
Q

How does a Thiessen polygon calculate catchment average of rainfall?

A

Works out what area of the catchment each rain gauge represents (midpoints found between rain gauges to create polygon)

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15
Q

How does a hypsometric curve calculate catchment average of rainfall?

A

One rainfall vs. altitude graph and one altitude vs. area < altitude graph combined

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16
Q

How does the isohyetal method calculate catchment average of rainfall?

A

Lines of equal rainfall through catchment (can be subjectively adjusted for underlying topography)

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17
Q

How does radar calculate catchment average of rainfall?

A

Reflectively calibrated against point gauge measurements

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18
Q

How does an evaporation pan measure open water evaporation (e.g. lakes)?

A

Measures how much water lost daily e.g. Symons tank

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19
Q

What is a disadvantage when using a Symons tank (evap. pan) to measure open water evaporation?

A

Has a smaller heat storage and higher advective transfers which creates artificially dry conditions above tin so “more evaporation”

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20
Q

How does the Penman method measure open water evaporation?

A

Measures controlling meteorological variables to estimate evaporation

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21
Q

What pieces of equipment are used to measure energy, wind speed, and saturation of the atmosphere relative to saturation pressure (max. holding capacity)?

A

Energy: Net radiometer
Wind speed: Anemometer
Saturation of atmos. relative to saturation pressure: Dry and wet bulb thermometers

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22
Q

What are the 2 controlling factors of wet canopy evaporation (interception loss)?

A
  1. Rainfall intensity
  2. Vegetation type (surface area) - use a canopy rain gauge (throughfall plot), stem flow collector, or a stem flow collar + throughfall trough
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23
Q

What are the 2 controls of transpiration?

A
  1. Stomatal loss
  2. Root absorption
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24
Q

What are the 5 ways of measuring transpiration?

A
  1. Porometer (leaf-scale)
  2. Sapflow sensors (tree-scale)
  3. Lysimeter water balance (few plants-scale)
  4. Long term catchment water balance (regional scale)
  5. Modified Penman method (above-canopy measurement)
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25
Q

Define storm hydrograph

A

A river’s response to precipitation, moderated by catchment characteristics (either flashy or damped)

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26
Q

What are 6 catchment characteristics which moderate a storm hydrograph?

A
  1. Shape (circular = flashy)
  2. Area
  3. Drainage density
  4. Catchment/channel slope
  5. Vegetation cover
  6. Infiltration
  7. Catchment storage (chalk aquifer = damped)
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27
Q

What are 6 ways of measuring river discharge?

A
  1. Volumetric gauging (volume over time)
  2. Float gauging (velocity of neutral buoyancy object x area, overestimate as surface is faster-flowing)
  3. Current metering (average velocity then average discharge over whole channel cross-section, channel divided into series at equally-spaced verticals, area between verticals = segment)
  4. Dilution gauging (constant injection method or integration method)
  5. Structural methods
  6. Slope area methods (measure slope on trashline which = slope on water surface during flood)
28
Q

What are 4 properties of a good water tracer?

A
  1. Chemically conservative
  2. Non-reactive
  3. High water solubility
  4. Relatively non-toxic
29
Q

What are 2 types of tracer that might be used in a larger river?

A
  1. Radioactive tracer
  2. Fluorescent tracer
30
Q

What are the 3 classifications of a stream?

A
  1. Perennial stream = flows all year
  2. Intermittent stream = seasonally dry
  3. Ephemeral stream = flows only during storms
31
Q

What is the difference between general and specific groundwater?

A

General = all water within ground (vadose (soil) and phreatic)
Specific = water in phreatic zone

32
Q

What are 2 reasons why groundwater is important?

A
  1. Contaminant transport
  2. Water supply
33
Q

What fraction of Lancaster’s water supply comes from groundwater in Millstone Grit?

A

1/3

34
Q

What is Representative Elemental Volume?

A

Pore volumes of water

35
Q

Define permeability and porosity

A

Permeability (Ks) = flow of water under unit area and unit hydraulic gradient
Porosity = volume of pores in a volume of soil/rock

36
Q

What are the 4 types of groundwater body and what are their differences?

A
  1. Aquifer - both transmits (permeable) + stores (porous) water
  2. Aquitard = transmits + stores only small quantities of water
  3. Aquiclude = stores but DOESN’T transmit water e.g. glacial till
  4. Aquifuge = neither transmits nor stores water e.g. granite
37
Q

What are the 2 types of aquifer and their differences?

A
  1. Confined aquifer = borehole level/piezometric surface higher than water table due to confining layer, can be Artesian (piezometric surface above ground)
  2. Unconfined aquifer = piezometric surface is equal to water table
38
Q

What is water flow in the saturated zone (below water table) proportional to?

A

Loss of energy/potential

39
Q

What are the 3 types of streamflow generation?

A
  1. Direct in-channel ppt.
  2. Infiltration-excess/Hortonian overland flow (when rainfall intensity > soil infiltration capacity)
  3. Subsurface pathways (transport rainfall in soil to rivers during storms)
40
Q

What are 3 reasons why Hortonian overland flow isn’t the dominant mechanism for routing ppt. to rivers?

