subsurface water
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Transcript of subsurface water
A presentation on Subsurface water
K.Bhargav Kumar154104063
Subsurface Waterunit volume of subsurface consists of
soil/rock, and pores which may be filled with water and/or air
total porosity= volume voids/total volumewater content=volume water/total volumesaturation=volume water/volume voidsdegree of saturation delineates various
zones of subsurface water
Definitionssoil water - Ground surface to bottom of root
zone depth depends on soil type and vegetation. May become saturated during periods of rainfall otherwise unsaturated (soil pores partially filled with air). Plants extract water from this zone. Evaporation occurs from this zone.
intermediate vadose zone - Between soil water zone and capillary fringe. Unsaturated except during extreme precipitation events. Depth of zone may range from centimeters to 100s of meters.
Definitions Continuedcapillary zone - Above saturated zone. Water
rises into this zone as a result of capillary force. Depth of this zone is a function of the soil type. Fractions of a meter for sands (mm) to meters for fine clays. All pores filled with H2O, p < 0. Effect seen if place bottom of dry porous media (soil or sponge) into water. Water will be drawn up into media to a height above water where soil suction and gravity forces are equal.
saturated zone - All pores filled with water, p > 0. Formations in this zone with ability to transmit water are called aquifers.
Unsaturated ZoneWater can exist in all its phases in the
unsaturated zone.Liquid water occurs as:
hygroscopic water - adsorbed from air by molecular interaction (H-bonds)
capillary water - held by surface tension due to viscosity of liquid
gravitational water-water in unsaturated zone in excess of field capacity which percolates downward due to gravity ultimately reaching saturated zone as recharge.
Unsaturated ZoneHygroscopic and capillary waters are held by
molecular electrostatic forces (between polar bonds and particles -- surface tension) in thin films around soil particles drier soil, smaller pores hygroscopic and capillary forces
Hygroscopic water - held at -31 to -10,000 bars. Water is unavailable to plants or for recharge to groundwater.
Capillary water - Held at -0.33 to -31 bars. More water filling pores but discontinuous except in capillary fringe. This water can be used by plants.
DefinitionsPermanent wilting point: tension (suction,
negative pressure) below which plant root system cannot extract water. Depends on soil and type of vegetation. Typically -15 bars (-15x105 Pa, -15000cm
Field capacity: tension (suction, negative pressure) below which water cannot be drained by gravity (due to capillary and hygroscopic forces) Depends on soil type. Typically about -0.33 bars
Typical Moisture Profilesrain after a long dry period
direction of moisture movement
moisture content
depth
root zone
hygroscopic
wilting point field
capacitysaturatio
n
Typical Moisture ProfilesDrying process
moisture
depth
field capacity
saturation
1 - Drying in upper layers by ET.2 - Bottom part of wetting front continues . Upper part continues to dry.3 - At some point and movement results in no moisture gradient4 - Dry front established. Lower zones are being depleted to satisfy PET at surface. Drying continues until capillary forces are unable to move water to surface.
Dacry-Buckingham lawFlow in unsaturated porous media governed by a modified
Darcy’s law called Darcy-Buckingham law :
- suction head (capillary head) or negative pressure head. Energy possessed by the fluid due to soil suction forces. Suction head varies with moisture content, n, 0, < n , is negative.
K() - hydraulic conductivity is a function of water content , K() because more continuously connected pores, more space available for water to travel through, until at = n, K(n) = Ksat
zh
zhKqz
Measuring Soil SuctionSoil Suction () head measured with
tensiometers, an airtight ceramic cup and tube containing water.
Soil tension measured as vacuum in tubes created when water drawn out of tube into soil. Comes to equilibrium at soil tension value.
Tensiometers often used to schedule irrigation.
