Hydrology Flashcards
What is the residence time of a component of the hydrological cycle
Amount of water in a resevoir / flux of water in or water out or amount
- Assume a stable state - what goes in is the same as what goes out
- E.g., in atmosphere
What is the water balance/ water budget?
P = E - R - dS/dt
- P = precipitation - adds water
- E = evapotranspiration - removes water
- R = runoff
- dS/dt = change in water storage on land
- Assume a stable state = no dS/dt
What is evapotranspiration?
Evapotranspiration = Evaporation (from water, wet leaves, soil) + transpiration (from plants - stomata)
What two different ways can we measure E?
Sap velocity:
- Measure rate at which sap ascends through plant stems
- Calculate sap velocity
- Errors can be caused by non-uniformity of flow across stem
Light vs heavy isotopes:
- Can separate evaporation and transpiration
- Use isotopes to determine how much is coming from transpiration/evaporation
- Transpiration makes up 60% of global E
How can we measure E?
- Eddy covariance
- Energy balance - Bowen ratio
- Remote sensing
- Surface water balance
How does eddy covariance measure E?
- Measures 3D wind sensor and IR gas analyser
- Sensible (H) and latent heat flux (lambdaE)
- Sometimes problems with energy balance closure
- Deployed on global FLUXNET network - but have some data deserts - need to expand
How can we use Bowen ratio to measure E?
- Simultaneously vertical measurements of temperature and humidity to partition surface energy into H and lamdaE
- Bowen ratio = H/lamdaE
How can we measure E using surface water balance?
- Only at catchment level
Requires observation of:
- Catchment precipitation (P)
- River runoff (R)
- Change in catchment water storage (S)
- E = P - R - dS/dt
- These variables are easier to measure
How can we measure E using remote sensing?
- Doesn’t directly measure E - estimates based on vegetation and other variables etc - and an algorithm to estimate E
- Good for measuring E over large spatial scales and in inacessible areas - spatially comprehensive
- Zhang et al., 2015
How can you measure catchment P?
- Gauge-based datasets
- Satellite retrievals
- Merged satellite and gauge analysis
Issues:
- Spatial and temporal coverage
- Uncertainty in measurement or retrieval
How can you measure river discharge (R)?
V notch weir:
- R is directly related to water depth (h) above bottom of the V
- Using rating curve - to estimate river discharge
How can you measure change in terrestrial water storage (dS/dt)?
GRACE satellite:
- Gives vertically integrated dS/dt for resolutions >400km so only possible for large river basins
How well do the different models estimate Amazon E?
- Catchment balance: shows lower ET in the wet season and higher in the dry season
- But other products such as (MODIS, P-LSH, GLEAM), reanalysis (ERA5) and Climate models CMIP5 and CMIP6 don’t capture this seasonality well at all
- Concerning as climate models cannot capture a fundamental component of the hydrological cycle - raises question of how reliable these models are for predicting future climate - especially over the Amazon
What has been happening to water throughout the year in the Amazon?
- Wetter seasons getter wetter and drier seasons getting drier
- Has been increase in frequency of Amazon droughts and floods from 1903 to 2015
What is precipitation recycling (P)?
- The contribtion of E to the precipitation in an area
- Important in Amazon hydrology
- Recycling ratio is defined as the ratio of locally derived P to total P
- 25-50% of precipitation in the Amazon is from E
- Tells us how dependent the Amazon is on precipitation itself - where up to 50% is coming from itself (E)
- Forests further inland are more dependent on water being recycled upwind
What controls evapotranspiration (E)?
- Meteorology
- Plant stomatal conductance
What factors influence E?
- Incoming energy
- Precipitation
- Soil moisture
- Stomatal conductance
E can be limited by the amount of water available or amount of energy
How does plant stomatal conductance control E?
Transpiration of water through stomata - open stomata to allow CO2 in
- But when plants open stomata - they lose water
- Evaporation dominant for unvegetated surfaces
- Transpiration more important as vegetation increases
What is water use efficiency (WUE)?
WUE: ratio carbon gain (A) to water loss (E)
- WUE = A/E
- To see how much a plant is trading off between save water and to photosynthesise
How does increase atmospheric CO2 affect WUE?
Increased CO2 - means plants open stomata less to get same amount of CO2 - is called the CO2 fertilization effect - positive use of WUE overtime
- This reduces water loss and increases WUE
What may long term changes in WUE do to precipitation?
- Climate models can predict changes in WUE - and how this will affect rainfall
- Suggests it will leave to large reductions in rainfall in the Amazon
How does LAI affect E?
Increased LAI - due to increased greening leads to increased surface area of leaves - hence increased E rates
How will temperature, precipitation and CO2 affect E rates in future?
Competing factors:
- Increased temperature = increased E
- Increased precipitation = increased E
- Increase CO2 = decrease E
How are E, H and R (runoff) different depending on the vegetation type?
Forest: High E, low sensible heat (H), low runoff (R)
Grassland: lower E, higher H, higher runoff (R) - as less water transferred to atmosphere
How does E in the amazon forest and pasture vary across the year?
Forest has higher E than pasture year round
- But difference in E is greatest in the dry months - because forests have deep roots - to get water and evapotranspire
- Whereas grassland cannot do this, so has to evapotranspire less
How do travelling parcels of air change depending what land they travel over?
Parcels of air travelling over more forest produce more rainfall - parcels take up more moisture - more precipitation downwind
- Spracklen et al., 2012
How do deforestation and afforestation affect E and their knock-on effects?
Deforestation: Lower E
- Less atmospheric water available for downstream rainfall
- Increased runoff
- Sudden event - changes will be fast
Afforestation: higher E
- More atmospheric water available for downwind rainfall
- Decreases runoff
- Slower - as forest recovers - changes will be more gradual
How good is the link between deforestation and floods?
- Bradshaw et al., 2007 - strong link between flood risk and deforestation - but study has been criticised for data used
- The effects are ambiguous and people are unsure
How does deforestation affect hydrology in somewhere like the amazon?
- Deforestation leads to reduced E, which increases R (runoff)
- Therefore lowers precipitation downstream
- So forest can no longer rely on itself for water - may reach tipping point where forest is unsustainable
