Environmental Flashcards
(241 cards)
1
Q
The water cycle?
A
Atmosphere ->
Precipitation & snow ->
Surface runoff, infiltration ->
Ground water flow -> ocean
Evaporation
2
Q
River discharge?
A
The flux of water through the river cross section of a point along a river
(L^3 T^-1) and (LT^-1) for runoff)
3
Q
Hydrograph?
A
Plot of discharge over time
4
Q
Q= vA (river)
A
River flux = average velocity * area of cross section
5
Q
River velocity meters? (3)
A
- valeport propeller meters
- acoustic Doppler velocity profilers (ADVP) larger rivers
- measure river height (stage) and use stage discharge graph to predict discharge
6
Q
Weir?
A
A rise in a channel bed which creates sub- critical upstream flow and super critical downstream flow with the critical section at the weir
7
Q
Where can you derive a reliable theoretical stage- discharge relationship?
A
Weirs
8
Q
How do you predict an end to a storm?
A
Identify the start of the stormflow by finding the inflexion point in the hydrograph. This often coincides with the start of the precipitation event and the start of the stormflow.
Then times the lag time (peak storm flow- peak precipitation) * 4(N) and then add this to the end of precipitation.
9
Q
How can you get base flow from a hydrograph with stormflow?
A
You can remove stormflow by using a straight line
10
Q
Catchment area?
A
The drainage area contributing to flow at a point on a river
11
Q
Precipitation? (2/4)
A
- rainfall
- snow
- sleet
- hail
12
Q
Hyetograph?
A
Plot of rainfall over time
13
Q
ΔS = P - E - Q - R
A
Change in internal catchment storage = precipitation - evaporation - river runoff - groundwater recharge
14
Q
Humidity?
A
The amount of water vapour in the atmosphere at a given point
15
Q
Absolute humidity (ρw) vapour pressure (e) [mb]?
A
The mass of water vapour per unit volume of air [g/ m^3]
16
Q
Dew point (Td)
A
The point where air parcels have cooled down through condensation enough to become saturated
17
Q
r= (ed/ ea) * 100
A
relative humidity = parcel’s vapour pressure / the saturation vapour pressure at the same temperature * 100
18
Q
What happens to the amount of water in air as temperature increases?
A
It increases too
19
Q
What impact does the pressure gradient have as air makes its way up the atmosphere?
A
It will expand and cool potentially generating precipitation
20
Q
What are four mechanisms that make vertical air movement?
A
- convection
- orographic ascent
- shear ascent
- frontal ascent
21
Q
Convection?
A
Localised heating at surface produces buoyant air parcels
22
Q
Orographic ascent?
A
Air forced to flow over an obstacle
23
Q
Shear ascent?
A
Differing wind velocities with height induce atmospheric turbulence in all directions, including vertical ascent
24
Q
Frontal ascent?
A
The meeting of air masses of different origins and properties results in the cooler ( more dense) air undercutting the warmer (less dense) air. This leading to widespread ascent, which, in turn, can also give rise to localised connective ascent
25
Three ways of measuring precipitation?
- manual rain gauges
- automatic rain gauges
- tipping bucket rain gauges
26
Three sources of errors for rain gauges?
- deflection of air by rain gauge ( horizontal flow)
- upward deflection over gauge (turbulent flow)
- downward deflection of air over gauge (turbulent flow)
27
How does a remote sensing weather radar work?
A narrow beam of microwave electromagnetic energy undergoes Rayleigh scattering when the beam encounters water droplets. A certain proportion of radiation is back scattered and can be measured
28
Spatial variability of precipitation*
Precipitation is often measured at a point scale. To calculate and analyse the water balance of a catchment, we need the flux over the basin area A. Therefore, it is often necessary to interpolate precipitation to obtain a spatial average. Many methods exist. But note that:
- the network density affects spatial accuracy
- the required distribution of stations is related to data requirements and physical features
- the quality of interpolation depends on rainfall type, typography, time scale
29
Isohyets?
Lines of equal precipitation across the catchment drawn by a skilled analyst
30
λE=Rn +C +V + G - Ps
λ= latent heat of vaporisation of water
E= evaporation
Rn= net radiation
C= sensible heat transfer
V= change in energy storage
G= energy exchange with the ground
Ps= energy consumed by photosynthesis
31
β= C/ λE
Bowen’s ratio = sensible heat transfer/ latent heat of vaporisation of water * evaporation
32
Factors of evaporation?
Meteorological:
- solar radiation
- relative humidity
- wind speed
Physical:
- salinity (sea water evaporates less than fresh water)
- water depth
- size of the water surface
33
Water cycle in plants?
Precipitation is collected
->
- Interception loss
-stemflow
- through full
34
Factors affecting interception loss?
- interception storage capacity of the vegetation cover
- type and morphology of the vegetation cover
- velocity of evaporation
- duration and intensity of the precipitation event
- precipitation event frequency
35
Evapotranspitation (Et)?
