GEOG220 Hydrology Flashcards
What happens when temperature increases (water, latent heat)?
Increased temp = more latent heat consumed, breaking down hydrogen bonds = availability for evapouration
How much radiation is re-emitted as longwave radiation?
15%
How much longwave radiation is converted into latent heat and sensible heat?
Latent Heat: 24%
Sensible Heat: 7%
Energy Balance Equation
Rn = LE + H + S
Rn (net radiation)
H (sensible heat)
LE (latent heat)
S (energy store/flux)
Sensible Heat vs Latent Heat
Sensible heat is the heat we can feel, latent heat is the energy used to break bonds between water molecules.
Arid Climate: Sensible v Latent Heat
Net radiation is converted into sensible heat, because water not available.
Humid Climate: Sensible v Latent Heat
Net radiation converted to latent heat for evap, because water is available.
Watershed
Area upstream of a point, known as catchment/basin/contributing area.
Pool
store of water (ocean, lake, ice, atmosphere, gw)
Flux
Way water moves between pools, like evaporation, precip, discharge, etc.
What is the main cooling mechanism?
Uplift!!!
Precipitation and Evaporation
Air temp controls the max amount of water vapor it can hold. Air cools (uplift) below dew point temperature = saturated = condensation = precip.
Water Balance Equation
P = Q + E + DeltaS
P = precip
Q = runoff
E = evap
DeltaS = chnage in storage
Precipitation
Flux of water from the atmosphere to the surface, in different forms (dew, mist, fog, rain).
Global Average Annual Precip
High over equator (where warm air holds more moisture) and low over poles/mid lat (cold air holds little moisture).
There are temporal variations (NZ has low variation, high supply).
Climate Controls on Aridity
Water vapour transports energy. Global circulation controls water availability.
Energy + water control aridity.
Energy + water high at equator, energy + water low near subtropical highs.
Saturated v Unsaturated Air
When air cools below its dew point = condensation.
If air unsaturated = evap (so long as there is energy to break bonds).
Effect is NON-LINEAR, getting stronger with warming air.
True of False, an air parcel can reach dew point temperature through warming and cooling
True, increase in temp = will reach dew point through warms. Decrease in temp = will reach dew point through cooling (like breathing out)
Convective Rainfall
Strong heating of land = air rise. Rising = gradual cooling = water vapour condenses. Produces heavy rain from quick rising.
short duration, intense. 30% of NZ rainfall.
Orographic Rainfall
Prevailing wind uplifted by topography. Rises, cools, dondenses, heavy precip on windward side.
Frontal/Cyclonic Rainfall
Precip caused by aur mass moving in.
Long duration. If cold front = more intense (displaces warm air and rapidly pushes it up to create intense precip).
What are the 3 types of rainfall?
Convective, Orographic, Frontal/Cyclonic.
Types of Precip Measurements
Direct, aerial, indirect.
Direct Rainfall Measurement (at a point)
Rain gauges
Little bucket, rain drops in, sensor tips, flicks switch. Flick switch frequency = amount of rainfall.
Aerial Rainfall Measurement
Array of rain gauges + interpolation
Thiessen polygons, inverse distance weighted, geospatial
Indirect Rainfall Measurement
Radar, remote sensing
Problems with Direct/At-a-point Rainfall Measurement
- Wind Turbulence
- Steep Terrain
- Forest Canopy
- Extreme Events
Wetting Loss
Water stays on funnel and can be lost to evap/not measured. To avoid, non-stick and steep funnels used.
Rain Splash
surface-level gauge is likely to over-measure the catch due to rain landing adjacent to the gauge - avoided by raising gauges.
What Increases Rainfall Measure Accuracy?
Number of gauges
Time Interval
Size of area
Thiessen Polygons
Polygons drawn by connecting nearest rain gauges to each other. Rainfall then spatially averaged.
