Hydrology Flashcards
(29 cards)
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
