Hydrology 1

7 Special Properties of Water

1) Abundance - covers 71% of the world

2) Found in all three phases on Earth's surface

3) High melting and boiling points

4) "Universal" solvent

5) Density decreases in freezing

6) High specific heat and thermal conductivity

7) Internal cohesion and surface tension

Water Vapour

The most energetic state water can be in...

30% of the energy required to circulate the atmosphere comes from the phase change of water vapour (to liquid or solid) in the atmosphere

Solid (ice) → add energy (80 cal) → liquid → add energy (~540cal) → vapour

What phase are clouds?

Liquid or solid - type of precipitation is largely dictated by surface temperature (so even if clouds are ice, precipitation can fall as rain)

Specific Heat Capacity

Water has the highest heat capacity of all liquids on the planet; this means it holds more energy, but also requires more energy to change its temperature - for this reason, water is a thermal regulator

Thermal Conductivity

Water, like most liquids, has a very low thermal conductivity (i.e. the amount of energy that can be transferred by convection - molecule to molecule contact)

Still, it has a slightly higher thermal conductivity than many other liquids

(3) Water Movement Exchanges

Exchange of energy - via latent heat and kinetic energy

Exchange of matter - via changes in state btw the hydrosphere and atmosphere

Exchange of force - via direct mechanical action or pressure caused by water flows or thermal expansion

Residence Time

Time required for water to move through a component of the hydrologic cycle

- Atmosphere - days

- River discharge - hours to days

- Deep ocean, glaciers, groundwater - 3,000-10,000 years

Water Balance

A mathematical statement of the hydrological cycle for an area of interest

Flux in = Flux out + Storage (Qin = Qout + ∆S)

Example application: local water budget

Precipitation = runoff + evaporation + storage

(P = Q + E + ∆S)

Soil Water

Everything from the surface down to the water table (i.e. any water present above the water table)

Big Water Issues

Water availability and aquatic ecosystem health - ex: droughts and floods, municipal water supply, productivity of lotic and lentic systems

Integrated watershed management in response to climatic variability and climate change - rivers, lakes, reservoirs, wetlands, groundwater, and cryosphere

Separating the impacts of human influence from natural climatic variability and climate change (ex: water abstraction and flow regulation)

Evaporation & Condensation

Getting water into the atmosphere (i.e. evaporation) requires energy; evaporation and condensation result in huge energy exchange in the atmosphere

~60% of all precip that falls over land will be re-evaporated

Controls on Evaporation (3)

1) Energy to break bonds - scales with available energy in the forms of radiative and sensible heat

2) Humidity (vapour pressure) gradient - drives moisture flux from high to low; scales with ∆q (i.e. qs - qa)

3) Requires removal of moisture so the air above the surface does not reach saturation - scales with wind speed

Surface Energy Budget I - Radiation

Net radiation at the surface

Q* = QS(in) - QS(out) + QL(in) - QL(out)

QS = shortwave (solar) radiation

QL = longwave (infrared) radiation

Surface Energy Budget II - 'Turbulent' Heat Fluxes

Sensible and latent heat

QH - energy (heat) flows from warm to cold via conduction (molecular transfer) and advection/convection (movement of the medium, e.g. water)

QE - energy transfer associated with phase change; consumed during evaporation, released during condensation

So, Net Energy QN = Q* + QH + QE + QG = 0

Modelling Evaporation/Sublimation

From a water/snow/ice surface, this typically takes the form:

E = Qe / ρ Lv

E - evaporation rate (m/s)

ρ - density of water

Lv = latent heat of evaporation

Measuring Evapotranspiration (ET)

Notoriously difficult to measure; a few techniques -

1) Evaporation pan - measure change in water height daily

2) Lysimeter - measure the change in weight of a soil or snow sample

3) Water balance - can work well for small, controlled basins; may also need to know about storage + precip

4) Energy balance - theoretical calculation

5) Hydrological model - the concept of potential evapotranspiration (theoretical calculation); there are different approaches (ex: Penman-Monteith equation)

Means of describing humidity (5)

Vapour pressure - Pa or mbar

Mixing ratio - g H2O / kg dry air

Specific humidity - g H2O / kg air

Absolute humidity - g H20 / m3

Relative humidity (RH) - %

Relative Humidity

Warmer air has greater capacity to hold water vapour, cooler air has lesser capacity

(Ex: the same amount of water vapour can mean 20% RH in a warm area and 100% RH in a cold area; specific humidity stays constant)