The combined process of direct evaporation and transpiration as plants enhance evaporation through root water uptake and transpiration
36
Factors of transpiration?
- number, size and distribution of stomata
- the thickness and permeability of the epidermis
- the area of internal surfaces exposed to intercellular spaces
- the arrangement of the vascular tissue
- solar radiation, stomata close at night, which reduces transpiration
37
Et0 =
(0.408Δ(Rn-G) + γ(900/(T+273)) u2 (es-ea)
———————————————————
Δ+ γ(1+0.34u2)
Et0 = initial evapotranspiration
Rn= net radiation at the crop surface
G= soil heat flux density
T= mean daily air temperature
u2= wind speed at 2m height
es= saturation vapour
ea= actual vapour pressure
Δ= slope vapour pressure curve
γ= psychrometric constant
38
Ep= Kc Et0
Ep= potential ET
Kc= crop or vegetation coefficient
Et0 = reference ET
39
Et = Kc Ks Et0
Et= actual ET
Kc= crop or vegetation coefficient
Ks = water stress coefficient
Et0 = reference ET
40
How is stem flow measured?
Using simple setups of water collection combined with a water flow meter
41
How is through fall measured?
With rain gauges installed under the canopy. Because of the spatial heterogeneity of the canopy, several rain gauges are required to get a representative sample
42
How can Et be measured?
With lysimeter experiements. A lysimeter contains a known volume of soil including a vegetation layer. The total mass of the soil volume is frequently weighed. By closing the water balance, the evaportranspiration flux can be obtained. This needs careful monitoring of precipitation and other changes in mass. Given the complexity of lysimeter experiments, set values are typically calculated using hydrometeorological measurements combined with empirical crop characteristics.
43
Unsaturated zone?
The portion of the subsurface above the groundwater table. The soil and rock in this zone contains air as well as water in its pores.
44
3 outgoing fluxes + 1 property of the unsaturated zone?
- overland flow
- recharge
- evaporation
- has water storage capacity
45
How is the unsaturated zone created?
The unsaturated zone soil is formed by disintegration and decomposition of rocks by physical and chemical processes, and by the activity and accumulation of the residues of numerous plants and animals. It is a complex mixture of solids both rocks and organic materials, liquid and gases
46
ρs = Ms/ Vs
Density of solids = mass of solids/ volume of solids
47
ρB = Ms/ VT
Dry bulk density = mass of solids/ total volume
48
ρT = Ms+ ML/ VT
Total bulk density = mass of solids + mass of liquids / total volume
49
ε = VL+ VG / VT
Porosity = volume of liquids and gas / total volume
50
e =VL + VG / VS
Void ratio = volume of liquids and gases / volume of solids
51
ε = e/ (1+e)
Porosity = void ratio/ (1+ void ratio)
52
θ= VL/ VT
Volumetric moisture content = volume of liquids/ total volume
53
θG = ML/ MS
Gravimetric moisture content = liquid mass/ solid mass
54
Fluid potential
Mechanical energy per unit mass of fluid
55
Capillarity force?
A result of the surface tension at the interface between the soil air, soil water and soilids
56
Absorption force?
Occurs on surfaces of soil particles due to electrostatic forces in which the polar water molecules are attached to the charged faces of the solids
57
Osmosis force?
Results from the presence of solutes in the water
58
Soil water potential?
The potential energy of soil water relative to that of water in a standard reference state. Thus is determined by capillarity, absorption, osmosis and gravity
59
h = 2*γ*cosθ / ρgr
h= height of water from capillary tube water top to water table
γ = the liquid air surface tension
θ= the contact angle
ρ = density of the liquid
g= gravimetric constant
r = radius of the tube
60
Explain the relationship between pore sizes and capillary forces?
Soil pore spaces act like capillary tubes. Therefore, the soil will retain water against the force of gravity. The effect decreases rapidly with increasing radius of the tube, and therefore water will be retained stronger in small pores than in longer pores.
61
Matric potential?
The potential energy of water related to the capillary retention (matric suction)
62
Ψ = Φ/ ρg
Water pressure head= matric potential / density * gravimetric constant (9.81)
63
What will the pressure head associated with capillarity always be?
Negative compared to the reference position outside the soil
64
Water retention curve?
Gives the empirical relation between ψ (pore pressure head/ matric suction). The higher the volumetric water content the lower the matric suction
65
What happens when the matric suction = 0
The soil is completely saturated
66
What happens when force is applied to saturated soil?
Air will replace the water gradually. Water with lowest tension (large pores) will be released first
67
Saturation point?
The maximum soil water content. All pores are filled
68
Field capacity?
Occurs when excess water has drained away. Beyond this point, the soil will not drain further because retention forces exceed the gravitational force
69
Wilting point?
Maximum suction that plants can apply to extract water from the soil
70
h= ψ + z
Hydraulic head = pore water pressure head (matric suction) + gravitational head
71
v= -k(θ) Δh
v= velocity
K(θ) = hydraulic conductivity
Δh= total hydraulic head gradient in 3 dimensions
72
Saturated conductivity Ks
The hydraulic conductivity in saturated conditions
73
Infiltration capacity?
