GEOG 408 midterm 2

saturated zone

zone where all available pores are filled by water

pressure head in the unsaturated zone

is negative and becomes more negative in drier soils

field capacity

is the water content at which the gravity drainage rate becomes negligible, it defines the “equilibrium of the soil

permanent wilting point

point where plant suction cannot overcome adhesive forces holding water to the soil grains

Available water content

water that is available to plants, difference between field capacity and permanent wilting point

groundwater zone

zone where water content is equal to porosity

Water table line

point where pressure equals atmospheric pressure

tension-saturated zone

zone where pores are filled due to capillary rise, negative pressure

intermediate zone

water content typically at field capacity

root zone

water content may become less than field capacity due to evapotranspiration

Leidenfrost effect

the hot surface creates an insulating vapor layer that prevents the liquid from boiling away rapidly

infiltration

the process by which water arriving at the surface as rain or snowmelt enters the soil

water input rate

rate at which water arrives at the soil surface

infiltration rate

rate at which water enters the soil

infiltration capacity

maximum infiltration rate

no ponding

infiltration rate equals input rate and is less than infiltration capacity

Ponding (saturation from above)

water input rate exceeds infiltration capacity

Ponding (saturation from below)

water table has risen to the surface, infiltration stops

Infiltration rate decreases over time

pores saturate, decreasing pressure head gradient; clay particles swell when wet, reducing pore space; raindrops can wash soil down, clogging pores

wetting front

the downwards movement of water, if it makes it to the water table it can “recharge” groundwater

Groundwater recharge

natural process whereby water enters the saturated zone

without recharge

the natural discharge of groundwater would result in a steadily decreasing water table

groundwater recharge pathways

infiltration/percolation, surface water, artificial

equipotential

an imaginary line (usually drawn on a cross-section or plan view) where all points have the same flow potential (hydraulic head)

we can only reliably draw streamlines between equipotentials if

the aquifer is isotropic and homogeneous

medium is homogeneous

if hydraulic conductivity (K) is the same at all points

medium is isotropic

if Hydraulic conductivity (K) is the same in all directions

groundwater flow systems

can be local, intermediate or regional

groundwater contributes to

baseflow and event flow

event flow

the surge in river flow observed during a rain event

baseflow

is a consistent, slowly varying contribution to the river

groundwater relationship to surface water

lakes, rivers, and wetlands are typically at groundwater discharge points

flow lines must be drawn

perpendicular to equipotentials to predict groundwater flow

hydrograph

a graph showing the rate of flow (discharge) versus time past a specific point in a river

storm hydrograph

plots precipitation and run-off/discharge over time, include baseflow with these

Runoff mechanisms: Channel precipitation

the fastest way for rain to get into the river is to fall directly

Runoff mechanisms: overland flow

rain rate exceeds inflitration rate creating ponding and overflow

Runoff mechanisms: throughflow

is flow through the unsaturated zone, once it breaks surface tension it flows with gravity downslope into the river

Runoff mechanisms: groundwater

a little bit of infiltration into the ground brings the soil up to saturation and allows more groundwater to start flowing

“flash flooding”, fast flow

promoted by steep slopes, sparse vegetation, impervious soil,

different hydrograph shapes

drainage basin conditions are reflected in the hydrograph shape, size, vegetation, slope, soil types

changing hydrograph

land use changes will change the hydrograph shape, removal of vegetation, less permeable surfaces

Unit hydrograph

a tool that allows us to predict the timing and magnitude of peak flow, in response to a precipitation or snow melt event, does not include baseflow

unit hydrograph assumptions 1

two storms with equal duration and equal rainfall excess produce the same hydrograph, regardless of intensity

unit hydrograph assumptions 2

the amount of discharge is assumed to be directly proportional to amount of excess rainfall

unit hydrograph assumptions 3

the timing of runoff is the same, regardless of antecedent conditions

key reasons for constructing a unit hydrograph

flood forecasting and management, extrapolation for various storms, design of hydraulic structures, model ungauged catchments

Volumetric gauging aka Bucket method

only works for very low discharge, directly measures discharge

velocity area method

classic discharge technique, measurements of stream velocity and channel area

challenges of velocity area method

channel geometry is variable, making area estimates tricky, flow velocities can vary significantly

velocity area method drawbacks

doesn’t work well in complex channels, difficult/dangerous in large rivers and during floods, time consuming

