Hydrology Flashcards
How is hydrology different from hydraulics?
It applies to processes which occur upstream of the point where detailed hydraulic analysis can be performed
What is interflow?
Lateral movement of water within unsaturated soil
What are the three mechanisms of water volume in soil increasing?
Precipitation
Surface water inflow
Groundwater inflow
What are the three mechanisms of volume of water in soil decreasing?
Surface water outflow
Groundwater outflow
Evapo-transpiration
How is a catchment defined?
The area of all points that can possibly contribute surface water to a particular river
How is flow direction algorithmically found?
The greatest difference between a cell and it’s 8 neighbours in a 9x9 square cell
How can cell algorithms predict where a flow channel will form?
By finding the cells with the most other cells flowing into them
What are the main two ways of measuring precipitation?
Storage gauges
Tipping bucket recorders
Where should a rain gauge be sited?
Level ground
Partially sheltered
Representative of the average for the area
What is the name of a graph of rainfall intensity?
A hyetograph
How does the isohyet method map rainfall?
By drawing contours on a map according to recorded rainfall data
What are the key advantage and disadvantage of an arithmetic average of rainfall?
Very simple
Very inaccurate
What are the key advantage and disadvantage of an Theisson polygons?
Improved accuracy for non-uniform rain gauges
Do not amount for topography
What are the key advantage and disadvantage of the isohyet method?
Most accurate
Highly time consuming - must be redrawn for every point in time
What 3 factors affect the rate of evaporation?
Energy supply for vaporisation
Ability for Vapor to move away from the surface
Supply of moisture on the surface
How is the evaporation rate defined (equation)?
E = -dh/dt where h is depth of water
What is Tp?
The dew point
What is lv?
Latent heat of vaporisation
What is the equation for rate of evaporation, assuming all energy goes into vaporisation?
Er = Rn / lv * ρw
What is Rn?
Net radiation
What is a lysimeter?
A large block of representative soil and vegetation, sealed at the sides and base by watertight barriers and a pipe to measure percolation
What are the two ways of modelling evapotranspiration?
Not limited by vapour transport rate
Not limited by energy supply
How is porosity (ω) defined?
Volume of void space / volume of rock
How is porosity of a large rock sample determined?
A Representative Elementary Volume (REV) is investigated and taken to be equivalent in porosity to the rest of the rock
How does the formation of igneous rock affect porosity?
Intrusive rock has low porosity
Extrusive rock can have very high porosity, but some, like basalt, form detached columns which individually have low porosity
How is effective porosity different from regular porosity?
It describes only the volume of pores through which fluid can flow
What is the equation of effective porosity?
Volume of through-flowing water / volume of rock
How do you measure rock volume not including voids?
Crush it into a powder
What is recharge water?
Water that infiltrates into the unsaturated zone and is not then lost due to evapotranspiration and will reach the water table
How is piezometric head expressed?
h = p/ρg + z
How is pressure head expressed?
H = p/ρg
What hydraulic head term is not included in piezometric head and why?
u^2/2g
Because flow velocity in porous rock is very slow
What is Darcy’s Law for saturated flow?
Q = AKΔh/L
Where Δh is from top of water to bottom of rock, L is simply depth of rock
What is K in Darcy’s Law? What is its equation?
Hydraulic Conductivity
K = kρg/μ
k is called intrinsic permeability
Define the 4 “Aqui-“ terms
Aquifer - Rock which is saturated with water and permeable such that extracting water is economically viable
Aquitard - A less permeable bed
Aquiclude - A bed with almost zero permeability
Aquifuge - Rock that is saturated but neither contains or transmits much water
How can unsustainable use of aquifers affect the local environment?
Removal of fluid can cause the rock to crush in on itself, permanently reducing porosity and causing subsidence
What is the probability of A & B occuring?
P(A) * P(B)
What is the probability of A or B occuring?
P(A) + P(B)
If it can only flood once per year, there are 3 ways that 2 floods can happen in 3 years. How do you find this as a probability?
The 3 ways are mutually exclusive, so sum each probability
What does
(n)
(k)
mean in terms of probability?
n!/((n-k)!k!)
How are probability and return period related?
If time period = T, probability = 1/T
How do you make use of a probability density function in hydraulics?
Integrate it between values of concern
What are the steps for creating a cumulative probability function?
- Create a series of yearly maxima over n years
- Rank them with 1 as the largest and n the smallest
- Cumulative probability is 1-(rank/n)
What distribution can often approximate the cumulative probability graph?
Gumbel distribution
How do you perform the Theisson method for rainfall analysis?
Draw lines connecting each rain gauge to all adjacent gauges
Bisect each of these lines and join to form a polygon
Weight gauge’s measurement by area of polygon
Why is k used as well as K in Darcy’s Law components?
K is dependant on rock and fluid properties
k is only dependant on rock properties
What is the probability of k floods in n years?
(n)
(k) P^k (1-P)^(n-k)
