Surface Hyrology Flashcards
What does surface hydrology allow us to describe?
The fate of a water droplet once it hits the earth (identify –> Characterise –> quantify
How much water is used per person per day in the UK?
140 Litres/day
How much rainfall does London recieve in a year?
650mm/year
What can the water availability in London be described as?
Water scarce
What is the average volumetric flow rate of the Thame?
65 cubic meters a second
How much water is needed to produce 1 kg of beef
15,000L
What is the main assumption in hydrology for the global water cycle?
The total amount of water on Earth is constant
What is the error of a ‘good’ measurement in surface hydrology?
5-10%
What is the easiest flux to measure in the hydrological cycle? Why?
River Discharge
One-Dimensional problem, all the discharge flows in a distinct channel, dimensions of channel can be measured.
Why are fluxes such as precipitation and evaporation so difficult to measure?
They are two-dimensional problems, measuring at specific points is accurate and straight forward, however, spacial averages are much more challenging.
Groundwater and soils have similair limitations but can also become three-dimensional problems
What is the direct method for meauring the volumetric flow rate (m^3 s^{-1})?
Q = change in Volume/ Change in time = Delta V/ Delta T
What is the indirect method for meauring the volumetric flow rate (m^3 s^{-1})?
Integrate the velocity over the cross sectional area (multiply the average velocity by the cross sectional area Q =UA (Y1 fluids))
Lecture 1 slide 19/39
Name one method for caculating the mean flow rate in a river.
Mean-section method
What are some ways of measuring the velocity profile in a river?
- Valeport propellor meter (density of data is very important)
- Acoustic Doppler Velocity Profilers
- Preasure transducer (automatic - multiple data points)
- Staff gauge (easy to check at regular intervals)
What measurement does the water stage give you?
The water height
How are the water stage and the discharge related?
Stage-Discharge relationship
- Often represented graphically
- Formed using manual volumetric flow rate measurements at different water stages
- result must be read off graph
Lecture 1 slide 24/39
What factors affect the stage discharge relationship?
- Vegetation cycles
- Ice cover
- Erosion or sedimentation of channel
What civil engineering structures can allow a reliable stage discharge relationship to be defined?
Weirs and flumes
Weir: rise in the channel bed, creates a sub-critical upstream flow and super-critical downstream flow. Flow over the top of the Weir is critical - allows a reliable relationship to form
Flume: induces a critical flow by reducing the width of a section of the river (sometimes raises the bed). Reduces the risk of the sedimentation behind the control. Critical flow allows for reliable relationship
Lecture 1 slides 26–> 28 /39
What are the main types of weir?
- Sharp-crested
- Broad-crested
- Crump weirs
- Special Weirs
Lecture 1 slides 26 &27/39
Why does the South Tyne have a larger normalized river flow than the River Kennet?
The River Kennet has a permeable chalk layer, water flows through into the aquifer, and so the overall flow rate is lower
What can be said about the dominant flow rate in a river? What does this say about the distribution of the flow rate when plotted on a histogram?
The low flow dominates.
This results in a positively skewed flowrate when plotted on a histogram
Lecture 1 slide 31/39
How do you determine the start of the stormflow?
The local minima after the start of precipitation
How do you find the time lag?
T_L = time@max flow - time @ max precip
How do you find the end of the stormflow
time at end of the preciptiation + the time lag*N
N is usually == 4
What is the ‘catchment area’?
The drainage area contributing to flow at a point on a river
What is ‘evaporation’?
Loss of water to atmosphere from land/water surface
What is the application of the conservation of mass in surface hyrdrology called?
What is it’s equation?
The catchment water balance
dS/dt = P(t) - E(t) - Q(t) - R(t) (1)
\DeltaS = S_t - S_0
= the integrtal between T and 0 of (1)
What is the application of the conservation of energy in surface hyrdrology called?
The catchment energy balance - measure of how much energy has been used to evaporate the water
Lecture 2 slide 8/27
What is the definition of humidity?
The amount of water vapor in a given point in the atmosphere
What is the dewpoint (T_d)?
The temperature at which an ‘air-parcel’ becomes saturated.
What is the definition of relative humidity?
The ratio between the vapour pressure and the saturation vapour pressure at the same temperature
r = (e_d/e_r)*100
What is condensation?
When air masses are colled down to their dewpoint.
What happens when air reaches its dewpoint?
Water starts to come out of the air as droplets
What happens to the pressure gradient in the atmosphere, and what does this mean for precipitation?
As altitude increases, pressure decreases. Because of this gradient, air that is higher in the atmohsphere has a lower dewpoint
Lecture 2 slides 13&14/27
What processes can cause atmospheric movements?
- Convection
- Orographic ascent
- Shear ascent
- Frontal ascent
Describe convection
Localized heating at the surface produdcing buoyant air parcels
Describe orographic ascent
Air being forced over an obstacle, such as wind passing over mountains
Describe shear ascent
Differing wind velocities with height (wind shear) producing atmospheric turbulence, can induce vertical ascent
Describe frontal escent
Air masses from different sources with different properties (warmer/ colder) meet, warmer air goes over the colder air, and leads to convective ascents
Describe the saturation point of a soil
The maximum soil water content, all pores are filled
Describe the field capcaity of a soil
the point at which the retention forces are greater than or equal to the gravitational forces
Describe the wilting point of a soil
the maximum suction that plants can apply to extract water from the soil
The soil moisture characteristic curve relates water content to what?
Matric Potential