Vapour Pressure

Dalton's Law - total pressure of a mixture of gases = sum of pressure of constituents

For the atmosphere, vapour pressure is the pressure exerted by water vapour (For example, 4% humidity at surface, P = 1000mbar, vapour pressure = 40mbar)

Air exerts pressure - standard atmospheric pressure = 101.325kPa = 1013.25mb

Vapour pressure - pressure resulting from water molecules

Saturation vapour pressure - partial pressure of water molecules when air is saturated; strong relationship btw saturation vapour pressure and temperature

Saturation

There is a continual process of evaporation and condensation at air/water interface

- Net evaporation - more water molecules enter the vapour phase than return via condensation

- Net condensation - more water vapour molecules condense than vaporize

- Balanced evaporation and condensation - this is saturation; the vapour pressure under this equilibrium is the saturation vapour pressure

Consider perturbations to this balance - colder → less KE, less evaporation → shift to condensation → lower vapour pressure and mixing ratio

Potential Evapotranspiration

A measure of what could evaporate if moisture was unlimited

What causes precipitation? (3)

1) Condensation nuclei

2) Water vapour (and continuous input of water vapour as precipitation forms)

3) Collision/coalescence, Bergeron process - something that will allow droplets to reach a sufficient size

What are the best (3) conditions for evaporation?

Evaporation rates are high when a lot of energy is available (net radiation (Q*), sensible heat (QH)), when atmosphere is dry (low moisture concentration), and when it is windy

Precipitation Growth Processes (2)

1) Collision and coalescence - most common method of precipitation growth

2) Bergeron process - nucleation/growth of snowflakes or ice crystals in cold clouds (-15°C to -40°C); growthe via:

- Riming (accretion) - supercooled water droplets freeze on impact

- Aggregation - like coalescence

Ice Particle Changes

As ice crystals fall and collide with supercooled drops, they get bigger by accretion; ice crystals colliding with each other form aggregates (of various shapes/sizes)

Ex: Graupel - ice crystals, when shape cannot be distinguished

Vadose Zone

Realm of soil moisture; also called the unsaturated zone or zone of aeration

Capillary Fringe

Subsurface layer in which ground water seeps up from the water table by capillary action to fill pores

Zone of saturation (phreatic zone)

Zone of groundwater

What is soil made of?

- Soil solids - minerals (45%), organic matter (5%)

- Pore space - water (20-30%), air (20-30%)

Water content and organic matter vary significantly

Organic matter includes humus (80%), roots (10%), organisms (10%)

Soil Particle Size Classification

Sand - 0.06-2.0mm

Silt - 0.002-0.06mm

Clay - < 0.002mm

Clay Fraction

Highly weathered particles

Mostly clay minerals (ex: plagioclase)

Absorbs soil water very effectively

Clay fraction 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

Porosity

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

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

Soil Properties

Grain size, grain type, sorting (variability in size), bulk density (packing), water content (degree of saturation), hydraulic conductivity, thermal state

Volumetric Water Content

The fraction of water in the soil (θ = Vw/Vs)

This tells us the amount of pore space filled with water

At saturation Va = 0 & θ = n

Gravimetric Water Content

The mass fraction of water in the soil (θg = Mw/Ms)

Gravitational water is the water in a soil that is freely available for gravitational drainage (i.e. free water in the pore space)

- Absorbed water/hygroscopic water

- Capillary water

- Gravitational water

Field Capacity

The water in soil that can be retained against gravity, through (a) absorbed water and (b) capillary pressure

Max. volume under capillary tension

Permanent Wilting Point

Water content at which plants cannot draw water out of the soil (θpwp); available water is the water available to plants (θa = θfc - θpwp)

Wilting point - volumetric water content not available to plants (i.e. out of reach)

Divisions of the Subsurface (3)

Vadose zone - Pwater < Patmos

- Unsaturated zone - root zone, intermediate zone

- Capillary fringe

Water table - Pwater = Patmos

Phreatic zone - Pwater > Patmos

- also called saturated zone or groundwater zone

Hydraulic Head and Total Potential

The idea of hydraulic head in soils translates to total potential - gravitational potential (elevation) and pressure/matric potential

In soils, matric potential = capillary pressure = suction

(Essentially a 'negative pressure')

Bernoulli's Equation

Define a parcel of fluid moving through a pipe with cross-sectional area A, the length of the parcel is dx, and the volume of the parcel A dx. If mass density is ρ, the mass of the parcel is density multiplied by its volume m = ρA dx.

