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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
Define storm hydrograph
A river's response to precipitation, moderated by catchment characteristics (either flashy or damped)
26
What are 6 catchment characteristics which moderate a storm hydrograph?
1. Shape (circular = flashy) 2. Area 3. Drainage density 4. Catchment/channel slope 5. Vegetation cover 6. Infiltration 7. Catchment storage (chalk aquifer = damped)
27
What are 6 ways of measuring river discharge?
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
What are 4 properties of a good water tracer?
1. Chemically conservative 2. Non-reactive 3. High water solubility 4. Relatively non-toxic
29
What are 2 types of tracer that might be used in a larger river?
1. Radioactive tracer 2. Fluorescent tracer
30
What are the 3 classifications of a stream?
1. Perennial stream = flows all year 2. Intermittent stream = seasonally dry 3. Ephemeral stream = flows only during storms
31
What is the difference between general and specific groundwater?
General = all water within ground (vadose (soil) and phreatic) Specific = water in phreatic zone
32
What are 2 reasons why groundwater is important?
1. Contaminant transport 2. Water supply
33
What fraction of Lancaster's water supply comes from groundwater in Millstone Grit?
1/3
34
What is Representative Elemental Volume?
Pore volumes of water
35
Define permeability and porosity
Permeability (Ks) = flow of water under unit area and unit hydraulic gradient Porosity = volume of pores in a volume of soil/rock
36
What are the 4 types of groundwater body and what are their differences?
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
What are the 2 types of aquifer and their differences?
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
What is water flow in the saturated zone (below water table) proportional to?
Loss of energy/potential
39
What are the 3 types of streamflow generation?
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
What are 3 reasons why Hortonian overland flow isn't the dominant mechanism for routing ppt. to rivers?
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
What 3 places is Hortonian overland flow localised to?
1. Tree stem bases 2. Compacted-surfaced roads 3. Desert surfaces
42
What are 3 water paths in riverside areas?
1. Channel inflow 2. Direct ppt. on saturated areas 3. "Return flow" (2 and 3 = "saturation-excess" overland flow)
43
What are 3 types of "return-flow"?
1. Shallow water paths (any layers with soil/underlying deposits direct water laterally) 2. Natural soil pipes 3. Wave propagation
44
What are the 2 controls of soil erosion?
1. Erodibility (soil properties) 2. Erosivity (kinetic energy of water flow)
45
What are the 4 erosion locations on slopes?
1. Rain-splash 2. Surface "sheetflow" 3. Subsurface 4. Gullys
46
How does rain-splash erode on a slope and what is evidence for this?
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
What are 2 methods of protecting slopes from rain-splash erosion?
1. Mulch tillage 2. Cover cropping
48
How does surface "sheetflow" erode on a slope?
Forms tiny channels called micro-rills (more conc. erosion, wouldn't have had same erosive power if water was in sheets)
49
What are 3 methods of protecting slopes from surface "sheetflow" erosion?
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
What are 7 causes of induration (hardening of rocks through cementation of soil/porous rock)?
1. Capping 2. Desiccation --> hydrophobicity 3. Burning 4. Salt precipitates 5. Salt deflocculates 6. Trampling 7. Vehicle compaction
51
What are the 2 subsurface processes that erode on a slope?
1. 'Sapping' at seepage face 2. 'Piping' (turbulent flows)
52
What are the 2 methods of protecting slopes from erosion via subsurface processes?
1. Afforest 2. Controlled drainage e.g. soil pumps, drains (dry out soil so water doesn't force pathways)
53
What are the 2 gully processes that erode on a slope?
1. Rill enlargement 2. Undercutting by scour (aided by soil fall)
54
What are the 2 methods of protecting slopes from erosion via gully processes?
1. Stop sheetflow/subsurface erosion 2. Bunds (putting things in gully to reduce water velocity so gully doesn't expand)
55
What is discharge (volumetric water flow underground) proportional to?
"Hydraulic" gradient - Henri Darcy
56
What 2 things does flow estimation need?
1. Total potential and how it changes (piezometer) 2. Permeability (very spatially variable due to soil layering)
57
What 4 factors affect permeability?
1. Particle size 2. Lithification 3. Soil aggregate (gaps between) 4. Layering + fracture
58
Permeability can't be estimated so what 2 things are used to measure it?
1. Core permeametry 2. Borehole tests
59
What are 2 additional processes to water flow in the unsaturated zone?
1. Change in water content 2. Flow only in some pores
60
What are 3 ways soil water content is expressed?
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
What are the 3 ways of measuring volumetric wetness (soil water content)?
1. Direct (gravimetric method) 2. Analogue (calibrated against gravimetric method, needs regular checking against water reference, not fully automatic) 3. Time domain reflectometry
62
Describe how time domain reflectometry measures volumetric wetness (soil water content)
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
What is the equation for estimating unsaturated flow (similar to saturated)?
Q = K(unsat)A(dH/L)
64
How is dH (pressure potential) measured?
Tensiometer
65
What sign is put before the value of a hydraulic gradient if the movement of water is going down?
The - sign

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