Water Resources

Question 1

how can we measure atmospheric moisture?

Answer 1

Direct measurement
- limited spatial and temporal coverage (singe point in time and space, how representative is this single sample however it is an accurate local precision)
Indirect measurement
- Repeat survey (daily at 250-m for MODIS)
- Estimation based on reflectance of radiation, can make a prediction of the amount of water in the atmosphere. Measures over a large area once every day.
both have important uncertainties
- These uncertainties mean that because estimates like these are balanced, they can lead us to think that we know more about the system than is really the case.

Question 2

Frequency and magnitude of hydrological cycle

Answer 2

-the size of the stores and the size of the fluxes
Residence times: how long the water is stored
-atmosphere 10 days
-polar ice 15000 years
-oceans 3600 years

Question 3

Calculating residence times at whole system scale:

Answer 3

-store= flow in- flow out
-If system is at steady state, then flow in = flow out
-So store/flow in=store/flow out = residence or turnover time

Question 4

How does water get back into the atmosphere?

Answer 4

-Evapotranspiration is a complex process (involving net radiation, soil heat flux, air density, air specific heat, air vapour pressure, surface and aerodynamic resistances and partial pressure of water in air). To measure we use a lysimeter.
Evapotranspiration = Precipitation – Deep Drainage – Change in Storage
rain gauge

Question 5

What is Charney hypothesis

Answer 5

• Charney (1975) suggested that changes in albedo as a function of vegetation growth has positive feedback on rainfall in the Sahel
• Vegetation has lower albedo ->more surface heating ->stronger land-ocean temperature gradients ->enhanced monsoonal circulation in the tropics ->more vegetation.
• Reduced vegetation ->increased surface albedo ->low-level cooling ->increased atmospheric stability ->low-level air subsidence drying ->reduced vegetation.

Question 6

Problems with the Charney Hypothesis:

Answer 6

-Jackson and Idso (1975) suggested that albedo changes in the US with vegetation change were inconsistent with Charney’s
-Wendler and Eaton (1983) found that the difference in albedo for vegetated and unvegetated sites in Tunisia was also insufficient for Charney’s model to explain patterns of precipitation change
An alternative explanation:
Entekhabi et al. (1992) have suggested that reprecipitation of moisture that is evapotranspired from vegetation is more likely to lead to the feedback at regional level

Question 7

Why does the hydrological cycle keep going?

Answer 7

-global energy budget (latent heat)
-evaporation into unsaturated air (Evaporation leads to latent heat flux)
-uplift and adiabatic cooling of water vapour (Uplift of water vapour provides potential energy)
-Condensational heating of air (Release of latent heat on condensation
At steady state should match energy from evaporation, but variable in space and time)
-Sinking of unsaturated air (Dry air sinks back to surface to replace moist air that is rising)
-precipitation of condensed water (Raindrops convert potential to kinetic energy as they fall, Drives sediment transport and continental change)

Question 8

movement of water through soil by throughflow:
Saturated Zone:
-Darcy’s Law

Answer 8

if saturated, the only thing in the soil that can change is its flow rate or flux
DL says that flux is a function of the pressure head (or gradient in potential energy) and a parameter
Discharge = saturated hydraulic conductivity x cross sectional area x pressure head

Question 9

movement of water through soil by throughflow:
Unsaturated Zone:
Richards equation

Answer 9

if unsaturated, then the flux may change but so may the ‘porosity’ (the amount of holes in the ground) so Darcy’s law is modified to allow for suction.
But difficult to solve numerically.
(3 phase system, water air and …)

Question 10

Dominant processes moving sediment in temperate, forested landscape.

Answer 10

• Creep
• Splash
• Tree throw
• All diffusive type processes.
• Convex slopes

Question 11

Dominant sediment-moving processes in a temperate, deforested landscape

Answer 11

• Splash at the top of slopes
• Water erosion on the lower part of slopes
• Movement of soil by ploughing
  à So a mix of diffusive (splash, ploughing) and advective (water erosion) processes
  Different shaped hill slopes causes different processe

Question 12

Type of flows

Answer 12

Macropore flow: Macropore flow refers to any flow which takes place outside of the normal pore structure of the soil, such as in wormholes or decayed roots. Due to root or animal activity
Pipeflow: a type of subterranean water flow where water travels along cracks in the soil or old root systems found in above ground vegetation. Often occur naturally in peat or marl soils
Artificial drainage pipes are commonly found in agricultural soils in the UK.