The saturated conductivity at the soil surface
74
Explain the relationship of hydraulic conductivity (K) and matric potential (ψ)
Water moves quicker through larger pores than though smaller pores. When a suction is applied, the larger pores will empty first, forcing water to flow through the smaller pores. The shape of the curve depends on the pore distribution in a type of soil. Sandy soils tend to have larger pores than clayey soil and will therefore have higher conductivity.
75
How do forces work in the unsaturated zone?
Flow in the unsaturated zone is predominantly vertical under the influence of gravity. It will recharge the saturated zone, causing the water table to move. This may cause pressure gradients which will force water to flow horizontally.
76
What acts as the storm flow and base flow in the unsaturated soil?
Storm flow is the runoff/ ground water flow. Base flow is the ground water recharge
77
TDR (Time domain reflectrometry)?
Used to measure soil moisture. The dielectric constant of soil is strongly dependent on its water content. The dialectic constant is determined by the travel time of an electromagnetic wave that propagates through two or more parallel metal rods in the soil
78
SMD?
Soil moisture deficit
= Σ Vg/VT
79
AWC?
Actual water content
= ΣVL/ VT
80
Potential evapotranspiration? Ep
Occurs when enough water is available
81
The actual evapotranspiration? Et/ Εa
May be lower due to water stress than potential evapotranspiration
82
Reference evapotranspiration Et0
The evapotranspiration of a sufficiently watered reference crop, typically grass
83
What two thinks is Et limited by?
Available water and available energy
84
Groundwater?
Subsurface water that occurs beneath a water table in soils and geological formations that are fully saturated
Subsurface water at or greater than atmospheric pressure
85
Aquifer?
A saturated permeable geological unit that stores groundwater and allows it to flow under normal conditions.
86
Aquitard?
A less permeable bed in stratigraphic sequence
87
Aquilude
A saturated geological unit that is incapable of transmitting significant quantities of water
88
Unconfined aquifer?
An aquifer with a free water surface as the upper boundary (the water table or phreatic surface) at which pressure is atmospheric
89
Confined aquifer?
An aquifer confined between two aquitards or aquicludes such that pressure in the aquifer are everywhere greater than atmospheric pressure
90
Valley aquifers?
- generally found in temperate climates where the soil is porous and permeable
- rainfall infiltrates and saturates the rock to a level called the phreatic surface or water table
- groundwater drains to topographic lows and exits as springs or streams
91
Valley aquifers in arid zones?
- In arid areas rainfall is much lower than potential evapotranspiration and surface recharge is almost zero
- however valleys may carry water from mountainous areas or from flash floods, which bring large quantities of water for a short time
- this water usually infiltrates through the river bed into the aquifer and constitutes the only recharge mechanism
- therefore the water table is higher beneath the valleys than elsewhere
92
Alluvial aquifers?
- unconfined aquifer situated in alluvial deposits found along the course of a stream or river
- water in aquifer is generally in equilibrium with that of the stream, which alternately drains and recharges it
93
Perched aquifer?
- a saturated lens of relatively low permeability bounded by a perched water table
- perched aquifers can provide minor sources of supply but can suffer rapid changes in water level since the storage involved is usually relatively small
- unconsolidated sediments are commonly interbedded, making the determination of the true water table sometimes very difficult in practice
94
Surface water vs groundwater dominated catchments?
In surface water dominated catchment:
- river flow reacts quickly to rainfall events
- therefore are periods of very low flow
In groundwater dominated catchment:
- river flow is less flashy
- substantial summer flows
- most of the flow (the base flow) is supplied by groundwater (see red line) through springs and upwelling in river channels
95
Groundwater characteristics*
- Areally distributed: In contrast to surface water, which is usually concentrated in streams. The relatively wide distribution of groundwater often makes it more readily available for development
- Time scale: groundwater is slow response, low velocity medium. Even small aquifers have time constants in the order of weeks or months. Large aquifers may have time constants of many years
- Economics: capital - groundwater usually requires relatively low capital investment. In contrast, costs for surface water are often very high when dams and other hydraulic structures are needed. Operational: pumping costs may form a substantial proportion of the cost of delivering the water. In general operating costs tend to be higher for groundwater than for surface water where gravity is often the natural delivering force
- Data: field data for modelling or analysis in groundwater are usually scarce, often at a few points only extensive interpolation and inference may be necessary, based on experience and judgement
96
Where is groundwater stored?
- In the voids or pore spaces in rock
- this may be in the minute spaces between the grains of a sandstone or in the small cracks and fractures that are more usual in limestones and chalk
97
n = volume of voids: total volume of the rock
Total porosity
98
ne = volume of voids accepting water/ total volume of the rock
effective porosity
99
nd= volume of voids drained by gravity/ total volume of the rock
Drained porosity or specific yield
100
nk = volume of flowing water/ total volume of the rock
Kinematic porosity
101
Why does not all volume of voids equate to volume of flowing water?