Surface Tension

A function of the interaction btw two materials at the interface

Fl = σ cosα x 2πr

Matric Potential (Suction, Ψ)

Can estimate Ψ as a function of pore size (Φ)

The drier the soil profile → the higher the matric potential

The smaller the pore size → the higher the matric potential

Different matric potential in every pore space - smaller pore sizes fill up faster

Matric Potential During Wetting/Drying

Suction is a function of

- Moisture content

- Where it is wetting or drying

- Antecedent conditions and soil type

- Pore structure

In general, when wetting - small pores fill first (high suction), so water content increases slowly; when drying - large pores empty first, so a rapid decline in water

Matric potential for sand will never be as high as clay because pore sizes are larger

When matric potential = 0 → saturation

Types of Water Flow in Soils

Ponded and non-ponded infiltration, macropore flow, fingering flow, funnel flow

Hydraulic Conductivity (K)

A measure of how effectively water moves through a substrate under a given hydraulic head (or potential) gradient (Units - m/s or cm/day)

Can be higher in a saturated environment

Permeability

Connectivity between pore spaces

K(θ) vs. θ

At high water contents, coarse soils have higher hydraulic conductivity than fine; at lower water contents - fine soils have a higher hydraulic conductivity than coarse

- At low θ, large pores in coarse soils are mostly empty per unit volume

- At low θ, more of the small pores per unit volume in fine soils filled with water

- At low θ, the tension head Ψ(θ) is higher (i.e. less negative) in fine soils

Soil Water & Plants

Gravitational water - drains right through (>field capacity)

Capillary water - water held in pores; plant roots can absorb this (i.e. available water)

Hygroscopic water - water adheres to soil particles (wilting point)

Water Use in Plants

Plants take up water and nutrients from the soil through osmosis

Mineral-rich water and nutrients used for:

- Fuel cell/tissue growth

- Photosynthesis (creation of C6H12O6)

- Cell respiration - production of energy from glucose

Excess water is brought up through this process, and plants evacuate this through transpiration; only 1-2% of soil water gets used for photosynthesis and plant tissue growth - more than 98% is 'waste'

Transpiration

Excess water is evacuated by opening of the stomata

Occurs during the day, when gas exchange and photosynthesis are active; also keeps plants cool

Trade-offs of stomatal opening - lets in CO2 as H2O transpires, but in a ratio of about 1:600 (driven by concentration gradients); "no free lunch"

Forces that draw water up (2)

1) Osmotic pump pulling water into the roots - this pressurizes and will push up water (overnight)

2) Evapotranspiration - creates suction forces that pull water up (day time); suction forces can be up to 1,500 kPa (Ψ = -15,000cm or pF = 4.2)

Plants can access water at a pF btw 2.0-4.2

- Less → gravitational water (pF btw 0-2)

- Greater → hygroscopic water

Penman-Monteith Equation

Recall the standard expression for evapotranspiration, given our limited ability to measure or predict it → equation to estimate this based on vegetation type and atmospheric conditions (temperature, vapour deficit, wind, net radiation)

Data needs - temperature, RH, four components of radiation, pressure, and surface (terrain, vegetation, soil) parameters

What is an Aquifer?

Must meet the following three criteria:

1) A geological unit

2) Saturated with water

3) Able to store and transmit water

2 Different Types - confined and unconfined

Based on structure of subsurface environment

Whether confined or unconfined, must have a confining layer beneath (not completely impermeable, but permeability contrasts)

Modern vs Young Groundwater

less than 50 years since recharge, most vulnerable to global environmental change

less than 100-years since recharge

Water Table

Top of groundwater within an unconfined aquifer (upper surface of the zone of saturation)

Can fluctuate throughout the seasons

Pressure at water table = atmospheric pressure

Unconfined Aquifer

Typically fluvial sediments (sands, gravels, silts) 'open' to the atmosphere, water table is free to fluctuate

Confined Aquifer

Saturated geologic zone overlain by a confining layer; pressures are greater than atmospheric pressure

Groundwater does not cycle as fast, groundwater ages can be >10,000 years

Receives water from a much smaller area

Over-harvesting can deplete the resource faster than it can recharge

Groundwater use in BC

25% of BC pop. and 45% of public water systems

impacted by quality (contamination) and quantity (drainage diversion, artificial recharge and withdrawal)

recharge is seasonally variable, land use changing recharge

shallow and confined aquifers vulnerable

Gaining Stream

stream is below water table and recharged by groundwater

Losing Stream

water table is below stream feature, losing stream water to the groundwater

Perched losing stream

Water table is not touching stream and losing water to the ground

Flow-through stream

water table is higher on one side causing stream charge, then loss in one direction

