LEC.275 Catchment Hydrology Flashcards
Which 4 catchment characteristics were used within the UK Flood Estimation Handbook to calculate peak flow in a UK river?
- Rainfall
- Lakes/reservoirs
- Soil, regolith, and rock type
- Contributory area
Define the water balance equation for a catchment
P = (Q + Et) + (dS/dt) + G
What are the 2 types of condensation nuclei?
- Hygroscopic particles (affinity for water vapour e.g. salt particles)
- Non-hygroscopic particles (need some degree of supersaturation e.g. dust, smoke)
Define 3 mechanisms whereby moist air is cooled to the dew point
- Adiabatic expansion of air forced to rise over high ground
- Warm air mass forced to rise above cold air mass at a front
- Contact of moist air mass and a colder object (e.g. tropical ground at night)
What are the 4 types of weather patterns producing precipitation?
- Convectional precipitation
- Waves in Easterlies and tropical cyclones
- Monsoon circulation
- Mid-latitude cyclones/depressions
Explain how a change in the monsoon winds gives higher rainfall in the summer months over India
During winter India fed by dry, cool winds moving southwesterly from Eurasian anticyclone but in summer warm, moist air from Indian Ocean drawn to low pressure area over northern India
What are the 10 methods of measuring precipitation?
- Storage rain gauge (not useful for floods)
- Chart recording rain gauge (hourly, weight-operated, expensive to turn chart into digital record)
- Tipping-bucket rain gauge
- Electronic weighing rain gauge (minimal calibration, anti-freeze)
- Capacitance rain gauge
- Drop-counting rain gauge
- Present weather detector (doesn’t tell you intensity)
- Disdrometer
- Ground-based rainfall radar
- Satellite-based rainfall
What are the 2 sensors on the TRMM satellite for rainfall measurement?
- TRMM Microwave Imager (TMI)
- TRMM Precipitation Radar (PR)
What happens to the direction and magnitude of skew in the frequency distribution of rainfall as the sampling intensity is increased from annual to daily?
From a normal distribution to a J-shaped distribution with high positive skew
Name a meteorological phenomenon that causes inter-annual (between year) cycles in rainfall totals
El Nino Southern Oscillation
Rainfall events can be characterised by rainfall intensity, duration, + spatial extent. Give example details of these 3 characteristics for typical afternoon rain events occurring within an inland catchment along the Equator
(Segama catchment)
1. Many events with >4mm/hr hourly-sampled intensity (UKMO ‘heavy’ class)
2. Mostly lasting for <15 mins
3. With short range of spatial correlation
What do Eo, Et, and Eref mean?
Eo: Open water evaporation (rivers/lakes)
Et: Actual evaporation or evapotranspiration (usually less than Eo, includes all pathways)
Eref: Reference evaporation (evaporation from a reference vegetation)
What is open water evaporation (Eo) not restricted by?
- Soil moisture availability
- Stomatal behaviour
What are the major limitations of evaporation pans?
Smaller heat capacity and raised temp + vapour pressure compared to larger water bodies, necessitating the use of a pan coefficient
What are the advantages of a water balance method of estimating actual evaporation?
Estimates can be made for very large areas using existing catchment infrastructure
What are the 2 key variables within the eddy covariance method of estimating actual evaporation?
- High frequency (upward) wind velocities
- High frequency humidity measurements
What are the 3 key advantages of the eddy covariance method of estimating actual evaporation?
- Can be very accurate
- Can sample over large areas
- Allows simultaneous measurement of sensible heat flux (as a check on energy balance)
Give a key limitation of determining evapotranspiration from direct measurements of the components of wet-canopy evaporation + transpiration
Only possible over small areas of forest
What are the 3 key terms within Dalton’s (1801) method of removing supersaturated water?
- Saturation vapour pressure
- Vapour pressure
- Wind speed
The net amount of energy available at a water surface (H) is the sum of which 2 terms?
Net radiation and ground heat flux OR latent heat flux and sensible heat flux
What are the 2 key assumptions made by Penman (1948) in the derivation of the Penman equation?