- flowing volume is dependence on the pore size distribution and often similar to the drainage porosity
- some water is held against gravity by surface tension
- some voids are too small to accommodate water molecules or isolated from other pores
102
The representative elementary volume?
Is sufficiently large so that the effects of the fluctuations from one pore to another are negligible
And sufficiently small so that large scale changes in the structure do not influence the result
103
How is porosity directly measured?
Measure the total volume from sample dimensions or by volume of liquid displaced after surface has been made impermeable. Dry the sample in an oven for 24 hours at 105C. Obtain the dry weight then inject a fluid. Determine weight of added fluid and hence the pore volume.
104
How is porosity indirectly measured?
With the exceptions of clays, most rock minerals are poor conductors and any flow of electricity occurs in the liquid phase. The resistivity is therefore dependent on porosity. A formulation factor F is defined by F = resistivity of the rock/ resistivity of water contained in the rock
105
Fluid potential?
A physical quantity capable of measurement at every point in a flow system whose properties are such that flow always occurs from regions in which the quantity has higher values to those in which it has lower regardless of the direction in space
Fluid potential for flow through porous media = mechanical energy per unit mass of fluid
106
w=mgz
w= potential energy
m= mass of fluid
g= gravitational acceleration
z= elevation
107
w = mu^2/2
w = kinetic energy
m= mass of fluid
u= velocity of fluid
108
w = m Σ dP/ρ
W = Elastic energy
M= mass
P = fluid pressure
ρ = density
109
Φ = w1 + w2 + w3
Fluid potential = potential energy m+ kinetic energy + elastic energy
110
h = z + u^2/2g + Σ dp/ρg
h = head
z= elevation
u= velocity
g= gravitational acceleration
P= pressure
ρ = density
111
What assumptions can be made for h= z + P/ρg = z + ψ
- In porous media velocities are generally very small u^2 = 0
- Incompressible fluid ρ =/ f(P)
- reference pressure set to atmospheric P0=0
Although these assumptions are used routinely in groundwater they are not always valid
112
q = -K dh/ds
q= volume flux
K= hydraulic conductivity
dh/ ds = hydraulic gradient
113
How is the observation well used to measure hydraulic head?
Essentially an open borehole. Water level in such a well represents the average hydraulic head over the length of the open section. In unconfined aquifers, the water level in an observation well is often assumed to be the level of the prevailing water table
114
How is a piezometer used to measure hydraulic head?
A device for measuring hydraulic head at a point in the ground. The water level in an open piezometer well is not necessarily the same as the water table level in unconfined conditions
115
h= zm- dw
h= hydraulic head
zm = elevation to flange cover from datum
dw= dip distance to water table level
Dip distance is the length of the cable from an electric water level meter or dipper
116
Barometric logger?
Logging air pressure at the same time stamps as the level logger
117
Level logger?
Measuring total pressures using a pressure transducer connected to a data logger logging at specified time intervals
118
qx = -Kxx dh/dx
qx = volume flux in x direction
Kxx = hydraulic conductivity in x direction
dh/dx = hydraulic gradient over x direction
Can be done for y and z
119
dh/ds = i = sqrt (A^2 + B^2)
i = hydraulic gradient
h = Ax + By + C
120
θ = arctan (A/B) + 180D
θ = direction of flow
h= Ax + By + C
121
Q’ = HK (h0-h1/L)
For steady flow in a confined aquifer
Q’ = volume flux
HK = transmissivity = hydraulic conductivity * height of aquifer
h0 = upstream height to water table
h1 = downstream height to water table
L= length between h0 and h1
122
h-h0 = (h1-h0) x/L
For steady flow in a confined aquifer
h= groundwater level
h0 = upstream height
h1 = downstream height
x= distance along length relating to h
L= length between h and h0
123
Q’ = W (x-L/2) + K/2L (h0^2- h1^2)
Steady flow in an unconfined aquifer
Q’ = volume flux
W= uniform force or weight
x= distance along L
L= length distance between h0 and h1
K= hydraulic conductivity
h0 = upstream height
h1 = downstream height
124
h^2 - h0^2 = W/K(Lx-x^2) + (h1^2-h0^2) x/L
Steady flow in an unconfined aquifer
h= height relating to x
h0 = upstream height
h1 = downstream height
W = weight / uniform force
K = hydraulic conductivity
L = length between h0 and h1
x = distance along L relating to h
125
What assumption can be made for unconfined aquifer of with no recharge?
W= 0
126
Homogenous?
Independent of position in a geological formation
K(x,y,z) = c (constant) = hydraulic conductivity
127
Heterogeneous?
Dependent on position K(x,y,z) =/ c (constant)
K= hydraulic conductivity
128
Isotropic?
Independent of direction?
K=/ f(θ)
K= hydraulic conductivity
θ = angle between the horizontal and direction of measurement
129
Anisotropic?
Dependent on direction?
K= f(θ)
K = hydraulic conductivity
θ = angle between the horizontal and direction of measurement
130
Transversely isotropic?
