Hydrological Cycle Flashcards
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Name the 3 main mechanisms of precipitation?
Frontal, Convective, Orthographic
Explain “Frontal” precipitation
Cold air pushes warm moist air up
Explain “Convective” precipitation
Warm moist air rises due to heated ground surface (occurs in summer from solar heating of the ground)
Explain “orthographic” precipitation
Moist air is pushed up due to topography of the land
Define precipitation
Any product of the condensation of water vapour that falls due to gravity (rain and snow)
Define infiltration
The portion of rainfall that enters the soil
Define surface runoff
The portion of precipitation that flows on the land surface
Define evaporation
Process of the hydrological cycle that causes a phase change of water from liquid to vapor, final product of process is vapour in atmosphere
Define Transpiration
The evaporation from plants through the stomata openings
True or false
We try to mimic previous Hydrology
True
Define watershed (catchment)
Area of land that collects water and drains to a single outlet
What are the advantages of indigenous knowledge
Indigenous knowledge has a holistic of community and environment
Source for adapting to climate change
Oberservation driven and passed on from word of mouth
List three ways of rainfall estimation/measurement, also say whether they are direct or indirect
Rainfall Gauges (direct)
Radar (indirect)
Satellite (indirect)
Problems with rainfall gauges
Accessibility Require matenance by skilled personnel Need to be powered (in some cases) Vandalism Exposure of gauge may effect accuracy Redundancy
Name the 4 main methods for estimating mean areal rainfall
Arithmetic method
Thiessen polygons
Isohyetal
Hypsometric method
Explain the arithmetic method for rainfall estimation and when this method should be used
You take the average of the measured rainfall values in the catchment area.
Use when:
- topography is flat
- the gauging stations are distributed evenly over catchment
- density of gauging stations is adequate
- not too much variability of rainfall
Explain the Thiessen polygon method
Divide catchment into set of polygons (one for each station)
The polygons represent the area of catchment nearest to station
Polygons are made by drawing lines joining stations then off those lines drawing perpidicular bisectors
List advantages of Thiessen polygon method
Simple
Stations outside catchments can be included
List the disadvantages of Thiessen polygon method
Not suitable for mountainous area (doesn’t take into account orthographic effects)
Have to draw new set of polygons if rainfall gauges are added or removed
Explain the Isohyetal Method
You draw an Isohyetal map, this map is made up of Isohyets which are contour lines that show equal rainfall depths. The mean areal rainfall is calculated by using the waited average rainfall depths between isohyets. NOTE I.E. You find the average of the rainfall depths over succsessive isohyets and multiply by area between isohyets. Sum of these is the mean depth
Disadvantages if Isohyetal method
Drawing of Isohyetal map can be tedious and time consuming. The resulting map is subjective so final results may vary.
What are the pros/cons of the hypsometric method
Pros takes into account orthographic effects
Cons requires a topographical map
What factors affect infiltration
Slope (large slopes less infiltration)
Land cover (vegetation causes higher infiltration)
Soil type (increased void space causes higher infiltration)
Soil temperature (frozen soils become dense and less permeable)
Initial moisture content(if the MC is high infiltration rate is lower
Depth of ground water table (linked to moisture content)
What are river flow measurements needed for
Design and water management of dams
Flood forecasting and control
Irrigation
Energy generation (hydro)
Management of water quality in rivers
Scientific research
Impacts of climate change on hydrological cycle
Change in average surface temperature (can change evaporation rates)
Change in average precipitation
Change in average sea level
Define remote sensing
Indirect rainfall measurement, radars/satallites
Uses of rainfall estimation
Riverflow forecastinf
Crop forecastinf
Famine warning system
Malaria warning system
Principle if estimation by radar
Radar emits electromagnetic energy which is partially scattered, absorbed and reflected when it hits the clouds
Some of the reflected energy returns to transmitter
The amount of energy reflected can be used to estimate rainfall
Indirect river gauging methods
Empirical rating curves (natural controls)
Theoretical rating curves (artificial controls such as weirs and flumes)
Direct gauging methods
Velocity - Area method
Dilution method
List the fundamental principles of unit hydrograph theory
Proportionality
Super position
Time invariance
Explain proportionality in terms of the Unit Hydrograph Principles
Two different effective rainfall intensities, but of equal duration produce hydrographs of storm runoff, the ordinates of which, at any given time are in the same proportion to each other as the effective rainfall intentisies.
What is the basic assumption of CN method
Runoff occurs immediately after initial abstraction volume is satisfied (once all soil voids are filled)
What are the principle assumptions made in frequency analysis
The sample is assumed to be a random samplr
The variation of the magnitude of the event can be described by a probability distribution
The relationship between the magnitude of the event and its return period do not vary with time
