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30 Cards in this Set
- Front
- Back
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water flows from |
high potential to low potential |
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total water potential = |
sum of all potential components -gravitation, matric, pressure, osmostic |
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hydraulic potenial in saturated and unsaturated soil |
= gravitational + matric in unsaturated soil
= gravitational + pressure (hydrostatic) in saturated soils |
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hydraulic potential in soil or aquifers determines.. |
direct of water flow |
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vadose zone in saturated flow = |
unsaturated zone and tension-saturated |
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perched water table (saturated flow) |
existence of a low permeability clay layer in a high permeability sand formation can lead to the formation of a discontinuous saturated lense, with unsaturated conditions existing both above and below
promoting the development of a perched water table and flow parallel to the upper surface of the confining layer - used in cover design |
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inverted water tables |
heavy rainfall and snowmelt may lead to the formation of a temporary saturated zone above ground surface
the lower boundary is an inverted water table underlain by unsaturated conditions |
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steady state saturated flow
volumetric flow rate (discharge) flux |
volumetric flow rate: Q (m3/s) = volume of water per unit time
flux: Jw (m3/m2/s) -> m/s = volume of water per unit area per unit time
flux is a vector, and its sign depends on definition of positive direction. |
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steady state saturated flow
gravitation potential hydrostatic potential matric potential osmotic potential
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gravitational potential - elevation of the point reference
hydrostatic potential - the height of standing water
matric potential - the effect of soil particle
osmotic potential - the effect of solute in solution |
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what is delta P / delta z |
hydraulic head gradient
dimensionless, and units cancel out
driving force causing the water to flow through the soil (high to low)
is a macroscopic driving force because of the complex pore geometry of the soil |
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darcy's law
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Jw = -Ks (delta Ph / delta z)
Ks or Ksat = saturated hydraulic conductivity (m/s)
negative sign - for a positive flux, water flows from high to low potential therefore the gradient is negative
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piezometers are installed in groups so that.. |
they can be used to determine the direction of groundwater flow |
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quantity of flow or volumetric flow rate |
is the volume of fluid passing through soil in an hour (cm3/hour) |
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water flux - 5 facts |
- volume of water passing a unit cross sectional area in a unit time - m/s - =Q/A where A is cross sectional area - has directions (positive or negative) - different from water velocity (or pore velocity) = pre water velocity x theta v
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5 steps os solving steady state saturated flow problems |
1) choose a reference elevation 2) evaluate potentials at each point 3)calculate hydraulic gradient 4)use Darcy's law to solve for Jw or Ks 5) check if your answer is reasonable (is flow from high to low, are the signs correct?) |
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compare hydraulic equillibrium, steady state, and transient flow |
hydraulic equilibrium : no differences in potential (hydraulic potential) between the two points of interest -> ZERO FLUX
steady state flow : constant hydraulic potential gradient -> CONSTANT FLUX
Transient flow : variable hydraulic potential gradient -> VARIABLE FLUX |
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Factors Affecting Ks |
Ks is soil specific and extremely variable
texture: porsity and pore size distribution: -large pores -> high Ks -Small pores -> low Ks
Pore geometry: tortuosity (having many turns)
presences of fractures or macro pores - preferential flow - root channels, burrows
soil structure and heterogeneity |
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Ks and pore geometry
result of - pore size, path length, friction |
pore size: larger the pore size, the larger the Ks
path length: longer the flow path, the smaller the Ks
Friction within the pores |
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what does steady state imply? |
a constant flux. if Ks is equal to all points in the soil, then hydraulic potential gradient is equal to all points in the soil |
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applications of saturated flow |
regional groundwater flow -1 directional flow problems -gravitational and hydrostatic potentials may act in all directions, fluxes may be 3 D -a very practical is to determine the direction and magnitude of groundwater flow |
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direction and magnitude of GW flow (in terms of saturated flow) |
must calculate vertical (nest of piezometers) and horizontal gradients (differences in Ph in different locations)
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characteristics of UNsaturated soils and flow |
Large Pores Empty First -capillary rise and pore size -macropores empty then meso, then micro. -pores between sand before clay -pores between aggregates first and in aggregates last |
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unsaturated flow in pores cont'd |
flow paths increase in length
there is less cross section flow
drag flow increases
Consequence: Hydraulic conductivity sharply decreases with the decrease in soil water content or matric potential |
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unsaturated flow
volumetric water content ______ with the ______ in matric potential or _______ in the soil water suction |
decreases
decreases
increase |
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unsaturated flow
flow inwet soils are ____ than flow in the drier condtions |
FASTER |
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How soil texture affects the soil hydraulic conductivity |
sandy soil adsorbs water more rapidly during infiltration
clay can sustain the evaporation longer |
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Darcy's law in unsaturated soils |
Jw = -K(Pm) [delta Ph / delta L]
where Ph = matric potential + gravitational potential |
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saturated soil
-> steady
-> transient
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steady
- theta = theta s - Jw = constant - constant head (leaching from lagoon ex)
transient
- theta = theta s - Jw = Jw (t) - falling head (falling water level of a slough) |
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unsaturated soil
-> steady
-> transient |
steady
- theta = constant < theta s - Jw = constant - Flow in deep soils
transient
- theta = variable < theta s - Jw = Jw(t) - flow in the root zone |
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unsaturated hydraulic conductivity principle |
only a function of matric potential or soil water content
at a soil water content or matric potential is the water flux at unit hydraulic gradient at the water content or matric potential |