A layered formation where hydraulic conductivity can be considered as having only horizontal and vertical components Kx= Ky =/ Kz
131
qx = -Kxx dh/dx - Kxx dh/dy - Kxz dh/dz
qx= volume flux in the x direction
K= hydraulic conductivity in direction
dh/dx = hydraulic gradient over x
132
What can you do if a layered formation is a sequence of discrete layer each homogenous and isotropic with hydraulic conductivity Ki?
The whole can be considered as a single homogenous anisotropic
133
Kz= d/ Σd/K
Total hydraulic conductivity over depth = total depth / sum of each depth/ each hydraulic conductivity
134
Kx = ΣKd /d
Total hydraulic conductivity in x direction = sum of each depths*each hydraulic conductivity / total depth
135
τ = μ du/dz
Shear stress = dynamic viscosity * velocity/depth
136
u’ = -1/4μ * dp/dx * (R^2-r^2)
u’= velocity
μ = dynamic viscosity
dp/dx = pressure gradient over length
R= radius
r= radius along radius
137
Q’ = -π R^4/8μ * dP/dx
Q’ = time averaged volume flux through one pipe
R= radius
μ = dynamic viscosity
dP/dx = change in pressure over length
138
Q= NQ’
Q= total flow through a block of soil
N= number of through a block of soil
Q’ = volume flux through one pipe
139
ne = vol of voids accepting water/ total volume of rock = πNR^2/ A
ne= effective porosity of the system
A= cross sectional area
N= number of pipes in block
R= radius of pipe
140
K= ρgneR^2/8μ = ρgk/μ
K= hydraulic conductivity
ρ = density
g = gravimetric acceleration
ne = effective porosity of the system
R= radius of pipes
μ = dynamic viscosity
k = intrinsic permeability
141
Q= NQ’ = ν π NR^2
Q= total flow through porous block
N= number of pipes
Q’ = volume flow through a single pipe
ν = mean pore water velocity
R= radius
142
Specific yield ? (Sy)
Applies to unconfined aquifers.
Is the volume of water released per unit area as a result of a unit fall in the water table elevation
The water arises from dewatering of the pores as the water table is lowered
143
Sy= ΔV / AΔz
Sy = specific yield
ΔV = change in volume of the moisture content in the elevation above water table
A= Area
Δz = change in water table
144
Storativity?
- applied to confined aquifers
- volume of water released per unit area over the entire aquifer thickness of the aquifer as a result of a unit fall in the potentiometric surface
- the water arises from the small reduction in the aquifer porosity and fluid density which result from the drop in fluid pressure
145
S= HSs
Storativity = aquifer thickness * specific storage coefficient
146
What causes the change in storage?
Effective stress and fluid stress
147
How does an increase in effective stress effect porosity?
A small decrease in porosity
148
What is flood risk?
Probability of the flood occurrence x consequences from damage if flooding happens
149
Four types of risks of flood risk?
Economic risk: economic impact of a flood
Health risk: health hazards caused by a flood
Social risk: stress caused by a flood
Environmental risk: environmental impact of a flood
150
Six types of flood?
1. Coastal flood
2. Fluvial (river) flood
3. Flash flood
4. Groundwater flood
5. Pluvial flood
6. Dam/ embankment failures
151
Coastal flood?
The coast is flooded by the sea
The cause of a surge is a severe storm
Tidal, storm or tsunami surges
152
Fluvial (river) flood?
Major rivers overflow their banks after extended period of rainfall
Relatively slow process, thus easier to alert people
153
Flash flood?
River overflows as a response to very high intensity rainfall
Fast process, water may carry away heavy objects
154
Groundwater flood?
Water rising up from the underlying rocks or springs
Most likely in low laying areas underlain by permeable rocks
155
Pluvial flood?
Rain ponds on the surface when drainage system capacity is over exceeded
Most likely to happen in urban environments
156
Flood risk management cycle?
Analysis: prevention & preparation
- risk mapping
- natural retention of water
- technical flood prevention
- land management
- preparation of defence and civil protection
- public awareness
- flood warning
Flood: Response and recovery
- flood defence
- emergency evacuation & civil protection
- reconstruction
157
Natural flood management?
When natural processes are used to reduce the risk of flooding and coastal erosion
Examples include restoring bends in rivers, changing the way land is managed so soil can absorb more water and creating salt marshes on the coast to absorb wave energy
158
WDIMOD?
Can analyse water supply and wastewater systems performance using river water quality as additional performance indicator and uncover unintended consequences of planning options
159
How would a coordinated strategy be more efficient?
Agricultural activities are found to drive river quality in wet periods while urban activities are the key source of pollution in dry periods.
A coordinated strategy (fertiliser reduction during wet periods and enhanced wastewater treatment during dry periods) performs comparably to the combined strategy but with higher overall efficiency
160
Flood frequency estimation?
Construct the flood frequency curve. This relates peak flow to some measure of frequency. Hydrology plot.