- Transport of sensible heat flux is by the same basic mechanism as latent heat
- The Bowen ratio is ratio of sensible heat flux to latent heat flux
What are the 4 key terms within Penman’s (1948) method of calculating open water evaporation?
- Net radiation
- Saturation vapour pressure
- Vapour pressure
- Wind speed
The saturation vapour pressure and vapour pressure are derived from which 2 measurements at a standard meteorological station?
Wet bulb temperature and dry bulb temperature (OR air temp and relative humidity)
Wet-canopy evaporation can be considered to be the product of the value of the evaporation from the Penman (1948) equation and the ratio of which 2 terms?
- Dynamic storage of the canopy surfaces (leaves, trunk etc.)
- Maximum water storage capacity of the canopy
What 2 ideas form the basis of cohesion theory?
- Liquid water existing in a continuum with the plant micro-capillaries
- Driving forces for transpiration are potential gradients
The a) canopy conductance and b) atmospheric conductance terms within the Penman-Moneith (1965) equation relate to which 2 processes?
a) Ability of plants to extract water from the soil, transport it via the xylem to the stomata, + release from the stoma
b) The ability of wind eddies to transport the evaporated water through the atmosphere
Give a significant limitation of the Penman-Moneith (1965) method
Error in the estimation of canopy conductance + atmospheric conductance terms, giving uncertainty in evapotranspiration estimates
What are the key problems with the gravimetric method of volumetric wetness measurement?
Soil disturbance during sampling + loss of CaCO3/organic matter during drying
Give the key advantages of the time domain reflectometry (Ghz) method of measuring volumetric wetness?
Safe to use and fully electronic (i.e. can be automated)
Combination of the Buckingham (1907) equation and water balance equation gives which equation describing water flow within unsaturated soil?
Richards (1931) equation
Name a method of determining the unsaturated hydraulic curve directly
Tension infiltrometer
Name 2 methods of determining pressure potential (h) directly
- Piezometer (when below water table)
- Tensiometer (in soil/regolith sometimes unsaturated)
Give 2 key problems with the ‘ring permeametry’ method of core permeametry
- Core extraction is difficult at depth
- Test volume limited to only 1000cm3
What is the runoff coefficient?
Proportion of the precipitation generating river runoff
What are the 3 steps for measuring river runoff?
- Measuring river level (stage) continuously
- Taking spot discharge measurements
- Stage-discharge rating
What are 9 methods of measuring river stage continuously (river runoff)?
- Stage board (manual)
- Float + chart
- Float + punched tape
- Float + potentiometer
- Float + shaft encoder
- Pressure transducer (water deforms it + changes its resistance to input voltage - depends on depth)
- Gas purge (bubbler) gauge
- Ultrasonic
- Radar
What is the key advantage of a gas purge (bubbler) gauge for measuring river stage?
Only a plastic pipe needs to be in the river so expensive sensor can be located away from risk of flood damage
What is the advantage of a river stage sensor using radar rather than an ultrasonic unit?
Not affected by air turbulence, temp., surface angle, + dust like ultrasonic units
What are 8 methods of taking spot discharge measurements (river runoff)?
- Volumetric gauging
- Float gauging (velocity-area methods)
- Impeller current meter (expensive)
- Electromagnetic current meter
- Ultrasonic (doppler) flow meter (gives numbers for backwards flow, works at low flow)
- Acoustic doppler current profiler (velocity at each increment instead of profile average)
- Dilution gauging
- Surface velocity + depth (continuous)
What is the principle of operation of an electromagnetic current meter?
Water flow cuts lines of magnetic flux, inducing an electromagnetic force (EMF) sensed by 2 electrodes
Many river gauging stations are designed to create a stable section with critical flow. Where on the specific energy diagram is the point of critical flow found?
Point of minimum specific energy on the river depth vs. specific energy curve
What is the theoretical relationship between river stage (h) and river discharge (Q) for a 120° thin-plate V-notch weir?
Q = 2.47h^2.5
What are the 4 key runoff generation pathways?
