Hydrology 2

Vapour Pressure Deficit

The difference between the actual water vapour pressure and the saturation water vapour pressure at a particular temperature. Higher deficit = more water absorbed from an evaporative surface.

Vapour Pressure

Pressure exerted within the parcel of air by having the water vapour present within it. The more water vapour is present the greater the vapour pressure.

Saturation Vapour Pressure

The maximum vapour pressure possible (i.e. the vapour pressure exerted when a parcel of air is fully saturated). The saturation point of an air parcel is temperature- dependent and hence so is the saturation vapour pressure.

Bergeron Process (Ice-Crystal Process)

The process of raindrop growth through a strong water vapour gradient between ice crystals and small water droplets.

Crystals grow via:

1. riming (accretion) = supercooled water droplets freeze on impact

2. aggregation = coalescence/stick together

Why is saturation vapour pressure greater over a water droplet than an ice droplet?

Easier for water molecules to escape from the surface of a liquid than a solid. Ice is colder, lower KE, less evaporation means lower pressure for condensation and evapo to be in balance

How does vapour pressure deficit relate to ice crystal processes?

Saturation vapour pressure is greater over a water droplet than an ice droplet. This creates a water vapour gradient between water droplets and ice crystals so that water vapour moves from the water droplets to the ice crystals, thereby increasing the size of the ice crystals

Ability of a soil to transmit water

Dependent on pore sizes within it and on the connections between pores.

Macropores

Pores greater than 30 microns (3mm) in diameter. Two types: large pores within the soil matrix, and pores that are essentially separated from the matrix.

Porosity

Empty space in the medium; Depends on the shape, arrangement and degree of sorting of constituting particles. (ex. poorly-sorted sand vs well-sorted sand). Specific yield + Specific retention (Water that is drained by gravity + water that is not drained by gravity = porosity)

Greater pore size = less porosity

η = (Volume of Water + Volume of Air) / Total soil volume

Specific Yield

Part of groundwater that will drain under the influence of gravity (opposite of field capacity)

Specific Retention

Volume of water which is retained by surface tension forces as films around individual grains and capillary openings, water on the surface of grains that will not flow through the material

Soil particle size classification

Sand = 0.06-2.0 mm

Silt = 0.002-0.06 mm

Clay = < 0.002 mm

Clay fraction

Highly weathered particles

Mostly clay minerals (ex. plagioclase)

Adsorbs soil water very effectively

(Clay fraction compared to sand & silt fractions determines a soil's hydrological properties)

Sand and Silt fractions

Less weathered particles

Less reactive/charged particles

Retain less soil water

Larger pores, higher hydraulic conductivity

adsorption vs absorption

Adsorption compounds cling to the surface of the molecule, whereas absorption substances enter the bulk phase of a liquid or solid.

Pore space in Sandy soil vs Clay soil

Sandy soil = larger pores, less total pore volume = less porosity

Clay soil = smaller pores, greater total pore volume = greater porosity

Measures of Soil Water Content

Field Capacity

Permanent Wilting Point

Available Water (to plants)

Gravimetric Water Content

Gravitational Water

Field Capacity

Ability of a soil to hold water against gravity, through adsorbed water and capillary pressure

Permanent Wilting Point

The water content at which plants cannot draw water out of the soil.

Gravimetric Water Content

Mass fraction of water in the soil

Gravitational Water

the water in a soil that is freely available for gravitational drainage and drains right through, i.e. 'free water in the pore space'

Capillary Water

Water held in micropores.

Available water = plant roots can absorb this

Hygroscopic Water

remaining water adheres to soil particles and is unavailable to plants

Groundwater management issues

Water quantity (floods, drought, water distribution)

- Changes in patterns of consumption: population increase, climate change, land use change, groundwater depletion

Water quality (drinking water, managing aquatic ecosystems, pollution/waste disposal)

Forces involved in water flow

Gravity = moves water down rivers, conversion of potential energy to kinetic energy/inertia

Friction = resists water flow

Types of flow in moving water

Laminar (regular/smooth) vs turbulent (irregular fluxuations)

Water table

An underground boundary between unsaturated zone and saturated zone of groundwater.

