Climate and water

Groundwater flow

Take away ideas and understandings

In recharge areas water is added to groundwater, in discharge areas water is lost from groundwater.
Topography and geology are the dominant factors controlling groundwater flow.
Storativity describes the property of an aquifer to store water.
Hydraulic conductivity is measured by performing a pumping test, i.e. by pumping one well and observing the changes in hydraulic head in neighboring wells.

What drives groundwater flow?

gravity is the dominating driving force
water flows from high elevation to low elevation and from high pressure to low pressure, gradients in potential energy (hydraulic head) drive groundwater flow
recharge and discharge (Fig 7.2)

in recharge areas water is added to groundwater
in discharge areas water is lost from groundwater
in recharge (discharge) areas, the hydraulic head decreases (increases) with depth
recharge occurs from the unsaturated zone or from surface waters
groundwater discharge occurs into rivers, lakes, springs, or by evapotranspiration
examples:

"Puszta" in Hungary: groundwater is discharging in the low lands of the Great Hungarian Plain and leaves the dissolved salts behind -> reduction of soil quality -> bad conditions for agriculture
example: evaporation in the Sahara, loss of valueable groundwater resources that were recharged in the last ice age (loss may be up to a few 10's inches per year)
example: springs, e.g. at Grand Canyon

we can draw flownets in a qualitative way if we know geology and topography, flow lines have to be parallel to no-flow boundaries
the hydraulic head along any equipotential is equal to the elevation of its intersection with the water table (Fig 7.3)

Regional groundwater flow

effect of basin aspect ratio (length to depth) (Fig 7.4)

basin yield higher in the deeper basin

effect of water-table topography (Fig 7.5)(Fig 7.6)

local, intermediate, and regional flow systems
if local relief is negligible, but a regional water-table slope exists, only a regional flow system will develop
if local hill-and-valley topography exists, but no regional slope, only local flow systems will develop.
if both local and regional topography exists in a basin, all three types of flow systems (local, intermediate, and regional) will develop
Explore 7

effect of heterogeneity

Well hydrographs

a well hydrograph shows the variation in water level in a well through time
water level in an unconfined aquifer in VA (Fig 7.9)

Storage of groundwater in aquifers

in many areas of the world the hydraulic head is declining with time because a lot of water is pumped out of the aquifer
storage in unconfined and confined aquifers is different

in unconfined aquifers the water pumped stems from drained void space
in confined aquifers the water stems from decompression of the water and the sediments.
the same change in water table represents a larger amount of water if taken from an unconfined aquifer as compared to a confined aquifer

storage of water in aquifers: yield per unit area and unit change in hydraulic head

unit: m³/m/m²(=> dimensionless)
in unconfined aquifers the storage coeff. is high, somewhat smaller than the porosity

for a 1-m decline in the water table, the volume of water produced per unit aquifer area is the specific yield, Sy. (Fig 7.10)

in confined aquifer much smaller ~10^-6

for a 1-m decline in the potentiometric surface, the volume of water produced per unit aquifer area is the storativity, S. The aquifer material is not drained and remains saturated.(Fig 7.11)

where is water being stored in confined aquifers? => compressibility of water and change in aquifer structure
land subsidence as a result of overpumping

examples:

Mexico City aquifer/Ogallala aquifer
the Dakota artesian basin: flowing artesian wells (hydraulic head above surface) are wells in which the water level is higher than the surface. A lot of wells were drilled into the Dakota basin, in South Dakota about 15000 wells. Most of them do not flow anymore
New Mexico, where an old school well was still flowing when visited, why did it break?

How to measure hydraulic head and hydraulic conductivity?

hydraulic head: install a well open to the aquifer only over a small distance (short screen), measure the level of the water in the well relative to a reference surface, for example sea level
hydraulic conductivity or transmissivity:

the change in water level in the pumping well, or in observation wells nearby, is referred to as a drawdown
the amount of this drawdown will decrease as one moves away from the pumping well, and the pattern that is produced is called a cone of

depression

we can measure the hydrualic conductivity by performing a pumping tests

shape of depression cone (Fig 7.13)
how does this cone look like in different geol. environments?
quantification (Theis equation)

What information can be drawn from the hydraulic head?

where the water is flowing
how fast it is flowing
how much water there is

Resources

Freeze, R.A. and Cherry, J.A. (1979) Groundwater. Prentice Hall, 604p.