Question 13

The effects of catchment characteristics on controlling water flows into channels:

Answer 13

1. Diffuse flows. Hillslope flows get faster as they get deeper (so we get more rapid runoff as the amount of runoff increases)
2. Faster flows with steeper slopes (more rapid runoff in steeper catchments) due to high kinetic energy.
3. Faster flows when the bed surface is smoother or less rough (so runoff is more rapid in urban areas like roads compared to rural areas like vegetation)
4. Deeper and faster flows will start to form channels (rills and gullies), which are more efficient at moving water

Question 14

What are diffuse and concentrated flows

Answer 14

Diffuse flow: water covering a large area, slow, vegetation intercepts the water.
Concentrated flow: flow of water in a small area that causes more erosion due to higher velocity and lack of interception from vegetation.

Question 15

Discharge calculation:

Answer 15

Discharge (Q) = width x depth x velocity
m3 s-1 m m m s-1

Question 16

What are the 2 critical processes in drainage networks

Answer 16

Flow accumulation
• The process of accumulation leads to increases in flood wave peaks – addition of Q. water discharge increases in volume further downstream but the peak of water in different catchments will take longer as the water has to travel further.
Flow attenuation
• Downstream movement of water is slowed due to secondary circulation, friction, storage etc. water volume will decrease further downstream due to lack of rain adding to the stream of water and so when it gets to catchment 2 the discharge will be lower and the peak last longer.
• The process of attenuation leads to decreases in flood wave peaks.
• (Attenuation is the inverse of conveyance)

Question 17

What causes attenuation?

Answer 17

-Different timings of sub-catchment response when two tributaries flood at the same time it can cause flooding but if you slow down the flood of one tributary moving the water into storage on a floodplain when the river is at high volume and releasing the water when the river level decreases, it can result in attenuation
-transport of water to floodplains
-momentum/energy loss within the water wave. Increase the roughness will slow the flow resulting in deeper flow. v=(d^(2/3) S^(1/2))/n v = velocity, d = depth, S = slope, n = Manning’s n

Question 18

What is attenuation

Answer 18

The waves are largest where they are formed and gradually get smaller as they move away. This decrease in size, or amplitude, of the waves is called attenuation.

Question 19

Why use attenuation in urban areas ?

Answer 19

• Attenuation is carefully managed in rivers:
• May be introduced immediately upstream of towns and cities to take the ‘tops’ of flood waves.
• May be used further upstream to slow flood waves (natural flood management)
But:
• If you defend a town/river by stopping water going onto the floodplain you locally reduce attenuation
•  has the effect of increasing flood risk downstream.

Question 20

Why dredge: pros

Answer 20

• dredging is the removal of sediments and debris from the bottom of lakes, rivers, harbour’s, and other water bodies.
• Pros:
• Improve land drainage and flow conveyance.
• Improve navigation.
• Economic use for sand and gravel extraction

Question 21

Why dredge: cons

Answer 21

• Can increase flooding downstream, not effective at reducing flood risk.
• Flow continuity/mass balance – floodplains cover large areas so channels would need to be much deeper/wider to contain the same amount of flow.
• Cost:
• Where to put dredged silt (may be contaminated)
• Increase subsequent erosion (and thus siltation) by destabilizing banks.
• Disturbance of fish spawning grounds/other parts of ecosystem (and thus related ecosystem service
• Erosion and deposition of sediment is going to happen after flood events and so the previous dredging of the river will be useless and so will need to be done after every flood event.