Can fluctuate throughout the seasons.

Water pressure = atmospheric pressure (state of equilibrium).

ex. 0m (sea level) = 1 atm, 10m below sea level = 2 atm

Aquifer

A geological unit, saturated w/ water, able to store and transmit water.

A layer of unconsolidated or consolidated rock that is able to transmit and store enough water for extraction

Serve both as reservoirs for groundwater storage and as pipelines for groundwater movement.

Confined aquifer

Has a flow boundary (aquitard) above and below it that constricts the flow of water into a confined area.

Saturated Geologic Zone overlain by a confining layer.

Pressures greater than atm. pressure.

Groundwater does NOT cycle fast, ages can be > 10,000yrs

Receives water from a much smaller area.

Over-harvesting of aquifers results in depletion that is faster than recharge.

unconfined aquifer

Has no boundary above it and therefore the water table is free to rise and fall dependent on the amount of water contained in the aquifer (Top of water table (saturated zone) is free to fluctuate)

Typically 'fluvial sediments (sands, gravels, silts)

Artesian well

A well in which water rises and reaches above the earth's surface because of pressure within the aquifer

Aquitard

Does not transmit significant amounts of water

Very low Hydraulic conductivity

Could have significant storage capacity/porosity

Aquiclude

Does not transmit any water

NO hydraulic conductivity

Little to no porosity

3 ways groundwater system 'recharge' inputs (infiltration & percolation) are transferred:

1. Flow to adjacent areas via throughflow

2. Re-surface as return flow (springs) or baseflow (rivers)

3. Long term storage in deep aquifers

Factors controlling quantity & quality of groundwater systems

- Climate (local water budgets, controls inputs & outputs)

- Geology (substrate properties control throughputs & storage)

- Land use (human activity affect infiltration & recharge)

Hydraulic head

Measure of mechanical energy at a location

A specific measurement of liquid pressure above a vertical datum

h = z + P/pg

hydraulic head (h) = elevation head (z) + pressure head (P/pg)

Elevation head

Equals the elevation above some chosen reference level

h = z + P/pg

hydraulic head (h) = elevation head (z) + pressure head (P/pg)

Pressure head

Common practice to define the water pressure at a free interface between air and water as zero.

Equals the length of the column of water above the screen

h = z + P/pg

hydraulic head (h) = elevation head (z) + pressure head (P/pg)

General rules for groundwater flow

1. Water table reflects topography (deepest below uplands, shallow in valleys

2. Equipotential contours Φ lines perpendicular to 'divides', parallel to boundaries

3. Streamlines perpendicular to equipotential contours Φ lines

4. Both horizontal and vertical flow components (general down in uplands & up in lowlands)

5. Flow nets complicated by changes in geology and topography

Equipotential contours Φ

Lines of equal hydraulic head, lines of same flow energy

What can you learn from an annual hydrograph?

Typical flow response patterns under variable conditions such as seasonal storms, snowmelt runoff, etc.

Timing of flow/runoff peaks correspond to pulses of precipitation

Rain-dominated: peak in late fall

Snow-dominated: peak in spring/summer

freshet

the flood of a river from heavy rain or melted snow

Hydrograph

plot of discharge over time

unit hydrographs used to predict discharge/floods

Baseflow

Discharge or part of discharge caused of processes gradually delivering water to the stream

Baseflow recession

Dependent on input/precipitation (may or may not be seen), gradually declining baseflow without new contributions

Rising limb

Start to see a rise in discharge rate after input (precipitation)

Peak

Max discharge during event

Quickflow/stormflow

the water that gets into the river as a result of overland runoff, the volume of water that steams from the process that is rapidly delivering water to the stream

Quickflow recession

declining flow after storm ends

Separation point

the point where quickflow ends & flow returns to baseline

Darcy's Law

A mathematical equation stating that a volume of water, passing through a specified area of material at a given time, depends on the material's permeability and hydraulic gradient.

