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EnviSci Year1 > LEC.174 Hydrology: Water in the environment > Flashcards

LEC.174 Hydrology: Water in the environment Flashcards

(95 cards)

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1
Q

Is spatial or temporal distribution of water more important for humans?

A

Temporal - natural hazard of flooding

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2
Q

If water controls the migration of pollutants and sediments what is an example of an issue it could cause?

A

Eutrophication (nitrates)

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3
Q

Water balance/catchment water budget equation?

A

P = E + Q ± ΔS

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4
Q

What part of the water balance equation can be ignored when over a period of a year or more?

A

ΔS as the change averages to zero

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5
Q

River catchments are normally defined by surface topography, assuming the groundwater divide is the same as the surface divide - what is wrong with this aassumption?

A
  1. The surface of the rock head may have a different shape to the ground surface
  2. Dipping impermeable strata e.g. Bowland Fells
  3. Karstic subsurface hydrology
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6
Q

Key features of a Met Office Mark II storage gauge?

A
  1. gauge orifice located 30cm above ground-surface to reduce rainfall hitting the ground and bouncing into the gauge (in-splash)
  2. 10cm between orifice and collection funnel to reduce out-splash
  3. orifice has a very sharp edge to reduce uncertainty in the collection area
  4. collection funnel has a narrow neck to recue E of collected water
  5. collection bottle sits within an outer can hthat will cactcg extreme rainfalls
  6. gauge is dug into the ground to give stability and reduce E
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7
Q

How does a siphoning tank recording gauge work?

A

Collects water in a funnel and directs this into a cylindrical tank, as the tank fills a float is raised which lifts a pen on a chart. Above a certain level the float-pen mechanism releases a catch and the tank tips over and empties from a pipe aided by a siphoning mechanism, once the tank is empty is rocks back on its pivot the catch engages and the tank again begins to fill

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8
Q

Disadvantage of a siphoning tank recording gauge?

A

Chart requires digitalisation to obtain the rainfall intensity data

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9
Q

How does a tipping bucket raingauge work?

A

Collects water in a funnel then delivers it to a pivoted double-bucket device. Water will fill one bucket first, it’ll tip and empty and now the second bucket is below the funnel. When the buckets tip a magnet passes a reed switch causing an electrical contact to close, time of voltage pulse can then be recorded. Normally set to tip after 0.5mm rainfall

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10
Q

Disadvantage of a tipping bucket rain gauge?

A

freezing during cold winter conditions, can be overcome with gauge heating but normally a storage gauge next to it (check gauge) is recommended to fill in records where needed.

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11
Q

What is the appropriate raingauge siting to avoid sheltering?

A

distance 2x height of nearest object/30° angle between gauge and nearest thing top

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12
Q

How to reduce turbulence at the gauge orifice?

A

Huddleston turf wall, institute of hydrology ground level gauge installation

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13
Q

How to site rain gauges in forests?

A

siting standard raingauges in clearings or by connecting a canopy-level funnel to a standard rainguage on the ground (canopy gauge).

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14
Q

6 methods of areal estimation?

A
  1. Arithmetic mean
  2. Thiessen polygons
  3. Hypsometric curve
  4. Isohyetal method
  5. Polynomial interpolation
  6. Rainfall radar
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15
Q

Areal estimation - arithmetic mean?

A
  • need good gauge distribution
  • bias where gauges are
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16
Q

Areal estimation - Thiessen polygons?

A
  • non-uniform dist. of gauges
  • uses midpoints
  • weighted representation
  • doesn’t take into account heigh variations
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17
Q

Areal estimation - hypsometric curve

A

Estimate catchment-average rainfall that is explicitly altitude-weighted.
Draw relationship between altitude and rainfall, then relation between altitude and area of catchment below that altitude (hypsometric curve), then use these relations to plot curve of precipitation.

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18
Q

Areal estimation - Isohyetal method?

A
  • can take altitude into account
  • isohyets subjectively adjusted for underlying topography
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19
Q

Areal estimation - rainfall radar?

A

Spatial patterns over large catchments but need to calibrate against point gauge measurements

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20
Q

Open water evaporation?

A
  • lakes, reservoirs, rivers
  • measure using an evaporation pan or penman method
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21
Q

Evaporation pan?

A

E for the day is E for the pan, once apply pan coefficient due to smaller heat storage of pan compared to lake.

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22
Q

Penman method?

A

Eo is dependent on 1. available energy for evaporation 2. saturation deficit (use dry and wet bulb thermometers) 3. the wind speed

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23
Q

Wetted-canopy evaporation?