Acoustic doppler current profiler (ADCP)

send high frequency sound pulses up (or down) through the water column and reflect off of moving particles

dilution gauging

based on C1V1, dump tracer at some point upstream, time to get downstream point, measure tracer concentration at downstream point

dilution gauging considerations

need to pick an appropriate downstream point

Dilution gauging tracer considerations

readily soluble, zero or low concentration in natural system, inert, easy to detect at low conc, harmless to environment

stage measurement

river stage (or water level/water height) can be monitored easily (and continuously)

stage measurements combined with other measurements

a rating curve can be developed

rating curve

shows the discharge of stream at various stages (depths)

stage measurement drawbacks

if streambed changes its geometry a new rating curve must be developed, it is difficult to get discharged data points for high flow (flood) situations makin the rating curve is unreliable

Weirs

modify river flow so that it travels through a known cross-sectional area with constant velocity

flumes

work similar to weirs but without the sitting pond

weirs/flumes drawbacks

generally only used on small streams, need to be sure all water is flowing through the structure, long-term weir may have sedimentation problems in the stilling pond, may have negative impacts on stream ecology

statistical experiment (or trial)

process or activity in which one outcome from a set of possible outcomes occurs

elementary outcome

each different outcome is known as an “elementary outcome”

sample space

all of the elementary outcomes

event

a subset of the sample space, or a collection of elementary outcomes

return period (recurrence interval or the annual exceedance)

the most common way to describe the probability of a hydrological event

risk

is the probability of at least one flood in a certain number of years

reliability

is the probability of no floods in a certain number of years

flow duration curve

reports how often a river will exceed a certain flow rate

isotopes

different versions of the same element, they contain same number of protons and electrons but different number of neutrons

unstable isotopes

some isotopes don’t like their configuration so they will tend to decay into other products and release energy (radiation)

isotopes of water

water is a combination of hydrogen and oxygen so you could get varying mixtures of these at any time

Isotope standards

Vienna Standard mean ocean water (VSMOW), a deep water collected from different spots around the globe that was mixed, distilled and then analyzed for isotopes

fractionation

any process that changes the isotope ratios

causes of fractionation

thinking of isotopes in terms of heavier and lighter, some travel faster through the water cycle than others,

Light isotopes will

evaporate easier and tend to be consumed first in biological processes

heavier isotopes will

fall faster as precipitation, condense first and form ice easier

isotope “thermometer”

warm places have ratios closer to SMOW but colder places have huge differences, can use isotopes as a “proxy” for temperature

equilibrium fractionation

once water vapour reaches saturation, the relative number of 18O and 2H atoms will also reach a balance, happens because heavier atoms get left in water during evaporation

equilibrium fractionation is

temperature dependent, warmer = less fractionation because warmer water has a better ability to boost heavier atoms out

2H and 18O ratio

is consistently 8:1, 2H depletion is 8 times greater than 18O no matter the temperature

storm tracking with isotopes

there are many different types of precipitation being driven by a warm or cold front resulting in distinct isotopic signatures which can be used to some extent to track storms

tracing groundwater with isotopes

groundwater flow can occur at very different timescales, each timescale has their own isotopic signature

Hydrograph separation with isotopes

following a storm hydrograph we can separate baseflow from event flow based on isotopes which will be different between the two flows

how to measure isotopes

standard is using isotope ratio mass spectroscopy but laser spectrometers are now becoming more common

endmember

a sample of one of the pure, unxed components

tracers

chemical characteristics measured on our endmember

good tracers

should be conservative (non-reactive) and should have contrasting values for the endmembers

we can unmix any number (n) of endmembers but

we need n-1 tracers

dissolved oxygen

the amount of O2 molecules dissolved in water (but not bubbles)

why is dissolved oxygen important

any aquatic creatures that breathe oxygen will be in trouble if DO concentration is low

How is DO measured

usually concentration (mg/L or ppm) or % saturation

why is temperature important

determines solubility of DO, warm water can’t hold very much oxygen

what can go wrong with water temperature

industrial discharge (power plants), low flow + high radiation, and climate change

what is turbidity

a measure fo cloudiness of water, caused by suspended solids and gas bubbles

why is turbidity important

high turbidity makes it difficult for fish/insects to breed, contaminants (including viruses and bacteria) attach themselves to suspended solids

nitrogen compounds

organic nitrogen, ammonia, nitrite, nitrate, fertilizer; measured by titration or spectrophotometry

Why are nitrogen compounds important

it is a critical nutrient for primary production

what can go wrong with Nitrogen compounds

runoff from agriculture land contributes nitrogen to watersheds, excess nitrogen can trigger massive algae blooms