Darcy's Equation

gives volume discharge rate (m^3 day^-1).

q = Q/A

Q = AK(Δh/L) or Q = -KiA

Q = Groundwater discharge (volume/time)

A = Area of flow path (length squared)

K = Hydraulic conductivity (length/time)

i = Hydraulic gradient = Δh/L

Δh = Difference of Hydraulic Head (m)

L = Distance between two points (m)

Hydraulic conductivity K

The measure of a soil's ability to transmit water, how effectively water moves through a substrate under a given hydraulic (head or potential) gradient

Units m/s or cm/day

At higher water contents, coarse soils have a higher K than fine soils.

At low water contents, fine soils have a higher K than coarse soils.

Hydraulic gradient i

the slope of the water table (indicates direction of movement), function of hydraulic head

i = Δh/L

Δh or dh = Difference of Hydraulic Head (m), change in head (elevation) between two points at the top of the groundwater table

L = Distance between two points (m)

Flow in +Q vs -Q

Negative Q value = flow moves right to left

Positive Q value = left to right

calculating ground water velocity

V = K*i/n

K = Hydraulic conductivity (length/time)

i = Hydraulic gradient = Δh/L

Δh = Difference of Hydraulic Head (m)

L = Distance between two points (m)

n = effective porosity

Vertical groundwater flow

Groundwater flows in all directions along potentiometric gradient from L

Vertical hydraulic gradient: i = dh/dz (determined from h levels between wells or piezometers)

Horizontal groundwater flow

Piezometric surface map of h contours

Horizontal gradient: i = Δh/Δx

Usually moves in the direction from a high recharge area to a low discharge area

Groundwater flow nets

Useful for determining Flow direction and distance, travel times, flow (discharge rates)

q = -KΔh(W/L)

K = Hydraulic conductivity (length/time)

Δh = Difference of Hydraulic Head (m)

W = width of stream tube (m)

L = distance covering a constant difference in hydraulic head

Conditions needed for effective recharge

- soils should be permeable to conduct infiltration

- The vadose (unsaturated) zone must be permeable and free from clay layers

- The aquifer to be recharged must be unconfined, permeable, and thick, to avoid groundwater mounds

- Groundwater table must be deep: beyond 10m below ground surface

Artificial recharge and Induced infiltration

The practice of artificially increasing the amount of water that enters a groundwater reservoir.

Used for waste disposal, secondary oil recovery, land subsidence problems, and water resource management

Engineering: injection wells, infiltration ponds, drainage ditches

Purposes of artificial recharge (7)

1. Conserve and dispose of runoff/flood waters

2. Supplement natural recharge of groundwater

3. Reduce or balance salt water intrusion

4. Suppress ground subsidence

5. Store water in off-seasons for use during growing seasons, can also reduce pumping

6. Geothermal applications

7. Remove suspended solids by filtration through the ground

Ground subsidence

if groundwater is removed in large quantities, surface may react by sinking to fill the new space

Geologically suitable sites for Infiltration

flood plains of rivers

alluvial fans

sand dunes

weathered zones

permeable vadose (unsaturated) zones

glacial outwash plains

Surfaces may be covered with a few metres thick clay layers, which can be removed. Infiltration basins/ponds may then be excavated in underlying permeable deposits

Infiltration rates of coarse sand, fine sand, loamy sand, coarse sandy loam

High rate, > 50mm/hr

Infiltration rates of sandy loam, fine sandy loam, loam

Intermediate, 15-50mm/hr

Infiltration rates of silt, loam, sandy clay loam, clay loam, salty clay, sandy clay, clay

Low, < 15mm/hr

Modern groundwater

Less than 50 years since recharge

Most vulnerable to global environmental change

Young groundwater

Less than 100 years since recharge

Groundwater use in BC

Derived mainly from infiltration, snowmelt

Recharge is seasonally variable (lag times)

Impacts of groundwater use in BC

Quality: contamination

Quantity: Drainage diversion, artificial recharge & withdrawal

Shallow & confined aquifers vulnerable

Flow routes, rates & residence times poorly understood and largely un-modelled

Landuse impacts changing recharge, storage, & discharge

Types of streams in groundwater

Gaining stream

Perched losing stream

Losing stream

Flow-through stream

Stage

water level

Froude number

dimensionless ratio between fluid inertial forces and fluid gravitational forces, describes different flow regimes of open channel flow.

