333 Flashcards
Inconsistency in Rainfall Data and how to fix it (4)
Site:
- condition of gauge
- condition of surrounding area
Data:
- check for obvious errors/patterns (time series plot)
- test consistency using Double Mass Curve.
Isohytel Method (2)
- Isohyets are contour lines of equal rainfall depths
- The mean areal rainfall is the weighted average of the rainfall depths between any two successive isohyets
Thiessen Polygon Method (2)
- catchment divided into polygons, one for each gauge station
- The polygon around each station contains that portion of the catchment which is nearer to that station than to any other
Hypsometric Method (4)
- takes into consideration of orthogonal effects
- plot elevation vs rainfall
- plot elevation vs area above specific rainfall
- divide the curve above into a number of different elevation zones and calculate using R=SUM(Ri*ai)
Arithmetic Method (1)
- an average of each rainfall depth at each gauge site.
Effects of time interval of variability (1)
- The variability increases as the time interval increases
Impact of climate change on frequency analysis (1)
changes in water level etc may cause non stationarity in the data of the time series plot.
Assumptions of frequency analysis (3)
- The series is a random sample from a population
- The population parameters can be estimated from the sample
- Stationarity of the X-T relationship
basis of construction probability paper (5)
- distorts the ordinate axis to obtain a linear relationship
- involves ranking the data according to magnitude
- the ranked data is plotted
- a line of best fit can be drawn
- slope and intercept parameters can then be used to get the parameters of the distribution
problems with the least squares method (2)
- easily influenced by outliers
- best for use of 2 parameter distributions
problems with unit hydrograph method (3)
- separation of base flow
- estimation of effective rainfall
- very subjective process
assumptions in Curve Number Method (3)
- assumes that runoff begins after some initial abstraction
- viewed as an infiltration loss model
- ratio between actual and potential quantities are equal
factors which affect soil infiltration (6)
- slope (larger slope, larger infiltration)
- soil type (coarse have larger infiltration than fine)
- land cover
- temp (frozen soils become too dense)
- initial moisture content (high initial moisture content means lower infiltration)
- depth of groundwater table (short depth, less infiltration)
calibration/verification of field data or V notch etc (6)
- collect synchronised discharge and stage measurements
- discharge can be measured using a variety of methods (velocity area, dilution gauging)
- stage can be measured using a staff gauge
- data collected should show a similar range of the table
- compare measured and theoretical data
- adjust parameters of the theoretical curve as required
factors to be included with a rainfall gauge installation (5)
- vandalism
- accessibility
- power supply (solar)
- redundancy (automatic or manual)
- surrounding (trees)
factors to be included with a river flow gauging (6)
- vandalism
- accessibility
- risk of failure during extreme events
- stability of river cross section (erosion)
- maintenance
- power supply
basis of satellites (1)
- the duration spent by an area having cloud top temps below 40 degs is the Cold Cloud Duration which can be used to calculate rainfall
basis of radar (3)
- radars emit electromagnetic energy and some can be reflected when it hits clouds
- the reflected energy returns to the transmitter
- this can be used to calculate rainfall
benefits of radars over rainfall gauges (1)
- better spatial and temporal resolution of rainfall
basis of velocity area method (3)
- divide the section into subsections
- measure velocity at one or more depths
- calculate discharge using numerical integration
basis or ordinary moments (4)
- Derive theoretical relationships between the moments and the parameters of the distribution
- Calculate sample estimates
- Equate sample estimates to the theoretical expressions
- Solve the resulting equations
basic principles of unit hydrograph (3)
superposition: A number of effective rainfall blocks equal to the sum of the runoff hydrographs produced by each block.
proportionality: Different rainfall intensities, but of equal duration are in the same proportion to each other.
time invariance: The effective rainfall vs storm runoff relationship does not change with time
impacts of urbanisation on the hydrological cycle (3)
- Increase in surface runoff
- Decrease in evapotranspiration
- Decrease in infiltration, groundwater recharge and baseflow
role of hydrology on storm water with water sensitive design (2)
- This approach uses distributed, de-centralized, small-scale on-site devices used to mimic pre-development hydrology of the site.
- Conservation requires delineation of hydrologically sensitive areas
advantages of indigenous knowledge (3)
- Co-produced knowledge
- Improve decision making
- Advance modern science
water year definition (2)
- 12 month period when the stream flow is minimum
- encompasses the wet season without dividing it
unit hydrogrpah theory (4)
baseflow: lower portion of the discharge hydrograph, which varies slowly through the year.
unit volume: 1 cm of excess rainfall over the catchment area
excess rainfall: the remaining rainfall after allowing for rainfall losses
duration of unit hydrograph: duration of effective rainfall