Problematic sharp borders
Areal Rainfall
Weighted mean, using Thiessen polygons, where the weight is based on size of each polygon
Isohyetal Method
Lines of equal rainfall on map and calculates areas between, giving spatial average.
Considers environmental factors, but subjective if manual and time consuming
Inverse Distance Weighting Method
Rainfall interpolated from surrounding gauges, where closer gauges = more weight and further gauges = less weight.
Sensitive to weighting and size of study.
Water
Water molecule consists of two hydrogen atoms bonded to a single oxygen via covalent bond (sharing of an electron from each atom). This is the strongest bond, making water a robust molecule - and is why it stays as water within our atmosphere.
Water molecule is bipolar, which means that there is a positive and negative side to the molecule, due to the repulsion and 105* angle between hydrogen atoms.
Evaporation
Loss of water from a wet surface through conversion into vapour.
Air temperature controls maximum amount of water vapour it can hold.
Evaporative demand when actual vapour pressure < saturation vapour pressure
Geographic Pattern of Potential ET
More solar radiation = warmer air, which can hold more water = greater water demand = greater PET (potential evapotranspiration)
Transpiration
Evaporation within leaves. Controls leaf temperature, nutrient delivery, and shares pathway for growth/photosynthesis (closes stoma under dry conditions to stop water loss).
Works with capillary action in plant, taking up water from soil and evaporating it from leaves at the stoma.
Factors Affecting Evapotranspiration
- Energy to supply latent heat of evaporation (net radiation, air temp)
- Capacity to transport vapour away from evaporative surface (wind, humidity such that vapour pressure deficit exists).
- Available water to supply evaporative demand (no water = no evap, controls how much AET meets PET)
Deep roots can increase available water (plants extracting from deeper, generally wetter soil).
Measurement: Evaporation Pan & Lysimeters
Pan: Scale/ruler put in large pan of water. Measured AET from pan, and PET from surrounding grass.
Lysimeter: isolate bucket of soil, measuring everything (weight, percolation, rainfall, etc) to determine evapotranspiration.
Bowen Ratio
Heat/sensible heat.
Where B > 1, Latent heat is higher than sensible heat = more evap
PET vs AET
Potential evapotranspiration: measure of ability of atmosphere to remove water
AET: quantity go water actually removed. Function of Potential Evapotranspiration and water availability.
Interception
Before reaching the ground, rain is caught by the vegetation and can be evaporated straight off.
Interception loss depends on:
Canopy structural factors
- Storage capactity/interception loss
- Drainage characteristics (stemflow)
- Aerodynamic roughness (turbulence)
Runoff
Movement of water to a channelized stream, after it has reached the ground as precipitation (not just overland, also through the soil).
Flow Pathways
- Channel and riparian precipitation
- Overland flow (infiltration excess, saturation excess, return flow)
- Throughflow/interflow
Baseflow
Overland Flow
Infiltration Excess, Saturation Excess, Return Flow
Infiltration Excess (overland flow)
Rainfall intensity exceeds infiltration capacity, so some rainfall must flow over the surface. Caused by intense rainfall, low permeability, or increasingly moist soil.
Infiltration rate > rainfall rate.
Saturation Excess (overland flow)
soil is/becomes saturated. Occurs where water percolates to a barrier, water table rises, water doesn’t have as far to percolate, and soil becomes fully saturated (water table meets surface).
Return flow (overland flow)
water is flowing underneath topsoil, but due to topography shape, water is pushed out into overland flow.
What moves water in soil?
Direction: gravity and pressure gradients
Rate: material permeability
Unsaturated soil water moves towards areas of greater suction (drier soil) and downwards (gravity). Field capacity = where these forces are in counterbalance.
Groundwater moves because of pressure and gravitational forces, from high to low pressure and downwards.
Throughflow/Interflow
Water infiltrates soil surface, moving laterally through unsaturated zone to stream. Slower than overland flow but faster than groundwater.
More rapid flow through preferential paths (macropores and horizon boundaries).
