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

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