1. Hydrology
Groundwater
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2. Groundwater Topics... General principles
Hydraulic head, fluid potential
Darcy’s Law, saturated groundwater flow
Hydraulic conductivity K
measurement of K
porosity
effects of heterogeneity on flow
groundwater flow patterns on a slope
R. Hudson - VFR Research

3. … relevant to Forest Hydrology
Unsaturated groundwater flow
hydraulic properties of unsaturated soil
drainage and infiltration
Interflow
Groundwater in relation to Forest Hydrology
How does forest harvesting affect groundwater
significance of those effects
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4. Hydraulic head Groundwater flows along an energy gradient
there are two possible energy gradients that affect groundwater flow: gravity and fluid pressure
z = z1 p1 p2
flow under fluid pressure
gradient where p1 > p2
z = z2
gravity drainage
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5. Groundwater head - energy for flow
z = elevation head
above reference
elevation (datum)
Y = pressure head (m)
= P/rg
where
P = fluid pressure
r = fluid density
g = acceleration
due to gravity
Groundwater head
is measured using
a piezometer. Y
h = z + Y z
datum
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6. Water table well vs. piezometer
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7. Darcy’s Law Groundwater flow is a function of hydraulic head gradient
total flow Q has units of volume/time
typically m3/s or litres/sec
specific discharge q is flow per unit area, units of length
the negative sign indicates
that flow moves in the direction
of falling head
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8. Hydraulic conductivity K
groundwater flow is driven by the hydraulic gradient dh/dl
K is a measure of the resistance to flow, is a property of the porous medium and the fluid
K has units of m/s or cm/s
k is permeability, is a property of the
medium related to diameter, packing,
shape and roughness of grains (m2, cm2)
m is the viscosity of the fluid (kg/m.s)
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9. Range of values of K
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10. Porosity
Porosity is another important property of porous media that governs water flow
porosity is a measure of the capacity of the medium to hold water
a volume VT of soil of rock is divided up into the volume of voids Vv and volume of solids Vs
porosity n = Vv / VT
void ratio e = Vv / Vs
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11. Range of values of porosity
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12. Relations between K and n
for soil , they are inversely proportional
for well sorted sediments, the finer grained they are, the lower K is and the higher n is
for poorly sorted sediments, smaller grains fill in voids between larger grains reducing K and n
for rock, K and n are related to structure
sedimentary rock, both n and K are less than that of parent sediments due to mineral deposition in voids
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13. Relations between K and n...
metamorphic and igneous rock have very low primary porosity, but K and secondary porosity are related to fracture spacing
porosity affects velocity of flow:
the lower the porosity, the greater the flow velocity:
v = q/n
flow velocity = specific discharge/porosity
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14. Heterogeneity Geologic formations are generally not homogeneous
in BC, most forested terrain is characterized by relatively thin (1-2 metre) coarse grained soils over basal till or igneous/metamorphic bedrock
the contact between soil and basal layer involves a sharp discontinuity in K such that the till or bedrock interface forms an impermeable boundary
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15. Flow in layered heterogeneity
Flow lines are perpendicular to equipotentials
increasing head
sand
clay
Flow will tend to go along the zone of higher K, and across
the zone of lower K. Thus preferential flow occurs in high K
zones.
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16. Flow in layered heterogeneity...
the grain size distribution of soil is generally not uniform, so there are variations in hydraulic conductivity
zones of relatively high K in soil become preferred flow paths - they carry more flow than zones of lower K
the distribution of K zones can be random, or K can decrease with depth in soil due to increasing clay content - the latter situation will result in more rapid groundwater flow as the water table rises
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17. Effects of slope steepness on flow
Slope gradients affect both direction and rate of groundwater flow
flow perpendicular to equipotentials
approx. lateral for steep slopes
dh/dl
dh/dl
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18. Groundwater flow on a slope
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19. Groundwater Recharge Groundwater flow follows hydraulic
gradient: total head
decreases with depth,
thus there is a downward
component to the
groundwater flow. This
is groundwater recharge,
and in the abcence of
water input, the water
table will fall.