161
What are the two ways of defining extreme values for flood frequency analysis?
1. Peaks over threshold
2. Annual maxima series
162
Peaks over threshold method (POT)
- choose threshold such that there is a given number of samples; or a threshold which has some physical significance
- discard the peaks which seem to be connected to another peak
- note that low flow years are excluded
163
Annual maxima series?
- the annual maxima approach is more simple
- annual maxima series comprises the largest flood drawn from each year
- in the uk the hydrological year starts in October
164
Annual maxima advantages and disadvantages?
Advantages
- easy to extract the data from a time series
- easy interpretation of P(q>qd)
Disadvantage
- often a small sample
165
Peaks over threshold advantages and disadvantages?
Advantage
- generally more samples
Disadvantage
- need to seperate out the inter- connected peaks
166
When do you estimate Q using POT and Am?
- POT data are used to estimate Q when, the POT record is the same length as the AM record and there are fewer than 14 years of record
- in all other cases, Q is derived from an annual maxima series
167
What are the rules of serially independent flood peaks for POT?
- two independent peaks must be separated by at least 3 times the average time to rise (difference between the start of the rising limb and the peak)
- the minimum flow between two peaks must be less than 2/3 of the first peak
168
P(q>qd) ?
1-P(q
169
T= 1/P(q>qd)
qd= exceedance value
T= average return period
170
What is the probability that that the flood will not occur in N successive years?
[P(q
171
P(x) = (nCx) p^x (1-p)^(n-x)
Probability that flood will occur x times in n years. p=P(q>qd)
172
What is T for POT?
T= n/t (1/ P(q>qd)
173
Pm(q
Weibull plotting position
174
Pm(q
Gringorten formula plotting position
175
Explain plotting positions?
This is because 1/(1-N/N) = inf which is wrong
Advantages
- they are simple
- based on experience that they give reasonable estimates of Pm(q
176
qd = Bz + α
qd= exceedence value
z= -ln { -ln[F(qd)]} (gumbel variate)
Gumbel distribution
177
Points on a soil moisture characteristic?
Saturation point - where matric suction =0
Wilting point
Field capacity
178
RAW = p TAW
Readily available water = this is the amount of water that a crop can extract from the soil without experiencing water stress
p= depletion fraction
TAW = total available water
179
TAW = 1000 (θFC - θWP) Zr
Total available water = the total amount of water available to plants between field capacity and wilting point
θFC= field capacity
θWP = wilting point
Zr= root depth in meters
180
How can you calculate the maximum time between two irrigation doses?
Maximum interval (days) = RAW/ ETc
181
Virtual water footprint?
Virtual water refers to the water used in the production of goods or services. It is an instrument to identify the water footprint of different industrial and agricultural activities, and to optimise global and local trade in terms of water sustainability. For example, the water footprint of a cup of coffee includes the water transpired by the coffee plant, the water footprint of a cup of coffee included the water transpired by the coffee plant, the water used in the processing of the bean, and the water used to produce the cup. A major weakness of the concept is that water is not chemically consumed during the processing and will continue to exist in another loacation, another state or simply be contaminated. To differentiate between these impact, the water footprint uses different colours: blue for water used from surface of groundwater resources; green for evaporated rainwater, and grey for water that is being contaminated.
182
Characteristics of groundwater that make it a useful resource?
- where appropriate geology present, it is an area resource rather than a linear one
- large storage means he is generally resilient to drought
- generally, low cost to set up and run
- good water quality, as soil zone acts as a natural filter
183
What infrastructure and measurements would you need to determine the pore water velocity in an aquifer?
- Need at least 3 observation wells screened across the water table in order to calculate the head gradient Δx,y(h)
- Need borehole logs in order to determine the elevation of the aquifer base zb
- Need to undertake a pumping test to determine aquifer transmissivity T
- Alternatively, use particle size analysis/ permeameter test to determine K (though not as effective)
- Use pumping test to determine specific yield Sy and treat this as an estimate of the effective porosity ήe which is turn is an estimate of the kinematic porosity ήk alternatively use a core sample to calculate the effective porosity
184
K= aL/At ln(h1/h2)
K= hydraulic conductivity
a= cross sectional area of the soil sample
A= cross- sectional area of the soil sample
L= length of the soil sample
t= time for the head to drop from h1 to h2
h1, h2 = initial and final heads at time t
185
B= (6^0.5 / π) * σ
Gumbel distribution from moment matching
σ = sample variance of data
186
A= μ - 0.5772 B
Gumbel distribution for moment matching
μ = sample mean
187
L1 = 1/n Σqj
L moment matching for Gumbel distribution
n= population
q= each flow value
188
L2 = 2/n Σ(j-1) qj /(n-1) - L1
L moment matching for gumbel distribution
n= population
j= number
q= flow
189
B = L2/ ln(2)
L moment matching for Gumbel distribution
Bx+a
190
a= L1 - 0.5772 B
L moment method for Gumbel distribution
191
DDF curve
Plots depth (y) vs duration (x) of rainfall
192
What is the unit hydrograph?
