Water Resources Flashcards
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)