- In-channel precipitation
- Infiltration-excess overland flow
- Saturation overland flow by direct precipitation
- Saturation overland flow by return-flow
Define precisely the conditions generating infiltration-excess overland flow
Rainfall intensity is greater than infiltration capacity
Which basin conditions favour the production of saturation overland flow?
A long, shallow contributing slope and a flat, wide streamside area
What are natural soil pipes?
Conduits formed/enlarged by liquefaction or sheer stresses of water flow
What does water flow via deep rock aquifers generate?
Slow hydrograph recessions
How is stage-discharge rating produced?
Plot log(Q) = log(C) + nlog(h-h0) as a straight line
(n = slope term and reflects geometry of channel e.g. rectangular channel section n = 1.5, concave section of parabolic shape n = 2, triangular/semicircular section n = 2.5)
Which answer best describes a first order, linear relationship between rainfall (R) observed at the last measurement time and river runoff at the new measurement time (qt)?
qt = bR(t-1) + aq(t-1)
Which 2 rainfall-runoff characteristics are readily calculated from a first order, linear relationship between rainfall and river runoff?
- Simulated runoff coefficient
- A time constant
With typical rainfall-runoff non-linearity, how will simulated river discharge look when compared to the observed river discharge?
River hydrograph peaks consistently under-estimated and low-flows consistently over-estimated
What does a short time constant (TC) for the rainfall-runoff behaviour of a catchment mean?
Relatively small change in basin storage but large change in river runoff with a unit input of rainfall
In the determination of the proportion of ‘old water’ in a river hydrograph, how is the concentration of a ‘natural tracer’ in the ‘old water’ typically estimated?
From either minimum concentration of a ‘natural tracer’ in the river prior to storm events or the concentration of a ‘natural tracer’ in a deeper piezometer
Which definition best describes a ‘Dirichlet boundary condition’ within a physics-based + distributed hydrological model?
The boundary of a ‘model discretisation’ that has a specific pressure potential
What is a model calibration?
Adjustment of the estimated values of model parameters to give the best simulation of one or more state variables (e.g. runoff time series)
Which water quality characteristics typically decrease as river discharge increases?
Magnesium and total dissolved solids
When sampling river water, which 2 procedures are normally followed?
- Sample bottle is rinsed with river water prior to sampling
- Head-space in bottle is minimised during sampling
What is the sampling frequency when determining water quality of a river?
Intensive sampling (every 5-10 mins) with automatic water sampler during storms combined infrequent (manual) sampling between storms
What are the 6 methods for analysing water quality in a lab?
- Gravimetric (suspended sediment)
- Titrimetric (alkalinity)
- Colorimetric (Cu/Pb)
- Chromatrographic (phenols, VOCs, pesticides, PAHs)
- Mass spectrometry (trace metals)
- Electrode-based (dissolved O2, pH, nitrate, K, NH3, Pb etc., different probe for dissolved O2)
Which description best describes the analysis of water samples for suspended sediment concentration (gravimetric)?
Filter sample through a pre-weighed filter paper then dry in an oven at 105°C, cool in a dessicator + re-weigh
What are the 8 methods for automated field monitoring of river water quality?
- Temperature probe
- Electrical conductivity probe
- Turbidity probe (calibration <1/yr)
- Dissolved O2 probe
- pH probe and ion-selective electrodes (re-calibration every 7 days)
- pH probe - waterworks standard (re-calibration every 2-3 months)
- Chlorophyll-a optical electrodes
- UV-Vis spectrophotometry for dissolved organic carbon, nitrate etc.
When comparing the frequency of re-calibration needed for use of dissolved O2 probe with a turbidity probe, which statement best describes the situation?
Dissolved O2 probe requires re-calibration on at least a weekly basis
What are the 2 problems of prolonged water storage, particularly in eutrophic reservoirs?
- Growth of algae (taste etc. problems)
- Thermal stratification (giving poor mixing of stored reservoir waters)
In the UK, ‘3 stage water treatment’ normally comprises of what?
A clarification stage and 2 filtration stages at different pH levels
Define the principle of pressure filtration
Filtration using coarse sand, but kept under pressure to minimise head loss in the system
