Lec 13: Soil water & groundwater III Flashcards
What are the 2 regimes of subsurface flow?
unsaturated flow (soil water flow)
saturated flow (groundwater flow)
What is saturated flow?
How does its flow velocity compare to that of rivers?
What does its flow velocity depend on?
Movement of water through saturated
pores and fractures
Relatively slow (cms to meters/day)
compared to flow of water in rivers
Flow velocity depends upon:
* Slope of the water table
* Permeability of the rock or sediment
in aquifers
Define aquifer
A permeable geologic unit that can transmit a significant amount of
water under an ordinary gradient
Define aquiclude
A geologic unit which does not transmit a significant quantity of water under
ordinary gradients
Aquiclude = confining bed = confining layer
What determines the type of aquifer?
What are some common types of aquifers?
The presence or absence of aquicludes determines the type of aquifer
The most common aquifer types are:
– Unconfined aquifers
– Confined aquifers
– Perched aquifers
Define unconfined water
… ground water in an aquifer with a lower barrier to movement caused by the
presence of an aquiclude below; however, there is no upper barrier, which allows
water to fluctuate over distances upwards
When does perched groundwater occur?
… occurs when an unconfined water zone sits on top of a clay lens or another quasiimpermeable lens, which separates it from the main aquifer below.
Define confined water
… ground water in an aquifer with a lower and upper barrier to movement, as there
is an aquiclude located immediately above and one located immediately below
the aquifer
What is hydraulic conductivity
measure of the ease with which water can move through
pore spaces or fractures in aquifers
Define groundwater recharge and discharge
Groundwater recharge is any water added to an aquifer, notably through
infiltration and percolation
- Groundwater discharge is any process that removes water from an aquifer
system (e.g., natural springs, artificial wells).
The presence and magnitude of subsurface water flow (saturated and unsaturated)
is largely determined by…
differences in subsurface water energy
What is hydraulic head a measure of?
what are the two forms of it?
Hydraulic head it sum of …
groundwater energy
- Elevation head (z): Height above a datum
- Pressure head (hP or ψ): height of water which would create a certain pressure
hydraulic head is total head, it is the sum of elevation and pressure head
What is hydraulic gradient (I)?
Is it negative or positive?
Hydraulic gradient: groundwater energy difference between two locations
It is the change in total head over a given distance
Negative, because it is defined from low to high hydraulic head. Groundwater flows in direction of decreasing hydraulic head (from higher to lower hydraulic head)
What does Darcy’s law state?
What is the constant of proportionality that is used?
States that the flux of water through a permeable medium is proportional to
pressure differences or energy differences within that medium.
The constant of proportionality that is used is the hydraulic conductivity (K)
What does Darcy velocity describe?
What is the problem with Darcy velocity?
The Darcy velocity attempts to describe the average behavior of all water
molecules, in macroscopic terms that are easily measured
Darcy velocity assumes that flow occurs across the entire cross-section,
whereas flow occurs only through interconnected pore channels