Question 22

The human basis of flooding:

Answer 22

• Flood events now impact >300 million people per annum, with financial losses of US$105 billion in 2021
• Flood hazard is projected to increase in the future because of increasing frequency of extreme precipitation events because of climate change
• Flood risk = flood hazard  vulnerability and exposure (people & assets)
• So risk also depends on demographic changes and economic development in flood-prone areas
• Globally, more than one-in-five people (1.8 billion people) are exposed to 1-in-100 year flood risk. Exposure is not evenly distributed; higher in lower income households (Rentschler et al., 2022)

Question 23

Factors determining wind:

Answer 23

1. Pressure gradient: Effect of Pressure Gradient Force (PGF) on Wind Speed. Steep pressure gradient= strong pressure gradient force, shallow pressure gradient= weak PGF
2. Coriolis effect:
3. Influence of Surface Friction Forces on Surface Wind:Friction reduces the wind speed, which reduces the Coriolis force., Reduced Coriolis force no longer balances the pressure gradient force, and the wind blows across the isobars toward or away from the pressure centre.
   4.Northern Hemisphere: surface winds blow counter clockwise and inward into a surface low, and clockwise and out of a surface high in the Northern Hemisphere.
   • Southern Hemisphere: Coriolis force acts to the left rather than the right. This causes the winds of the Southern Hemisphere to blow clockwise and inward around surface lows, and counter clockwise and outward around surface highs

Question 24

Nature of the atmospheric circulation

Answer 24

-3 cell model of atmospheric circulation (polar, Ferrel, Hadley)
-seasonal heating and circulation; polar high-seasonal expansion and contraction of cold air, ITCZ movement with seasonal change of sub-polar point and zone of maximum heating, Seasonal movement of Subtropical High Pressure systems.
-seasonal variations of polar front

Question 25

What causes jet streams:

Answer 25

-Jet streams are relatively narrow bands of strong winds in the upper westerlies (troposphere) that blow from west to east at an altitude of 7000 - 15000m.
-Jet streams are caused by a combination of the planet’s rotation on its axis and atmospheric heating (by solar radiation).
-The jet stream results from latitudinal and vertical large gradients of temperature and pressure at the intersection of a colder air mass from the north with a warmer air mass from the south (Northern Hemisphere case).
-The combination of temperature and pressure differences in the air mass, plus some help from the Coriolis force (Coriolis acceleration) creates the acceleration of winds into the jet stream.
-The meanders in the upper westerly circulation are known as Rossby waves.
-Within the upper westerlies strong ribbons of wind form. These are known as jet streams.

Question 26

What are air masses

Answer 26

-Large volumes of air with homogeneous thermal, moisture and stability characteristics
Formed by air sitting over an ocean or land surface for several days

Question 27

Development of Mid-Latitude Cyclones:

Answer 27

-Mid-latitude cyclones form when freezing air masses from the Arctic meet with warm moist air from the tropics. These air masses begin to rotate and increase in energy, thus forming a large low-pressure centre or cyclone.

Question 28

What is a drought

Answer 28

• To a meteorologist, it is below-normal precipitation
• To a hydrologist, it is below-normal water supplies (either surface or groundwater, depending on location)
• To a farmer, it is insufficient water to grow a specific crop
• To local residents, it is a lack of water supplies for daily activities (socioeconomic drought)

Question 29

Meteorological drought:

Answer 29

What is below-normal precipitation?
- in the UK, it is formally 15 consecutive days with < 0.2 mm precipitation
- OR < 50% rainfall in three months
- OR 15% lower than average over two years

Question 30

How to calculate/recognise droughts:

Answer 30

-spatial patterns of anomalies of places with more rainfall and others with low rainfall anomalies eg/ south drought north normal.
-standardised precipitation index
-standard deviation: One approach is to compare the long-term deviation from the mean using z scores

Question 31

Palmer Drought-Severity Index:

Answer 31

-Wayne Palmer developed his index in 1965 as a way of comparing drought across the highly different climates of the USA
-The index uses temperature and precipitation data and a simplified representation of the hydrological cycle – (i.e. uses an understanding of process and a simple mass balance) – to produce a standardized range of values

Question 32

Agriculture drought:

Answer 32

• Reflects lack of availability of soil moisture for crops of a specific type (types of plant need different water levels
• It may still occur even if there is no meteorological drought
• Different crops have different water requirements, so adjacent fields growing different crops may not both exhibit agricultural drought
• Plants may have different water requirements at different points in their life cycles, so agricultural drought is relative to their current needs not average conditions
• Some have argued that the assumptions made by Palmer make the PDSI more relevant to agricultural than hydrological drought

Question 33

What is quantity and quality of water

Answer 33

Quantity: total amount of water globally
Quality: quality refers to the biological or chemical content of the water. Sediment can significantly affect quality as a result of flood events.

