how can we measure atmospheric moisture?
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.
Frequency and magnitude of hydrological cycle
-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
Calculating residence times at whole system scale:
-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
How does water get back into the atmosphere?
-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
What is Charney hypothesis
- 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.
Problems with the Charney Hypothesis:
-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
Why does the hydrological cycle keep going?
-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)
movement of water through soil by throughflow:
Saturated Zone:
-Darcy’s Law
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
movement of water through soil by throughflow:
Unsaturated Zone:
Richards equation
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 …)
Dominant processes moving sediment in temperate, forested landscape.
- Creep
- Splash
- Tree throw
- All diffusive type processes.
- Convex slopes
Dominant sediment-moving processes in a temperate, deforested landscape
- 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
Type of flows
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.
The effects of catchment characteristics on controlling water flows into channels:
- Diffuse flows. Hillslope flows get faster as they get deeper (so we get more rapid runoff as the amount of runoff increases)
- Faster flows with steeper slopes (more rapid runoff in steeper catchments) due to high kinetic energy.
- 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)
- Deeper and faster flows will start to form channels (rills and gullies), which are more efficient at moving water
What are diffuse and concentrated flows
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.
Discharge calculation:
Discharge (Q) = width x depth x velocity
m3 s-1 m m m s-1
What are the 2 critical processes in drainage networks
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)
What causes attenuation?
-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
What is attenuation
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.
Why use attenuation in urban areas ?
• 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.
Why dredge: pros
- 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
Why dredge: cons
- 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.
The human basis of flooding:
• 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)
Factors determining wind:
- Pressure gradient: Effect of Pressure Gradient Force (PGF) on Wind Speed. Steep pressure gradient= strong pressure gradient force, shallow pressure gradient= weak PGF
- Coriolis effect:
- 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
Nature of the atmospheric circulation
-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
