Peter Dahlhaus SCGEO 2106 Week 4. PrecipitationEvapotranspirationPond Storage Overland...
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Transcript of 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
Ch
an
ne
l sto
rag
e
Ru
no
ff
Gro
un
dw
ate
r d
isch
arg
e
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.