Peter Dahlhaus SCGEO 2106 Week 4. PrecipitationEvapotranspirationPond Storage Overland...

Peter Dahlhaus

SCGEO 2106

Week 4

Evapotranspiration

Pond Storage

Overland Flow

Throughfall

Interception

Interception Storage

Infiltration

Soil moisture storage Interflow

Throughflow

Groundwater recharge

Groundwater storage Baseflow

Return flow

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Unsaturated zone(Vadose zone)

Saturated zone(Phreatic zone)

When does water become groundwater?

Soil moisture

pressure

+’ve-’ve(suction) (pressure)

Hydrostatic increase

Water rises in a column of soil due to capillarity. The ‘suction’ is due to the surface tension between the water molecules and the soil particle surfaces.

Small diameterGreatest height

Surface tension

Capillary Fringe:

Silty Clay ~ 1 metreFine Sand ~ 0.1 metreGravel ~ 0.001 metre

Groundwater is stored in the spaces and voids in the rock mass, such as the pore spaces between the grains and particles, or in fractures or in cavities.

For groundwater to move, the spaces or voids need to be interconnected. The pathways can be very torturous and complex, like a three-dimensional maze.

Groundwater moves at varying speed but is usually very slow. Velocity ranges from a few microns per year (in a clay) to hundreds of metres per day (in a very open-fractured rock).

“Underground rivers” don’t really exist. (Rivers might disappear underground into a cave, but that’s not the same as groundwater. Deep Leads are buried rivers, but the surrounding rocks are saturated with groundwater as well).

Groundwater storage

Volume of voids (Vv) Total volume (Vt)

Porosity (n) =

Effective porosity (permeability) enables an aquifer/rock unit to store, transmit and release water

Primary porosity is made at the same time as the rock – sands, gravels, sandstone, limestone

Calcarenite (dune limestone)Barwon Heads

ScoriaMt Buninyong

Secondary porosity is made when rocks are fractured or “dissolved” by later processes

Limestone cavePort Campbell

Fractured rhyoliteWannon

Fractured basaltDunnstown

Specific Yield is the ratio of the volume of water drained under gravity to the volume of saturated rock.

Groundwater storage

Specific yield (Sy) = Volume drained (Vd)

Total volume (Vt)

Specific Retention is the ratio of the volume of water retained after gravity drainage to the volume of saturated rock.

Specific retention (Sr) = Volume retained (Vr)

Total volume (Vt)

Specific yield + specific retention = porosity

Sy + Sr = n

A core one metre long and 10cm diameter

is extracted from an aquifer.

Saturated weight is 19.65kg

It is left to drain (by gravity) and then

weighed as 17.29kg

It is then oven dried (105oC) to a constant

weight of 16.90kg.

Calculate specific yield and specific

retention and porosity.1 m

0.1 m

Groundwater storage

1 m

0.1 m

A core one metre long and 10cm diameter is extracted from

an aquifer.

Saturated weight is 19.65kg

It is left to drain (by gravity) and then weighed as 17.29kg

It is then oven dried (105oC) to a constant weight of 16.90kg

Total Volume (Vt) = 0.00786m3

Weight of water drained = 2.36kg

Volume of water drained (Vd) = 2.36L = 0.00236m3

Specific yield (Sy) = 0.00236/0.00786 = 0.3 = 30%

Volume of water retained (Vr) = 0.39L

Specific retention (Sr) = 0.00039/0.00786 = 5%

Porosity (n) = Sy + Sr = 35%

Groundwater storage

The Saturated ZoneThe watertable is usually a subdued replica of the land surface

Springs, seeps, swamps, rivers & lakes occur where the groundwater intersects with the land surface

Unsaturated zone(Vadose zone)

Saturated zone(Phreatic zone)

Water tables fluctuate with seasonal input (recharge)

The amount of groundwater in storage changes with the seasons

Movement of waterGroundwater flows from higher elevations to lower elevations.It travels from where it enters the system (recharge) to where it leaves the system (discharge)

Unconfined aquifer- Open to the surface- Broad recharge area

- Includes most aquifers

Aquifer conditions

Unconfined – open to the surface.

Confined – sandwiched between less

permeable beds.

Fractured rock – water stored in fractures.

Confined aquifer- “Sandwiched” between less permeable beds - Recharge area is limited to aquifer outcrop- Source of artesian water

Covering layer Aquifer type

Impervious Confined

Semi-pervious, negligible horizontal flowSemi-confined

Less pervious than the main aquifer, significant horizontal flow

Semi-unconfined

Aquifer – carries water in useable quantity

No covering layer = Unconfined

Confining beds make up the non-aquifers and may be referred to as:

aquifuge - an absolutely impermeable unit that will not transmit any water,

aquitard - a low permeability unit that can store groundwater and transmit is very slowly, and

aquiclude - a unit of low permeability located adjacent to a high permeability layer.

http://campuswaterquality.ifas.ufl.edu/images/floridianaquifer.jpg

An aquifer system is the complete 3-d package of aquifers and confining beds

Total head Static head

PressureHead

ElevationHead

Groundwater Head

Australian Height Datum (AHD)

ground level

groundwater bore

Hydraulic Gradient (i) = (h1 – h2)/L

Hydraulic gradient shows flow direction

Three point ProblemsThree points are needed to fix a plane in space

N

Scale

0 100m

Bore CRLgw = 39m

Bore AElevation = 52mSWL = 10mRLgw = 42m RLgw 50m

RLgw 45m

RLgw 40m

Bore BRLgw = 49m

Flow

dire

ctio

n

ΔL =

100

mΔH

= 5

m

Hydraulic gradient = 0.05

Vertical Gradients

Groundwater flow is three-dimensional

DARCY’S EXPERIMENT

Q Cross sectional area (A)

Q to the head loss over a distance (i)

Darcy’s Law

Q = kiA

k = Hydraulic Conductivity

Transmissivity (T)Hydraulic Conductivity (k)

m mm m km

Constant Head Permeameter Falling Head Permeameter

Hydraulic conductivity is often varied in a single aquifer

Homogeneity / Heterogeneity

http://www.regione.emilia-romagna.it/wcm/geologia_en/Sections/Water_resources/rel_scentifiche/094_err_case_study/fig_01.jpg

http://ess.nrcan.gc.ca/gm-ces/bulletin/bulletin_v3_2_e.php

Deltas, alluvial plains, lacustrine deposits, paludal deposits and glacial sediments are examples of heterogeneous aquifers.

Isotropy / Anisotropy

IsotropicHomogeneous

IsotropicHeterogeneous

AnisotropicHomogeneous

AnisotropicHeterogeneous

http://www.kgs.ku.edu/Hydro/Publications/2005/OFR05_29/gifs/fig12.jpg

Reality:

Most aquifers are heterogeneous and anisotropic in three dimensions.

The degree of variation depends on the scale of the investigation. As you zoom out the variations become less important.