The unit hydrograph is the storm flow hydrograph u(D,T) resulting from a unit depth of net (effective) rainfall uniformly over a catchment at a constant rate for a unit duration D.
193
UH?
Specified by the depth and duration of the net rainfall
194
r= n-m+1 ????
r = no of UH ordinates is (number of times you move it along)
n= number of stormflow values
m= number of rainfall pulses
195
How can you tell if a vegetation experiences some water stress?
Ks<1
196
What is the Kc for large vegetation
> 1
Et based on evaporation of grass therefore Ea higher for larger vegetation
197
Runoff ratio?
Mean (Q) / Mean (P)
198
Explain the potential impacts of deforestation?
- reduction of transpiration which may lead to higher runoff, and potentially higher river discharge into the hydropower reservoir
- part of the increase in runoff may arrive as fast surface flow, so higher peak inflows can occur
- deforestation may reduce the soil infiltration capacity and reduce base flows. This may make streamflow more variable, and hence a larger storage reservoir may be needed to buffer streamflow variability
- deforestation may also lead to soil degradation and erosion, which means that higher sediment loads can be expected. They may fill up the reservoir but also damage turbines and clog water intakes.
- deforestation may free up space for other land use activities, such as agriculture. Given that water stress already occurs in the basin, it is likely that irrigation may be needed. This could reduce water availability in the river if water is abstracted from surface water sources
199
The climate model predicts that the impacts on precipitation and evapotranspiration will be strongest during the summer months. Discuss the implications for the water resources of the basin, and how this trend fits in a global pattern of climate change impacts on the water cycle?
- precipitation is lowest and evapotranspiration is highest in summer, so this is the period when water availability is already most limited. The impact of climate change will further aggravate water scarcity through the combined impact of a reduction in precipitation and an increase in evapotranspiration.
- this enhancement of the extremes is broader phenomenon which is sometimes referred to as drier regions getting drier and wetter regions becoming wetter and can be observed clearly at a global scale, where wet climate such as the humid tropics have a tendency of becoming wetter, and dry regions such as the worlds major desserts will become even drier.
- this trend is worrisome from a water management perspective, as it increases the risk for both floods and droughts and reduces global water security.
- as a side note, however, care should be taken not to overinterpret a single climate model. Climate projections have very large uncertainties and therefore typically an ensemble of climate models is used, as a method to quantify uncertainties in future predictions and take this into account when designing climate change adaptation strategies. In the context of the basin, this means that the water authorities in the basin should plan for a wider range of possible futures than what this single model predicts.
200
What causes the change in mass in a aquifer?
1. An increase in fluid density due to compression from an increase in fluid pressure
2. An increase in porosity from rock matrix expansion from a decrease in effective stress
201
Ss = nρg( cR + cF)
Ss= specific storage coefficient
n= porosity
ρ= density
g= gravitational acceleration
cR= 1/n dn/dP (rock compressibility)
cF= 1/ρ dρ/dP (fluid compresibility)
202
s= he- h = Qw/2πT ln(re/r)
S= drawdown
he = distance from datum to maximum water table
h = distance to water table at point r along
Qw= total flow in a confined aquifer
T= aquifer thickness * hydraulic conductivity
re= radius of influence to maximum water table point (he)
r= distance to point h
203
he^2 - h^2 = Qw/πK ln(re/r) + W/2K (r^2-re^2)
he= far point height datum to water table
h= close point height datum to water table
Qw = total flow in a unconfined aquifer
K= hydraulic conductivity
re= far radius (he)
r= close radius (h)
W= recharge rate
204
Pumping near hydrological boundaries?
- Hydrogeological boundaries in aquifers are physical boundaries to a groundwater flow system such as streams or impermeable barriers
- the well flow equations given so far have been developed for aquifers of infinite extent and physical boundaries represent finite constraints on such flow systems
- however the boundaries can be replaced or represented by an imaginary equivalent flow system that will allow the infinite aquifer equations to be used
- in other words an image well can be used to simulate an aquifer boundary, therefore transforming a semi-finite aquifer flow system into an infinite one in which the radial flow equations can be used
- this is known as the method of images and makes use of the principle of superposition, which can be used with linear solutions
205
When do you + or - wells
Abstraction wells +
Image wells for rivers -
Image wells for impermeable boundary +
206
what do you do for geothermal energy?
In open loop geothermal systems water is pumped from one borehole and passed through a heat exchanger. The heated/ cooled water is then pumped back into the aquifer. This means that there is no net loss of water from the aquifer.
Take away borehole +
Put back in point -
207
s= Qw/4πT * W(u)
u= r^2 S/ 4Tt
W~ ln(1/u) - 0.5772
s= drawdown at a distance r from the well at time t
Qw= pumping rate
T= transmissivity of the aquifer (hydraulic conductivity * thickness)
W(u) = Theis well function
r= distance from the well
S= storage coefficient
t= time since pumping began
208
Key assumptions for Theis solution?