A
  • Precipitation above the canopy - throughfall plus stemflow
  • Canopy raingauge or clearing raingauge
  • throughfall measured with throughfall troughs
  • stemflow measured with stemflow collars connected to volumetric or tipping bucket
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24
Q

Transpiration?

A
  • Water lost from stomata
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25
How to measure transpiration ?
Porometer - individual leaf enclosed in chamber and the moisture lost into the chamber measured Lysimeter water balance - block of soil containing one or more whole plants is isolated and the net precipitation, drainage and changes in soil-water-storage are measured and calculated
26
7 factors affecting river response to precipitation moderated by catchment characteristics?
1. basin shape 2. basin area 3. drainage density (length of river channel per unit catchment area) 4. basin or channel slope 5. vegetation type 6. infiltration 7. catchment storage
27
Volumetric gauging as Q measurement?
Small river flows, or indirectly in combination with float gauging or current metering
28
Float gauging as Q measurement?
- orange or dog biscuit - velocity x cross sectional area - surface faster than avg - reduce by coefficient - changes along the river - not entirely accurate
29
Current metering as Q measurement?
- rotation of current meters impeller gives local water velocity (post application of calibration eqn) - measurements at diff depths - across river split into sections, average for each section
30
Why is NaCl a good tracer?
chemically conservative, high solubility, relatively non-toxic, can be measured in field, cheap, available
31
Alternate to NaCl in large rivers?
Alternative tracers (in ppb levels) e.g. fluorescent Rhodamine WT
32
Dilatation gauging - constant injection method?
tracer into river at a fixed rate (q) using matiotte device (C1), downstream the background conc (C0)measured followed by conc mixed with tracer (C2)
33
Dilatation gauging - gulp method?
addition of known vol and conc of tracer (V1) into river in one go, downstream conc is measured (C2), background subtracted (C0) and the area beneath C2-C0 versus time (t) curve calc
34
Subcritical/tranquil flow has a Froude number of?
<1
35
Supercritical/fast/shooting flow has a Froude number of?
>1
36
What is the structural method of measuring Q when there are continuous traces of river discharge?
Relationship between height of water in the river and several spot discharge measurements is established and called rating curve
37
What do weirs do?
Force the river to generate a hydraulic drop, which allows accurate discharge trace to be calculated
38
When using a weir, where does the water fall from?
Stilling pool
39
What do flumes use to create a hydraulic drop?
Constriction
40
Ephemeral ?
channel normally dry unless big rainfall or flooding event (e.g. desert wadi)
41
Intermittent ?
dry for one or more seasons (e.g. chalk streams in S England)
42
Perennial ?
Flow all year
43
When are slope area methods used?
If peak discharge during a flood is required for flood prediction at a site where only the max height of the flood is known or where the river gauges has been over topped
44
If water-slope and hydraulic mean depth increase what happens to the river velocity?
Increases
45
If the channel roughness increases what happens to the river velocity?
It decreases
46
River generation pathways?
Direct in-channel precipitation, infiltration-excess overland flow (hortonian overland flow), subsurface stormflow
47
What is infiltration excess overland/hortonian overland flow?
Rainfall intensities > infiltration capacity of the soil
48
Problems with the hortonian overland flow as a pathway of precipitation?
- Measured soil infiltration capacities normally seen to be grater than rainfall intensities. - Temporal dynamics of conservative ions dissolved in rainfall should be very comparable t those in river water, if river water is generated only by HOF, but the dynamics of conservation ions in river are very damped compared to those in rainfall, which indicates to storage of rainfall. - Extensive HOF is rarely observed in most catchments even during storm events, therefore HOF is restricted to semi-arid areas, urban areas, and other localised small scale areas.
49
How can water move from near-river soils to the river?
1. flowing up through the river bed 2. falling onto saturated soils and then moving overland 3. subsurface flow being forced out of the ground (return flow) as the water table rises and then flowing overland
50
How can subsurface water move rapidly to riverside soils?
1. shallow water paths (topsoil) 2. natural soil pipes 3. wave propagation
51
What are the two stages of soil erosion by water?
Detachment and transport
52
Rain-splash erosion (soil erosion by water)
- raindrop falling at terminal velocity can splash water - with a slope lots of material splashes down slope
53
How to protect against rain-splash erosion (soil erosion by water)
Covering the ground with organics
54
Surface 'sheetflow' erosion (soil erosion by water)