Tensiometer
Why different flow equations?Steady-state Transient
Saturated
Unsaturated
Darcy’s law
Darcy’s law (with
K(q))
N/A
Richards’ equation
Darcy’s law:L
AKq
q changes with time
No K(q)
No DqNo q(y)
Equation of Continuity(Conservation of Mass)
Steady-state TransientSaturated
Unsaturated
Darcy’s lawDarcy’s
law (with K(q))
Richards’
equation
Input – Output = Change in Storage
xq
=t
txq
Richards’ equation
LKq
Given Darcy’s law:
xK
xxq
Let things change from place to place (say, in the x-direction)
txq
We also want
conservation of mass
xK
xt So we substitute
it in to the left-hand side
Richards’ equation
xK
xt
Remember that the
potential gradient, ,combines elevation, osmotic, pressure, and matric components (among others).
x
Sometimes it’s convenient to separate out the elevation part:
1
xK
xt Vertical
0
xK
xt Horizontal
Just remember that this y doesn’t include elevation!
depth
Wetting Zone
TransmissionZone
Transition ZoneSaturation Zone
Wetting Front
InfiltrationGeneral
Process of water penetrating from ground into soil
Factors affectingCondition of soil surface,
vegetative cover, soil properties, hydraulic conductivity, antecedent soil moisture
Four zonesSaturated, transmission,
wetting, and wetting front
InfiltrationInfiltration rate, f(t)
Rate at which water enters the soil at the surface (in/hr or cm/hr)
Cumulative infiltration, F(t)Accumulated depth of water infiltrating during
given time periodt
dftF0
)()(
dttdFtf )()(
t
f, F F
f
InfiltrometersSingle Ring Double Ring
http://en.wikipedia.org/wiki/Infiltrometer
Infiltration MethodsHorton and Phillips
Infiltration models developed as approximate solutions of an exact theory (Richard’s Equation)
Green – AmptInfiltration model developed from an
approximate theory to an exact solution
Horton Infiltration Model• one of earliest infiltration equations developed
(1933) and the most common empirical equation used to predict infiltration if ponding occurs from above:
• Instantaneous infiltration
• Cumulative infiltration
• fc, minimum infiltration capacity (approximately saturated hydraulic conductivity)
• fo, maximum infiltration capacity (function of saturated conductivity and soil tension)
• k constant representing exponential rate of decrease of infiltration
ktcc ffftf exp)()( 0
t Ktcoc
KfftfdftF
0)exp1()()(
Horton’s Infiltration Model
• All are empirical parameters which must be fit to each soil type using data from a ring infiltrometer experiment
• Horton’s equations are only valid after ponding. Therefore all water the soil has potential to infiltrate is available at soil surface. Ponding will only occur if i > f(t). Should only be used during very high intensity precipitation events over small areas
fc
fo rate of decay governed by k,increase k, increase rate of decay
(analogous to Ksat)
t
F(t)f(t)
(time after ponding)
Green-Ampt Assumptions
Wetted Zone
Wetting Front
Ground Surface
Dry Soil
L
ni
z
= increase in moisture content as wetting front passes
= Suction head at “sharp” wetting front
Conductivity, K
L = Wetted depth
K = Conductivity in wetted zone
Ponded Water 0h
0h= Depth of water ponding on surface (small)
Green-Ampt soil water variables
Wetted Zone
Wetting Front
Ground Surface
Dry Soil
L
ni
z
r e
i = initial moisture content of dry soil before infiltration happens
= increase in moisture content as wetting front passes
= moisture content (volume of water/total volume of soil)
r = residual water content of very dry soil
e = effective porosity
n = porosity
Green ampt equation:
Infiltration rate:
The cool thing is, though, that what we want (F or f) is a function of only things we can figure out (porosity, initial moisture content, soil conductivity, and soil capillary pressure). The problem is that you can’t easily put F on one side, and all the other stuff on the other. This inability to separate the equation means that the equation is nonlinear.
Ponding timeElapsed time between the time rainfall
begins and the time water begins to pond on the soil surface (tp)
Ponding Time
Up to the time of ponding, all rainfall has infiltrated (i = rainfall rate) if
1
FKf
ptiF *
1
* ptiKi
Potential Infiltration
Actual Infiltration
Rainfall
Accumulated Rainfall
Infiltration
Time
Time
Infil
trat
ion
rate
, fC
umul
ativ
e In
filtr
atio
n, F
i
pt
pp tiF *
)( KiiKt p
Referencesenchartedlearning.comtutor.comHuggett, J. (2005) Fundamentals of Geomorphology,
Routeledge,Horton, Robert E (1933)
"The role of infiltration in the hydrologic cycle" Transactions of the American Geophysics Union, 14th Annual Meeting, pp. 446–460.
Horton, Robert E (1945) "Erosional development of streams and their drainage basins; Hydrophysical approach to quantitative morphology" Geological Society of America Bulletin, 56 (3): 275–370. doi:10.1130/0016-7606
Thank you