A
  1. Rarely seen/measured in significant quantities
  2. Soil infiltration capacities are usually > rainfall intensities
  3. Conservative tracers are very variable in rainfall but damped in rivers, implying water enters soil + mixes with subsurface water in storage
41
Q

What 3 places is Hortonian overland flow localised to?

A
  1. Tree stem bases
  2. Compacted-surfaced roads
  3. Desert surfaces
42
Q

What are 3 water paths in riverside areas?

A
  1. Channel inflow
  2. Direct ppt. on saturated areas
  3. “Return flow”
    (2 and 3 = “saturation-excess” overland flow)
43
Q

What are 3 types of “return-flow”?

A
  1. Shallow water paths (any layers with soil/underlying deposits direct water laterally)
  2. Natural soil pipes
  3. Wave propagation
44
Q

What are the 2 controls of soil erosion?

A
  1. Erodibility (soil properties)
  2. Erosivity (kinetic energy of water flow)
45
Q

What are the 4 erosion locations on slopes?

A
  1. Rain-splash
  2. Surface “sheetflow”
  3. Subsurface
  4. Gullys
46
Q

How does rain-splash erode on a slope and what is evidence for this?

A

Water droplets dislodge particles if no vegetation (net loss if on slope)

Evidence: ‘soil pillars’, particles washed away by sheetflow, ‘capping’ of soil (rain drops orient particles on surface –> sealing –> more sheetflow)

47
Q

What are 2 methods of protecting slopes from rain-splash erosion?

A
  1. Mulch tillage
  2. Cover cropping
48
Q

How does surface “sheetflow” erode on a slope?

A

Forms tiny channels called micro-rills (more conc. erosion, wouldn’t have had same erosive power if water was in sheets)

49
Q

What are 3 methods of protecting slopes from surface “sheetflow” erosion?

A
  1. Increasing infiltration (organic matter improves soil structure, plough surface induration - semi-permeable)
  2. Reduce velocity (terracing, contour bands, mulch tillage)
  3. Reduce net ppt. (afforest)
50
Q

What are 7 causes of induration (hardening of rocks through cementation of soil/porous rock)?

A
  1. Capping
  2. Desiccation –> hydrophobicity
  3. Burning
  4. Salt precipitates
  5. Salt deflocculates
  6. Trampling
  7. Vehicle compaction
51
Q

What are the 2 subsurface processes that erode on a slope?

A
  1. ‘Sapping’ at seepage face
  2. ‘Piping’ (turbulent flows)
52
Q

What are the 2 methods of protecting slopes from erosion via subsurface processes?

A
  1. Afforest
  2. Controlled drainage e.g. soil pumps, drains (dry out soil so water doesn’t force pathways)
53
Q

What are the 2 gully processes that erode on a slope?

A
  1. Rill enlargement
  2. Undercutting by scour (aided by soil fall)
54
Q

What are the 2 methods of protecting slopes from erosion via gully processes?

A
  1. Stop sheetflow/subsurface erosion
  2. Bunds (putting things in gully to reduce water velocity so gully doesn’t expand)
55
Q

What is discharge (volumetric water flow underground) proportional to?

A

“Hydraulic” gradient - Henri Darcy

56
Q

What 2 things does flow estimation need?

A
  1. Total potential and how it changes (piezometer)
  2. Permeability (very spatially variable due to soil layering)
57
Q

What 4 factors affect permeability?

A
  1. Particle size
  2. Lithification
  3. Soil aggregate (gaps between)
  4. Layering + fracture
58
Q

Permeability can’t be estimated so what 2 things are used to measure it?

A
  1. Core permeametry
  2. Borehole tests
59
Q

What are 2 additional processes to water flow in the unsaturated zone?

A
  1. Change in water content
  2. Flow only in some pores
60
Q

What are 3 ways soil water content is expressed?

A
  1. Mass wetness (mass water/mass dry soil)
  2. Volumetric wetness (volume of water/volume of undisturbed soil) - most important
  3. Saturation wetness (volumetric wetness/porosity)
61
Q

What are the 3 ways of measuring volumetric wetness (soil water content)?

A
  1. Direct (gravimetric method)
  2. Analogue (calibrated against gravimetric method, needs regular checking against water reference, not fully automatic)
  3. Time domain reflectometry
62
Q

Describe how time domain reflectometry measures volumetric wetness (soil water content)

A

Sends signal down to wave-guides (rods) in soil –> speed of noise/reflection depends on dielectric content of material around rod (changes with moisture content) - can be data-logged

63
Q

What is the equation for estimating unsaturated flow (similar to saturated)?

A

Q = K(unsat)A(dH/L)

64
Q

How is dH (pressure potential) measured?

A

Tensiometer

65
Q

What sign is put before the value of a hydraulic gradient if the movement of water is going down?

A

The - sign

BSc Environmental Science (13 decks)

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