- micro-rills due to impermeable surface
55
How to protect against surface 'sheetflow' erosion? (soil erosion by water)
- reduce velocity of overland flow 1. promote infiltration (ploughing) 2. slope terracing 3. increasing evapotranspiration loss
56
Subsurface erosion? (soil erosion by water)
- soil loss because particles moving as subsurface low discharges from the ground surface - piping erosion, water in soil turbulent, erosive
57
How to protect against subsurface erosion? (soil erosion by water)
reduce the amount of water moving in natural soil pipes - afforestation - drainage
58
Gully erosion? (soil erosion by water)
Can develop from sheetflow or subsurface erosion. - Rills that cannot be removed by ploughing
59
How to protect against gully erosion? (soil erosion by water)
Dams across gullies to slow the waterflow
60
What is the vadose zone?
Subsurface soil-water
61
What is the phreatic zone?
Subsurface ground-water
62
Why is it important to consider subsurface water?
- contaminant migration - development of groundwater resources
63
Permeability? (subsurface)
flow of subsurface water under unit area and unit hydraulic gradient (Ks)
64
Porosity?
volume of pores in a volume of rock or soil
65
Permeability and porosity of an aquifer?
High permeability high porosity
66
Permeability and porosity of an aquitard?
some permeability, some porosity
67
Permeability and porosity of an aquiclude?
low permeability, high porosity
68
Permeability and porosity of an aquifuge?
no permeability, no porosity
69
Confined aquifer?
less permeable layer above aquifer, allows pressure to build, borehole can puncture it, water level in borehole is higher than the water table e.g. London basin
70
Unconfined aquifer?
water level in borehole in an aquifer same as water table, no confining layer above aquifer
71
Total potential H?
The slope on an underground water table reflects a loss of pressure and elevation energy
72
Change in potential between two points equation?
(p1 + z1) - (p2+z2) = H1-H2 = dH
73
Darcy's law what does it state?
Changes in groundwater flow are directly proportional to changes in hydraulic gradient
74
Darcy's law equation for groundwater velocity?
Q = Ks(dH/L)
75
Darcy's law equation for groundwater volumetric discharge?
Q = AKs(dH/L)
76
How do you get the measurement of pressure and elevational pressure for a estimate groundwater flow?
Borehole/piezometer
77
Measuring Ks - core permeametry/ring permeameter?
undisturbed cores extracted from the ground and Ks determined by measuring all opther terms in Darcys eqn as water is ran through the core. Ring permeametry dH/L calc for whole length of core
78
Measuring Ks - borehole tests
water pumped from a borehole and the effect on water-level in two nearby boreholes is observed. With measurements the Ks can be determined
79
What is the problem with estimating groundwater flow? (permeability)
Huge spatial variability in permeability
80
η what does this symbol represent?
porosity
81
Soil water content - mass wetness θm
mass of water/mass of dry soil - changes as the dry bulk density of the soil varies spatially
82
Soil water content - volumetric wetness θv
= vol water/vol soil -> = (mass water/mass dry soil) x (mass dry soil/volume soil)
83
maximum θv is = ?
porosity
84
θv/η = ?
saturation wetness θs
85
Saturation wetness? θs
volumetric wetness/porosity
86
Measuring soil water content using gravimetric determination?
Assessing weight loss of soil after drying at 105°C for 24hrs
87
Measuring soil water content using neutron moderation (indirect 'analogue')?
- radioactive Am-Be source into ground with aluminium access tube - Fast neutrons generated by source - Fast neutrons bombarded with large ions in soil (H+ from H2O) and makes a cloud of slow neutrons - cloud of slow neutrons can be measured with a BF3 detector also within the access tube
88
Measuring soil water content using time-domain-reflectometry TDR (indirect 'analogue')?
Speed at which reflection of EM signals are returned along pairs of metal rods which are buried in soil. For very high frequency waves the resultant value of dielectric constant is strongly correlated only with volumetric water content
89
Equation for velocity of water in unsaturated soil?
Q = Kunsat(dH/L)
90
How to measure pressure potential in an unsaturated soil?
Tensiometer
91
Why does a tensiometer have a ceramic tip?
It acts as a permeable interface between the tensiometer's water-filled tube and the soil water, allowing water to move freely between the two. This movement of water creates a vacuum inside the tube, which is then read by a gauge, indicating the soil's moisture content.
92
What happens in a tensiometer when the tip is above the water table?
partial vacuum or negative pressure potential
93
What happens in a tensiometer when the tip is below the water table?
positive pressure potential
94
The Darcy eqn gives an avg velocity over a unit area of soil, what represent this in other eqns?
QDarcy
95
How to calculate pore-water velocity?
Qpore = QDarcy/η

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