20. Groundwater discharge
At riparian sites, ground-
water discharge often occurs.
In this case, head increases
with depth, resulting in an
upward component to ground-
water flow. In the example
shown, under high flow condi-
tions the water table rises
to the surface near the stream,
groundwater discharges out
of the soil and enters the
stream by overland flow.

21. Later that year... ...at the same site, under low
flow conditions, the water table
and the stream stage have
dropped. Groundwater is still
discharging to the stream
channel, but not at the soil
surface. Total head is now
independent of depth within the
soil. There is no longer an
upward component to ground-
water flow. Discharge to the
channel is essentially horizontal.

22. Occurrence of groundwater
Saturated vs. unsaturated
Define q as water content of soil
Saturated: all the void spaces are filled with water: qs = n
Unsaturated: void spaces are only partially filled with water: q < n
K is reduced because cross sectional area for flow is less than saturated cross section: K is now a function of moisture content
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23. Saturated vs. unsaturated flow
below water table above water table
Saturated: void spaces
filled with water: q = n,
Y > 0
Unsaturated: voids partially
occupied by air: q < n,Y < 0.
K is reduced; K = K(Y) or K(q)
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24. Soil drainage and infiltration
If pressure head increases with depth, then why does soil drain?
recall, there are two components of head: pressure head and gravity head
soil drains under gravity when elevation gradient (dz/dl) > pressure gradient (dY/dl)
drainage will continue until equilibrium is reached
equilibrium may never occur
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25. Infiltration
Initially, moisture content at the surface is low, hence K is low
When water is supplied to a dry soil, initially the water is absorbed, raising the moisture content and hence increasing Y
this creates a head gradient that drives water down towards the water table.
water moves down under large head gradient at the wetting front , overcoming the fact that K is low for dry soil
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26. Infiltration rates
Over time, infiltration rate will tend towards the saturated hydraulic conductivity of the soil
Initial infiltration rates for dry soil can be up to 5 times Ks for very dry soil
typical infiltration rates for forest soil are in the range of 50 to 300 mm/hr depending on the soil and its moisture content
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27. Macropore flow
It is generally accepted among forest hydrologists (if not hydrogeologists) that macropore flow is a significant component of runoff from forested catchments
Darcy’s Law does not describe macropore flow
difficulty is in defining a representative dimension for a macropore
macropores can form a large interconnected network
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28. Macropore flow (interflow)
they are formed from the rotting out of dead tree roots, aminal burrows, cracks in soil resulting from blocky structure, etc.
how to define a representative dimension for such a feature?
subsurface flow through macropore networks is much faster than soil matrix flow
often called interflow
we still do not know how to describe it mathematically
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29. Forest harvesting and groundwater
Forest harvesting alters groundwater levels (thus, groundwater flow) by altering the water balance
increase in water available for infiltration due to decreased interception, increased snowmelt
decrease in extraction of water from the soil due to decreased evapotranspiration
Related activities can also alter soil structure
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30. Effects of ground skidding and roads
ground based yarding can result in soil compaction, thereby reducing infiltration capacities
exessive access roads
ground skidding
these effects would tend to result in increased runoff, hence reduced infiltration
Road cuts on steep terrain can interrupt subsurface flows
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31. Effect of road cut on groundwater flow
Before: ground-
water flow on
treed slope
After: potentially
increased flow, inter-
ception by road cut,
conversion to
ditch flow
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32. intercepted flows can either be routed to the stream channel thereby altering streamflow hydrograph, or can be routed back onto slope below the road by way of culverts or cross ditches
in many cases, poorly placed culverts and inadequate culvert density have resulted in concentration of ditch flows onto unstable slopes, resulting in landslides
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33. Improper culvert placement
A plan view schematic of a road cut showing water flow pattern
hillside above road
cut bank
road surface
Too few culverts and poor placement results in flow disruption

34. Landslides and pore pressure
Increased pore pressures at the failure plane of potential instability results in reduced frictional contact between soil grains
this results in a reduction in the forces that keep the soil on the hillside.