Aquitard vs. Aquiclude

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

Groundwater Storage

Aquifers serve both as:

1) Reservoirs for groundwater

2) As pipelines for groundwater movement (not flowing quickly, but flowing)

The amount of groundwater stored in a saturated material depends upon its porosity; recall that porosity depends on the shape, arrangement, and degree of sorting of constituting particles

Specific yield vs. Specific retention

Specific yield - the volume of water that can freely drain from a saturated rock or soil under the influence of gravity

Specific retention - the volume of water, which is retained by surface tension forces as films around individual grains and capillary openings

For example - clay or chalk has high porosity value but low specific yield

3 ways Groundwater “recharge” inputs are transferred and controlling factors (3)

Transfers

1. flow to adjacent areas via throughflow

2. re-surface as return flow or baseflow

3. long term storage in deep aquifers

Factors

• climate (local water budgets, inputs and outputs)

• geology (substrate properties control storage and throughputs)

• land use (human activities and land use affect infiltration/recharge)

Hydraulic Conductivity

Hydraulic Conductivity (saturated) - relevant for pore (matrix) flow in a saturated, porous medium

Saturated hydraulic conductivity > unsaturated hydraulic conductivity

General Rules for Groundwater Flow (5)

1) Water table reflects topography - deepest below uplands, shallow in valleys

2) Equipotential contours (Φ) are lines of equal hydraulic head - lines perpendicular to 'divides', parallel to boundaries

3) Streamlines perpendicular to equipotential contour Φ lines

4) Both horizontal and vertical flow components - generally down in uplands and up in lowlands

5) Flow nets complicated by changes in geology and topography

(More common to deal with m/day in groundwater flow, rather than cm)

Hydraulic Gradient

A function of hydraulic head

Gradient slope of the top of the groundwater table (indicates direction of movement)

Piezometer measurement components (3)

hydraulic head - mechanical energy of water at a location

elevation head (z) - elevation above reference level 0

pressure head (P/pg) - water pressure at free interface between air and water, equals length of column above the screen

Conditions for effective groundwater recharge (4)

• 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 10 m below ground surface

Groundwater Chemistry

Point sources - easier to identify and deal with (ex: landfills, gas stations, mines, fire sites, train wrecks, waste disposal sites, etc.)

Non-point sources - hard to pinpoint source, protect against, or legislate (ex: agriculture/feedlot runoff (manure, fertilizer), herbicides/pesticides, urban runoff, etc.)

EPA estimates on average over a 1-year period - 1,800 Olympic swimming pools of contaminants enter the groundwater supply

Groundwater Pollutants

Inorganic Pollutants - minerals, heavy metals (such as mercury, lead, arsenic)

Organic Pollutants - two principal sources of toxic organic chemicals in water:

1) Improper disposal of industrial and household wastes

2) Pesticide runoff from farm fields, forests, roadsides, golf courses, private lawns, etc.

Artificial Recharge

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 resources management

Purposes for artificial recharge:

- Conserve & dispose of runoff/flood waters

- Supplement natural groundwater recharge

- Reduce or balance salt water intrusion

- Supress ground subsidence (which reduces groundwater storage)

- Store water in off-seasons for use during growing seasons; can also reduce pumping (effective technique in areas with dry seasons)

- Geothermal applications

- Remove suspended solids by filtration through the ground

Wetlands

"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;

spans a continuum of environments where terrestrial and aquatic systems intergrade" - US Fish & Wildlife Service

Freshwater reservoir - approx. 20% of freshwater

Surface water drainage in Canada from most to least

1. Hudson Bay

2. Arctic Ocean

3. Atlantic Ocean

4. Pacific Ocean

5. Gulf of Mexico

Major rivers in Canada

by mean discharge

1. St. Lawrence

2. Mackenzie River (but longest and largest area)

3. Fraser

4. Columbia

5. Nelson

6. Ottawa

Characteristics of Moving Water

•Energy

•Types of flow

•Flow characteristics

•Measuring discharge

•Hydrographs

Wetlands in Canda

Approximately 14% of global wetlands

- Peatlands - cover 12%

- Wetlands - cover 14%

This corresponds to ~14% of global wetlands (though declining)

