Flashcards in previous exam Qs Deck (28)
explain the different mechanisms of precipitation
1. frontal (approaching cold air forces warm air up and rain falls)
2. convective (warm air rises from all directions and rain falls)
3. orthographic (moist air rises and rain falls on mountains, dry air blows down other side of mountains)
Catchment is divided into polygons, one for each station. Polygon around each station contains the portion of the catchment closest to that station. mean area rainfall is calculated as a weighted average with each polygon.
Hypsometric method of calculating mean area rainfall
orthographic effects are taken into account and a topographical map is required.
Station elevation is plotted against rainfall.
Then, elevation is plotted vs. area above specific elevation = hypsometric curve.
Divide the hypsometric curve into elevation zones.
Calculate the mean area rainfall as the sum of the products of the average rainfall and the fraction of the area accounted for by that zone.
three main reasons for data errors
1. transcription and transmission errors
2. recording errors
3. measurement errors
general framework for identifying errors
1. site inspection: condition of the gauge (maintenance, leaks), condition of surrounding area.
2. data inspection: look for obvious errors/patterns (time series plots) and test consistency (changes in data collection procedures or other local conditions) using the DOUBLE MASS CURVE (cross check with other stations).
causes of errors (abrupt and gradual)
abrupt: change in location, damaged instrument, change of instrument, change in observational procedure, change in condition around gauge.
Gradual: growth of vegetation around instrument, gradual damage (rust)
effect of time interval on variability of the different mean area rainfall estimates obtained by different techniques.
in 1 year all methods yield comparable values within 10% deviation. As the time period increases, the methods deviate even less, showing that the simpler methods are practically as good as the complex ones. for areas with a lot of variation, the hypsometric curve method is preferable.
explain the basis of estimating rainfall with radars
radars emit electromagnetic energy which is partially scattered, absorbed and reflected when it hits a cloud. Some of the energy is reflected back to the transmitter.
the average power of the reflected energy is related to the radar reflectivity, which is in turn used to estimate the rainfall.
explain the basis of estimating rainfall by satellites
When the cloud temperature drops below -40 degrees celsius, there is rainfall occurring below. These clouds are known as cold clouds. Cold Cloud Duration (CCD) is the time spent by an area having cloud-top temperatures below this threshold.
CCD is related to rainfall.
Discuss the factors to be considered when installing a rainfall gauging station.
- maintenance requires skilled personnel
- availability of personnel to read gauges
- power supply
- transcription and transmission errors
- surroundings (e.g. exposure of gauge to wind).
factors to consider when installing river flow gauging station
- risk of failure during extreme events (use instrument that can withstand extremities)
- stability of river cross-section (sedimentation and erosion)
- power supply
how do you calibrate the v-notch weir for field applications?
- collect synchronous measurement for stage and discharge data
- stage measured with staff gauge
- discharge measured by velocity area or dilution discharge
- data collected should be representative and cover range of data shown in the table
- compare measured discharges with those obtained using theoretical rating curve
- adjust parameters of theoretical curve (discharge coeff. and exponent) as required
factors which affect soil infiltration
- slope (slope increases, infiltration decreases)
- soil type - pore size and distribution (coarse have higher capacity)
- initial/antecedent soil moisture content
- soil temperature (frozen soils are dense and less permeable)
- land cover (vegetation roots perforate soil and increase infiltration capacity)
basic assumptions made in the CURVE NUMBER (CN) method for estimation rainfall losses
- infiltration loss model
- allows for initial abstraction losses (interception and depression storage and infiltration before storm runoff occurs)
- this method assumes that the runoff starts after the initial abstraction volume is satisfied.
- P > Ia, F/S = Pe/(P-la)
main problems with the unit hydrograph method
- time invariance: unit hydrograph assumes it is the same regardless of when it occurs. This is a problem because the relationship might change over time as a result of: climate change, initial soil moisture content (storms close together, higher runoff in 2nd storm), land cover (deforestation), and temperature.
- unit hydrograph is only applicable to one catchment
- uniformity in time and space are also rarely met.
impact of climate change on the principal assumptions made in frequency analysis of hydrological data
- sample is random from the population
- parameters of the distribution can be determined from the sample
- the magnitude to return period relationship doesn't vary with time.
Climate change could affect randomness of the population. It may cause non-stationarity in the statistical properties of the time series.
explain the basis used in constructing probability plotting papers and how they can be used for frequency analysis of hydrological data
Probability plotting paper is one on which the graph of a particular cumulative density function, F(x), plots as a straight line. This is achieved by distorting the scale in a plot of F(x) vs. x. The goal is obtain a relationship of the form x = ay + b, where y is a function of F(x). The use of probability paper in hydrology involves ranking data according to their magnitude and assigning probability plotting positions (exceedance probabilities) to the ranked data. The ranked data is then plotted against the plotting position in the probability paper. A line of best fit is drawn and the parameters of the distribution are estimated based on the slope and intercept of the best fit. Usually used in conjunction with a 2-parameter distribution.
Discuss the main weaknesses of estimating the parameters of probability distribution using the least squares method.
- They are very sensitive to errors and outliers!
- the probability is suitable for 2-parameter distribution.
- Extrapolating the graph can cause discrepancies
- higher values of least squares are inaccurate.
Discuss river flow measurements based on using the velocity area method
measurements of the flow cross sectional area and the flow velocity are taken at several locations across the river.
the discharge of each segment is calculated using Q = VA.
Total discharge of the river is found by summing the discharge of each segment in a section of the river.
describe the types of current meters used for measuring velocity in rivers and the principals involved in their operation
- rotating current meters: rotating element placed in flow. it revolves with an angular speed proportional to the velocity of the flow.
- electromagnetic current meters: water flows through magnetic field produced by sensor, EMF induced in the fluid which is proportional to flow velocity
principal assumptions made in frequency analysis
- sample assumed to be random sample from the population
- variation of the magnitude of the event can be described by a probability distribution (e.g. normal, gumbel, etc)
- relationship between magnitude of an event and its return period doesn't change with time (stationarity of X-T)
explain the basis of estimating the parameters of a probability distribution by the method of ordinary moments
- theoretical relationship between moments and the parameters of the probability distribution is obtained
- calculate sample estimates from the moments
- evaluate sample estimates to the theoretical relationship
- solve resulting equations
discuss the role of catchment slope and soil permeability in rainfall-runoff relationships
- increased slope = less infiltration and more run-off. Also larger peak runoffs, and reduced time to peak runoff
- increased permeability = more infiltration and less run-off
3 fundamental principles implied in the development of the unit hydrograph
1. superposition: two successive blocks of effective rainfall intensities can be obtained by adding the two individual storm runoff hydrographs
2. time invariance: unit hydrograph is the same regardless of when it occurs
3. proportionality: two different effective rainfall intensities, but of equal duration, produce storm runoff hydrographs which at any time the coordinates are in the same proportion to each other.
causes of inconsistency in rainfall records at a particular station
- instrument damage
- change of instrument
- change in condition around gauge
- change in location
- change in observational procedure (observer)
advantages of estimating rainfall with radards over rainfall gauges
- can provide real-time information
- more detailed than gauge estimates
- more valuable in extreme cases
- provides measurements of temporal variability
- has greater range and can cover a larger area
discuss impacts of urbanisation on the hydrological cycle and its effects on estimation of runoff using the curve number method
urbanisation decreases the possible infiltration of rainfall into the soil, as the soil percolation decreases; therefore, surface runoff increases. A decrease in infiltration will increase the CN value. urbanisation also decreases the number of plants thus decreasing evapotranspiration