Baseflow
Sustained flow from groundwater discharge or seepages.
Groundwater continues to discharge as baseflow because of the new recharge of rainwater into the landscape.
Gaining vs Losing Stream
Gaining stream: groundwater discharge adds to stream
Losing stream: groundwater recharged by stream.
Quickflow vs Baseflow
Quickflow: Event flow, where water enters stream promptly after water input events.
Baseflow: water from persistent slowly varying sources. Maintains streamflow between water-input events.
Runoff Generation Depends On…
- Rainfall intensity or duration
- Antecedent conditions (pre-existing conditions)
- Permeability (soils + vegetation)
- Depth to water table (topography)
- Geology
- Land use
River Discharge
Q = wdv
At-a-Station Hydraulic Radius
At-a-Station: at a point in the river.
Hydraulic Geometry: Increase of w,d, and v with discharge (Q) . Varies with channel shape and friction.
In cannals, an increase in discharge cannot be accomodated by increase in width - uptaken by increase in depth
In wide channels, an increase in discharge can be accomodated by increase in width
In gentle slopes, flow velocity remains the same and depth/width increases
In steep slopes, flow velocity increases.
Thalweg
Fastest flow near centre and surface of river, which shifts bends in river.
Flow resistance is highest at bed and banks, where water is most in contact. When graphed, it shows a peak just before the surface - air has friction on surface water.
Continuity of Mass
Where mass is not being lost or added, Q1 = Q2 (discharge in two different places).
If discharge is being lost or added, the river would change to accommodate.
Hydrographs and Catchment Slopes
Steeper basin = contribute much more quickly = higher, shorter peak
Gentle basin = longer contribution time, smaller, longer peak
Hydrographs and Catchment Shape
How elongate the catchment is affected where the tributaries meet the channel, and how quick water reaches.
Very elongate = sustained addition within catchment = sustained, small curve
Intermediate shape = somewhat sustained, with some water at same time = higher peak, shorter duration
Circular = water added almost at the same time rom tributaries = higher peak, shortest duration
Hydrographs and Drainage Density
how close are the streams in neighbouring valleys
Fine density = sustained = small curve
Course density = peak = large curve
What Happens to Q Downstream?
Q does not increase in losing streams (streams recharging groundwater).
But, generally, Q does increase as we move downstream (tributaries > loss to groundwater). Accommodations come from
Mostly w, some d, least v
Velocity increases the least as discharge (q) increases downstream. This is because valleys get less steep, meaning there is less gravitational slope to increase velocity.
How do you compare upstream and downstream flow, given changes in discharge, velocity, width, and depth?
Bankfull flow
Hydraulic Radius
Channel cross-sectional area/wetted perimeter
Deep flow relative to wetted perimeter length: friction from the channel boundary has smaller effect.
Shallow and/or wide flow relative wetted perimeter length: friction has a longer distance to act on flow, and will slow the flow more.
Manning’s N (0 - 0.1)
Resistance increase with the size of friction elements (rocks, vegetation). Manning’s n quantifies resistance.
Smoother and less vegetation = smaller Manning n. Coarser and more vegetation = larger Manning n.
Fr Number
Compares flow velocity to depth (ratio of inertial to gravitational forces)
Fr > 1. Subcritical. Flow is slower than waves
Fr < 1. Supercritical. Flow is faster than waves.
Fr = 1. Critical. Flow is equal to waves.
Froude Number: Why Does It Matter?
(Fr < 1) Subcritical (90% of flows): where there is an obstruction, flow will back up behind it - leading to a gradual raise in water.
(Fr > 1) Supercritical: where there is an obstruction, flow upstream has a hydraulic jump but water upstream does not move upwards.
Shear Stress
How much the river pulls at the base of the bank. Low shear stress = deposition.