On average, 6% of global land surface is wetlands

Marshes

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

Swamps

also wet most of the time, but with woody plants, often treed; deeper water (often >1m), shrub and forested swamps (ex: mangrove swamps), nutrient rich

Bogs

water from precipitation, mostly; thick mat of vegetation (peat) rather than soil, often acidic, soft, spongy, organic (usually ~1 foot of peat)

Fens

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

Lakes

Uncounted in Canada, but >32,000

Mostly less than 100km2, but seven >10,000km2

Lake Baikal (Russia)

About 20% of the world's lake water, reaches depths of 1,637m

Estimated age (of water) 25-30 Ma

Forces involved in surface water flow

1) Gravity - moves water down rivers; drives a conversion from potential energy to kinetic energy/inertia (mv^2)

2) Friction - resists water flow, slows water down (creates different speeds at different levels; i.e. slower along river bottom → causes shear, and shear results in turbulence)

Thus, float method probably gives the maximum flow velocity (because water at the top of the stream will be moving faster than water at depth, due to friction)

Types of Flow

Laminar vs. turbulent

Steady vs. unsteady (steady → dv/dt = 0)

Hydrographs

Response of a stream to additional inputs (precip, snow melt, etc.)

Many factors influence how water moves toward stream channel - topography, soil characteristics, vegetation, rainfall intensity, etc.

Quickflow - flow associated with precip or other input events; includes contributions from surface runoff and/or throughflow (unsaturated zone)

Baseflow - groundwater flow (below water table); baseflow line can be a known value with regular (i.e. repeated) observations

Hydrograph character depends on:

- Precip characteristics - magnitude, intensity, duration, distribution, phase

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

Unit Hydrographs

Used to predict discharge/floods

What would the expected response be in a specific channel at a specific location with x amount of input? (generally 10mm, 100mm, etc.)

Extremely useful when trying to report likelihood of events (ex: floods)

"A predictive hydrograph for storm events/flooding"

Perennial vs. Ephemeral Streams

Perennial - streams that would be expected to have flowing water year-round (however, can still go dry in drought conditions)

Ephemeral - a stream that only flows after a precipitation event; water flows for hours, days, maybe weeks (not likely groundwater-fed)

Infiltration & Runoff

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

1) Infiltration excess overland flow - occurs when soil is not saturated; soil properties or land cover do not allow infiltration to keep up with rainfall or snowmelt rates (aka 'Hortonian flow')

2) Saturation excess overland flow - occurs when the soil becomes saturated and there is no longer space for water to infiltrate

Interflow - horizontal movement through the unsaturated zone (throughflow)

Factors influencing overland flow (6)

Precipitation intensity

Snow melt rate

Pre-existing soil saturation level

Soil type (hydraulic permeability)

Ground cover

Topography / terrain characteristics

Flooding

A rising 'stage' level associated with excess discharge in response to storms or seasonal melt - when water level exceeds banks

A natural process - required to maintain river form and function

Useful to understand for - designing bridges, culverts, sewers, dams, spillways; land-use zoning and planning; floodplain delineation; insurance?

Flood Prediction Methods (3)

1) Hydrographs

2) Recurrence interval (R) - annual peak records used to predict flood probability (P) based on record; plot flood frequency curve

3) Rational method - many rivers not properly gauged, limited historical data; estimate peak annual discharge using empirical formula and data on rainfall and basin statistics

What causes flooding in BC/North America?

BC: high intensity precipitation + rapid runoff in mountainous, low permeability rock terrain, debris torrents and mass wasting common

• rain on snow melt events

• autumn and winter frontal rains

• spring melt or freshet

• heavy summer rains (convectional and frontal)

Pluvial vs. Nivel systems

Pluvial system - more rain driven

Nivel system - more snow-melt driven

(Also hybrid systems)

Snow Hydrology

Understanding and predicting the physical processes of snow accumulation, ablation, melt water runoff, etc.

Estimation Problems in Snow hydrology (4)

1) Quantity of water held in snow pack

2) Magnitude and rate of water lost to the atmosphere by sublimation

3) Timing, rate, and magnitude of snowmelt

4) The fate of melt water

Spatial scales of snow distribution (3)

1) Macroscale - areas up to 106 km², characteristic distances of 10-1000km; larger scale meteorological effects are important

2) Mesoscale - characteristic distances of 100m-10km; redistribution of snow along relief features due to wind, deposition and accumulation of snow may be related to terrain variables and vegetation cover

3) Microscale - characteristic distances of 10-100m; differences in accumulation results from variations in air flow patterns and transport