Question 34

Climatic Variability:

Answer 34

-Climate variability refers to variations in the mean state and other statistics (standard deviation, the occurrence of extremes, etc.) of the climate
-Climate variability is often measured by comparing the observed conditions to the long-term mean conditions – ‘climate normal’ (e.g. WMO: 1961-1990. 1971 – 2000).
-Variability may be due to natural internal processes within the climate system (internal variability), or to external variations in natural (e.g. sun) or even anthropogenic forcing.

Question 35

Climate Variability: Spatial and Temporal Characteristics

Answer 35

-Variability ranges over many time and space scales
-Variations in
-small-scale phenomena: wind gusts, localized thunderstorms and tornadoes
-larger-scale features: fronts and storms, hurricanes, to even more prolonged features such as droughts and floods

Question 36

Modes of climate

Answer 36

-variability are semi-regular fluctuations of global or regional scale climate variables (natural variability, known as modes or oscillations)
- Such fluctuations arise from the interaction of the components of the climate system (atmosphere, hydrosphere etc) on all time scales
- Their phases and states can be monitored using measurements of critical climate variables (e.g. SST, atmospheric pressure) which are indicative of changing ‘behaviour’.
- There are many known climate oscillations or modes of climate variability which extend over large geographical areas, with some having near global scale impacts

Question 37

Modes of Climate Variability and Teleconnections:

Answer 37

-Modes of climate variability or climate oscillations can produce recurring and persistent atmospheric and ocean circulation features that occur simultaneously over vast geographical regions (e.g. the Pacific Basin)
-These may result in climate anomalies (abnormal climate patterns) in different parts of the world.
-Sometimes climate anomalies that occur in different parts of the world are related and can be opposite to each other (hot in one place, but cold in another, and vice versa)
-Associations of climate between places separated by large distances are called teleconnections
-Teleconnections occur because the climate in two separate locations are related/linked via the atmospheric and ocean circulations

Question 38

El Nino:

Answer 38

-is the opposite of “normal” or long-term average climate patterns and contrasts with La Nina
-Characterized by unusually (anomalous) high SST in central and eastern Pacific
-Often referred to as a warm phase or warm event because of unusually warm SST.
-Climate impacts: if a place is normally warm and moist or cool and dry then the reverse applies during an El Niño

Question 39

La Nina:

Answer 39

-Is opposite extreme of El Nino
-Basically is an exaggeration of the “normal” or long-term average climate patterns
-Often referred to as a cool phase or cold event because of unusually cool SST in central and eastern Pacific.
-Climate impacts: If a place is normally warm and moist or cool and dry then it becomes even more so during La Niña

Question 40

The walker circulation:

Answer 40

• the pattern of air rising in the west and falling in the east with westward moving air at the surface is referred to as the Walker Circulation.
• El Nino: WC displaced to the east with subsidence over W Pacific and strong ascent over Central Pacific
• La Nina: WC maintains position with strong ascent over W Pacific and strong subsidence over Central Pacific

Question 41

El Nino Southern Oscillation (ENSO):

Answer 41

-The El Niño/ Southern Oscillation (ENSO) cycle = a global-scale, naturally occurring phenomenon, linked with variations in SST and atmospheric pressure across the Pacific Basin
-El Niño (EN) is the ocean component (SST) and Southern Oscillation (SO) is the atmospheric component (atmospheric pressure)
-El Nino and La Niña episodes typically last approximately 9-12 months.
-Often begin to form during June-August, reach peak strength during December-April, and then decay during May-July of the next year.

Question 42

What is the effect of vegetation on water cycle

Answer 42

-precipitation tends to be similar in grass land and forest
-grass land has more run off then forest
-variation in loss of water into the atmosphere. More evaporation in forestry as water is remaining on the vegetation causing evaporation plus evapotranspiration is higher in forestry
-less tress more water yield increases (more water runoff goes into streams and rivers)