We can calculate:
- Entrainment/deposition
- Riverbed incision (water surface will drop) + aggradation (water surface will rise)
- Riverbed stability + habitat longevity
- Habitat diversity + refugia (diversity of flow good for diversity of life)
Flow Monitoring
How do we measure velocity, to measure discharge?
- Manning equation - Float method (can estimate velocity using a float, measure 10x the stream width (to get average flow), and time how long it takes). - Velocity area method (splits the river into blocks, calculating separate velocities - errors accumulate quickly) - Acoustic Doppler (uses sound to measure velocity, where pings of sound ricochet off suspended sediment and bounces back to sensor - limitations include too much/too little sediment) - Dilution gauging (introduce a tracer (salt, dye) and measuring its arrival downstream - LSPIV (take a photo of river, and another quickly after,
Why/how do we measure water level (stage)?
It is impractical to measure Q continuously. We measure water level (stage). Then, a rating curve is built to relate Q and stage.
Measurements for water level include:
- Staff
- SR50 Ultrasound
- Floatation device
- Pressure transducer (most common in NZ)
Drought: Contributing Factors
- Precipitation (no rain = low soil moisture = bigger gap between actual evap and potential evap)
- Soil moisture
- Evapouration
- Soil moisture
Potential Evapotranspiration Deficit
PET - AET.
Soil moisture deficit
The amount of water the soil is short of full capacity.
SMD = 0 when soil is full of water.
Yesterday’s deficit = SMD + E - P
Flood Definitions
Overbank Flood: Where flow exceeds river banks/channel.
Recurrence Interval: River flow of a given recurrence interval, which doesn’t necessarily leave the channel. Often referred to in instance over time (1 in 50yr flood)
Flood Factors
- Intense precipitation (infiltration excess overland flow as water table rises)
- Sustained precipitation (sustained, leading to saturated excess overland flow as soil is saturated)
- Infrastructure failure
- Snow melt (two main controls: temperature and evapouration)
- Soil moisture
Water table (higher water table = more saturation excess overland flow, high water table raising level of lake = backflow effect and raising level of river, tides = more or less pressure on groundwater = table fluctuates)
Influences on Flooding
Precipitation characteristics vs catchment characteristics.
Precipitation characteristics: type, form, speed of storm
Catchment characteristics: antecedent (pre-existing) conditions, shape, drainage pattern, density, etc.
Flood/Drought Frequency Analysis
Involves analysing probability - examination of frequency-magnitude event.
Flood frequency analysis –> peak flows
Drought frequency analysis –> lowest flows
Ground Is…
Largest liquid component in freshwater system (10,000,000 kilometres cubed, over 1000yr residence time)
Subsurface Zones
Soil Zone, Intermediate Zone, Capillary Fringe, Water Table, Saturated Zone,
Soil Water vs Groundwater?
Soil Water/Vadose Zone: everything unsaturated. Biologically modified. Porous bedrock. Variably saturated. Negative water pressure when unsaturated (soil pores have demand for water, pulling water in)
Groundwater/Phreatic Zone: Porous bedrock, saturated. Positive water pressure (soil/rock pores filled).
Factors that Control Water
- Hydraulic conductivity (fluid properties, porosity)
- Water content
- Hydraulic gradient
- Boundary conditions (inputs, outflows (leaks, abstraction))
Hydraulic Conductivity: Porosity
Solid that contains holes, where porosity = % which is void.
Pores can be:
- Connected
- Disconnected
- Interstitial (between grains)
- Planar crack-like
True or False, pore space controls water storage while
Connectivity controls water movement.
True.
Darcy’s Law
- Rate of water flow through a bed is proportional to the difference in height of the water between two ends (Hydraulic Gradient)
- The flow rate is inversely proportional to the length of the flow path (distance between measurements)
- The quantity of flow is proportional to a coefficient, K, which is dependent on the nature of the porous medium (porosity and connectivity of pores)
Darcy’s Experiment
Water is forced in, moving through clay and gravel. Change in length of tube impacts time to move through. Effect of gravity affects speed of flow/movement.