Fr = flow velocity/surface wave velocity

or

Fr = flow velocity/(gravity*hydraulic depth height)^2

Overland flow

Non-channeled water flow over the ground's surface

Factors influencing overland flow

- precipitation intensity

- snow melt rate

- Pre-existing soil saturation level

- soil type (hydraulic permeability)

- ground cover

- topography/terrain characteristics

Two types of surface runoff that can occur during rainfall or snowmelt

1. Infiltration over excess overland flow "Hortonian flow": Occurs with unsaturated soil. Soil properties or land cover does not allow of infiltration to keep up with high rainfall or snowmelt rates

2. Saturation over excess overland flow: Occurs when the soil becomes saturated and there is no longer space for water to infiltrate

Hydrograph character depends on:

- Precipitation characteristics (magnitude, intensity, duration, distribution, phase)

- Basin characteristics (slope angle, slope shape, soil type, soil thickness, initial soil moisture conditions, anthropogenic impacts, basin size, basin shape)

Rainfall rate ≤ Infiltration capacity

No surface runoff

Rainfall rate > Infiltration capacity

Surface runoff

Interflow

the lateral movement of water in the vadose (unsaturated) zone, that returns to the surface or enters a stream.

Flooding

A rising 'stage' level associated with excess discharge in response to storms or seasonal melt

A natural process required to maintain river form & function

Defined geomorphically by channel cross-section and bank height

Useful to understand for designing bridges/dams/sewers, land-use planning, floodplain delineation

Bankfull discharge

Threshold stage before river flows over banks.

Occurs frequently with recurrence interval (R) of 1.5yrs (Q1.5)

Q1.5 maintains channel form: most efficient at moving water & sediment, determined from channel surveys or rating curves

Rating curve

a graph of discharge versus stage for a given point on a stream

ex. Rating curve shows initially rapid increase in stage with little increase in Q, then a more rapid increase in Q. Occurs due to increase in cross-sectional area with stage, causing greater amounts of water to flow through = higher Q.

Flood prediction

1. Hydrographs

2. Recurrence interval (R): annual peak discharge records used to predict flood probability (P) based on record. Plot flood frequency curve (log-normal)

R = (n+1)/m

P = R^-1

n = number of ranked observations

m = rank of the observation concerned

3. Rational method: estimate peak annual discharge using empirical formulae and data on rainfall and basin statistics

Qpeak = CiA

C = runoff coefficient = 0.278

i = rainfall intensity (mm hr^-1) at time of concentration

A = basin area

Wetlands

a lowland area, such as a marsh or swamp, that is saturated with moisture, especially when regarded as the natural habitat of wildlife.

(Land where an excess of water is the dominant factor determining the nature of soil development and the types of animals and plant communities living at the soil surface)

Marshes

wet most of the time; grassy or reedy vegetation; can be salty (i.e. tidal) or fresh; shallow; most common wetland in N. America

Swamps

woody plants, often treed; deeper water (often > 1m); shrub and forested swamps (e.g. mangrove swamps) ; nutrient rich

Bogs

water from precipitation, mostly; thick mat of vegetation (peat)

rather than soil; often acidic; soft, spongy, organic

Fens

like bogs, but fed more from surface or groundwater; more nutrients, higher pH; often associated with glacial kettles

Peatlands

wetlands with soil formed mostly from decomposing plants

SMAP

Soil Moisture Active Passive; a satellite mission dedicated to monitoring soil moisture levels

phreatic zone

saturated zone, realm of groundwater

Capillary fringe

region above the water table with water drawn up by capillary action

Vadose zone

unsaturated zone, realm of soil moisture

Why study soil water?

flood prediction

contaminant migration

erosion

agriculture

geotechnical engineering

What is soil made of?

25% air, 25% water, 45% mineral, 5% organic

Total soil mass

Solid mass + Water mass

Total soil volume

Mineral volume + Water volume + Air volume

Total pore volume

Water volume + Air volume