Darcy’s law
Confined aquifer
Homogenous and isotropic aquifer
Uniform initial condition
No recharge
Infinite aquifer
Infinitesimal well
Fully penetrating well
Constant pumping rate
209
What is a pumping test?
A test whereby an aquifer is pumped and its response to the pumping is monitored
210
Step drawdown tests?
Pumping rate and drawdown are plotted against time after pumping started
Well is pumped until drawdown in the well reaches a quasi- steady state.
The flow rate is then increased then held constant until a new QDS is reached
211
Formation loss?
The drawdown represents the energy loss of the formation ( ie the energy required to move the flow of water from the radius of influence to the well screen) and is the formation loss
In practice this loss indicated the energy required to pump the water from the well to the original water level
The linear nature of the loss arises from the fact that the flow through the aquifer is laminar
212
Well loss?
As the water flow approaches well, however, there is convergence in the stream lines which result in a significant increase in velocity in the immediate vicinity of the well. This may result in the flow through the gravel pack surrounding the well screen, along with the flow with the well screen itself becoming turbulent. This effect leads to an additional non linear energy loss which varies according to power of Qw
213
sw = AQw + BQw
Drawdown= formation energy loss + well loss
Where A is the Theis or Thiem equation in booklet
B= varies with well
214
Specific capacity = Qw/ sw
Specific capacity = discharge capacity of pumped well/ drawdown
215
E = 100 AQw/sw
Well efficiency = 100 AQw/ sw
216
E= 100 1/(1+BQw/A)
E= well efficiency with head loss
Qw= flux
Where s= AQw + BQw
217
Slug tests?
In site investigation work, particularly where there is contamination present, performing a pumping test may not be practical. An alternative approach is a slug test. Where a known volume of water is either instantaneously added to or taken from the well. In practice this is achieved using a solid volume which is inserted into or withdrawn from the water column.
218
Ex- situ tests?
Pumping test has the advantage of being done in situ. This means that there is limited suburban to the aquifer formation.
Pumping also sample a greater volume of the aquifer and therefore give an upscale value for either T or K
There are two alternative methods for estimating hydraulic conductivity based on ex situ methods 1) falling/ constant head permeameter 2) particle size analysis
However these effectively provide a point sample and therefore may not be representative of the aquifer as a whole. Also it is likely that the sample with have been disturbed during the drilling process, further affecting the validity of the result
219
Falling permeameter
220
h= H0 exp - (AL/aL) t
h= hydraulic head at time t
H= initial head at t=0
A= sample cross section area
K= hydraulic conductivity
a= area of pipe
L= sample length
t= time
221
Representative elementary volume?
Represents a volume which is sufficiently large so that the effects of small scale fluctuations are negative
Sufficiently small so that large scale changes in structure do not influence the result
222
Explain scales with aquifers? *
All aquifers are heterogeneous at the small scale, even though they may be homogenous at the large scale. However, characterising these properties depends on the scale of the measurement method. The small the measurement scale the more data are needed to obtain effective parameters compared to the larger scale pumping test. Advantage of smaller scale tests, however, is that they can be used to characterise the heterogeneous properties, which is important when considering contaminant transport
223
Water stress?
Occurs when the demand for water exceeds the available amount during a certain period of time
224
Virtual water?
Water used in the production of goods or services
225
Green water?
Volume of rainwater evaporated
226
Blue water?
Volume of surface or groundwater evaporated
227
Grey water?
Volume of water that becomes polluted
228
P= εt εg h ρ Q g
P= generated power
εt= turbine efficiency
εg = generator efficiency
h= head
ρ= density of water
Q= flow
g= gravity acceleration
229
230
RR= Q/P
Runoff ratio = long term average runoff/ long term average precipitation
Used when discharge is unkown
231
Purpose of irrigation?
Aims at Et= Ep
232
I= Ep - P + R
Irrigation = potential evapotranspiration - precipitation + recharge
233
What does Kc depend on?
Crop type and climate such as wind speed and humidity.
Time in the growing year
234
When does a crop cycle graph act linearly
For crop development and late season
235
TAW = (θFC - θWP) Zr
TAW= total available water
θFC= water content at field capacity
θWP= water content at wilting point
Zr= root depth
236
RAW = p TAW
RAW= readily available water
p = depletion factor
TAW= total available water
237
Volume of net rain?
Volume of stormflow
238
UH cons?
- assumes linearity and stationary
- based low needs estimated separately
- effective rainfall needs estimated separately
239
S curve? From large (2 hr 10mm) to small (1 hr slots)
- lag UH by 2 hr slots until the row is 0
- sum up all lags
- lag this by one
- find the difference between last two
Final UH = difference * T1/T2
- Times by mm or time slots to get to different mm/h
- sum with lags these times to get value
- add base flow to get total flow
240
Shortcut lag ?
T2/T1 -1
241
What’s used for lead times between 2-6 hours for forecasting floods?
NWP - numerical weather predictions
Quantified used for 1-2 days
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