Pressure of force through tube, when punctured, will cause water to move upwards (pressure head). Pressure is higher at bottom, cancelling out gravitational effect. So, when we add gravitational head and pressure head, we get total hydraulic head. Difference between them = hydraulic gradient.
Hydraulic Gradient
Change in hydraulic head/distance the water is flowing.
Piezometric/Hydraulic Head
Level water rises in well = pressure head. Sum of both = hydraulic head.
Datum is often sea level.
For what type of flow can we use Darcy’s Law/Experiment?
Laminar
When Do We Get Zero GW Movement?
No hydraulic gradient (if water table is flat, no pressure difference, K = 0)
Piezometric Head Map
Add gravity and pressure heads, to give map of direction groundwater will flow (like contours).
Groundwater and Topography
Groundwater usually flows away from topographic highs, and towards topographic lows.
Groundwater discharge zones are topographically low spots (although permeability may affect this)
Groundwater Budgets
Inflow - Outflow = charge in storage.
Pumpage = increased recharge (decrease hydraulic head and increased gradient) + water moved from storage + decreased discharge
Aquifer Types
Artesian Aquifer: high pressure, where natural crack or introduction of well will see water move to surface - no pumping required. Creates artesian well.
Confined aquifer: surrounded by impermeable rock
Unconfined aquifer: able to percolate outwards, sometimes leading to no definitive catchment.
Hydraulic Head: Water Table
When could hydraulic head be at the water table?
- Unconfined, no extra pressure, all lateral movement the same as water table on each side.
When could hydraulic head be above the water table?
Confined, additional pressure, lateral movement different to water table on each side.
Hydraulic head
measure of the potential energy that causes groundwater to flow.
Sum of gravimetric + pressure head
Gravimetric/elevation head
defined as soon as a datum is set
Pressure head
For any given fluid, pressure head (m) is proportional to pressure (Pa).
Pressure head can be calculated from the height that water rises in a piezometer.
Pressure head = 0
At water table
+ and - pressure head
+ pressure head is a direct measure of the depth of water standing above instrument.
- the subsurface is unsaturated.
Field Capacity
At field capacity, hydraulic head = 0. As soil dries, pressure head becomes more negative and a larger gravitational head is required to balance it out.
Hydraulic Conductivity vs Water Retention
Hydraulic Conductivity: rate water moves through soil (reflects porosity + connecivity of pores)
Water retention How much water is actually held in soil.
Soil Water
Different stages of water in soil
- Hygroscopic Water
Water adheres to soil particles, inaccessible to plants. Wilting point.
- Capillary Water Water held in large pores. Best for plant growth. - Gravitational Water Water, and nutrients, drain through soil profile. Suction and gravity balance. BAD for crops.
Physical Properties of Soil
- Texture (grain size, compaction)
- Structure (plants, rock)
- Density (comapction)
Porosity (grain type)
Soil Moisture Retention Curve
Relationship between water content and soil water pressure.
As soil moisture potential increases (drier soils gets), the stronger suction gets. This shows how dryness impacts the moisture content.
More moist = less suction force.
Different Soils: Which is Best?
Coarser soils/large pores = empty under low negative pressures
Smaller soils/pores = need increasing suction to empty.
We need a medium for best plant growth, where there is available water but low plant stress (Loam)
How Do We Measure Subsurface Water?
Unsaturated:
- Tensiometer (negative pressure, have porous cup that reaches equilibrium with soil)
- TDR (moisture content, probe bounces EM waves)
- Satellites (active and passive, gives top 5cm)
Saturated:
- Dye traces (can see finger flow of runoff)
Variable head permeameter
Where Are The Chemicals?
Chemicals in water are in two different phases:
- Aqueous Phase Solutes: dissolved in a liquid.
- Solid Phase: locked up (absorbed by other particulates), but particulates move
Dissolved vs Particulate
Depends on:
- Temperature
- Ionic change (negative chemical meets positive clay particle)
- pH
Composition of rocks/solid (clays quite polarised, bond)
To track the movement of water through the hydrological cycle, we can use:
“Tracer” Contaminants - very soluble, travel with water.
“Highly Sorbed Particles” - stick to soil, transported with sediment
Effects of Sorption: Retardation
Each particle has its own trajectory. Particles are slowed by intermittent absorption (solid phase). Collective effect across millions of molecules leads to retardation,
Retardation: slowed movement of particles through water.
Chemical Evolution and Groundwater Age
Groundwater chemistry evolves over time due to water-rock interaction (eg, filtering from clay)
- TDS increases (total dissolved solids)
- Ratios of major ions change
Oxygen is depleted, redox potential decreases
Groundwater Dating: Tracer Methods
Over the last 100yrs, we have released chemicals that can be traced.
- Tritium (nuclear bomb tests in Pacific, nuclear energy)
- CFCs (aerosols, refrigerants)
SF6 (electrical switchgear)
Radionuclides as Subsurface Water Tracers
Rye crop on top, measuring how quickly rye crop was creating negative pressure (soil water deficit) and causing suction. Three radionuclides added (radiochlorine (moves with water), radiocaesium (sticks to soils), radiosodium (in-between)). All move at different rates due to retardation effect. More soluble contaminants = more mobile (travel further in soil)
True or false, plants create negative pressure (soil water decifit) and subsequent suction
True
Water Quality vs Pristine
Water Quality: how safe it is to drink
Pristine: water in natural state, not always safe to drink
Palatable vs Potable
Palatable: aesthetically pleasing to drink. Considers presence of chemicals/materials that do not pose a threat to human health, that affect colour, smell, taste, etc.
Potable: safe to drink, not necessarily aesthetically pleasing. Affected by:
- Microbes (Giardia)
- Organic chemicals
- Inorganic chemicals (lead, mercury)
Radionuclides
Size of Particles in Water
Size of pores between rocks are good for filtering, especially for microbes. So, we can purify water by particle size.
GW and Delays
Caused by long residence times and effects of retardation due to soprtion.
Mean we ahaven’t seen full consequences
How Would Hydrological Fluxes Change Across Vary Across Arid, Humid, and Snow Climates?
Atmospheric moisture: less in cold climate, can hold less moisture
Precipitation: less in arid and snow
Evapotranspiration: PET similar in humid and arid climate, but AET will be more in arid (more potential)
Reservoirs: bad in arid regions, high evap = no water storage.
Groundwater recharge: depends on whether ground has frozen, and if water is percolating through snow
Arid Zone
By annual rainfall, aridity index, extreme spatial and temporal variability of rainfall
Biocrusts
Hydrophobic, limits infiltration in arid regions.
- Rough crust might slow runoff + encourage infiltration - Smooth curst encourage runoff + decrease infiltration
Humid Regions - Forest Hydrology
Trees thrive when it’s humid, and trees allow humid regions if they access deeper groundwater.
- supply water to meet PET demand
What is a Humid (or forest) Zone?
Heavy rainfall, humid, productive, 18-35*C
Roots, Deforestation + Hydrological Cycle
Roots give access to deep water.
- Faster water cycling
- More ET
- Less Rn (runoff) converted to sensible heat
Areas where deforestation occurs slow fluxes, changing energy and water balances (less moisture into atmosphere = less precip, depending on scale - large-scale suggests decrease)
Snow Interception
28-65% of cumulative seasonal snowfall can be intercepted and stored in coniferous canopies. Can be stored for several months.
Snowpack
non-homogenous porous matrix, storing water (if rainfall is not too warm).
- Runoff mainly at upper and lower surfaces
- Thermal quality depends on water in interstices (how old, frozen, porous, water availability, saturation, etc)
- Substantial changes over time (after melt/freeze)
