Groundwater Hydrology

Calculating recurrence interval (T)

(T) = (n+1)/m

n = number of records (annual flood)

m = the rank of the particular event, 1 being the largest.

Where do we get annual flood data?

Network of measuring stations

Stream Gage

tracks surface height of river (stage), measured relationship of stage to streamflow.

Flood prediction:

1. Gather maximum annual streamflow or water level.

2. Calculate recurrence interval (T)

3. Use relationship to predict gage height or flood dischage.

Hydrographs

a graph that plots the rate of water flow (discharge) or water level (stage) over time in a river or stream. 

Flood behavior 

Permeable surface

Vegetation

Drainage density

Storm path

Rainfall intensity

Prior saturation of soil/rocks

Reduce flooding

Dam construction

Pros - mostly works

Low GHG emissions energy 

water supply and recreation

Cons - failure is catastropic

construction displace people 

Geopolitical concerns 

River erosion changed significantly

Can trigger (earthquakes) 

Artificial levees

Problems with levees

• increase flooding somewhere else in stream 

• They break 

• Isolate natural floodplains 

• 1. reduce groundwater recharge 

• 2. pollutants deposited stay in river 

• 3. change riverine habitats 

Channelization

Friction slows water flow

Reduce friction, straighten the path. 

Increase stormwater infiltration

improves retention by allowing rainwater to seep into the ground.

Where does groundwater come from?

30% Earth’s freshwater

68% glaciers

2% rivers and lakes

Water Cycle

Properties influencing infiltration

Porosity and Permeability

Porosity

fraction of the rock volume that is open space

Permeability 

ease with which fludis can pass through a porous material. 

Permeability depends on

v = velocity of fluids

u = viscosity of fluids 

change in pressure = change in pressure across given distance x 

Water table / unconfined aquifer

• surface

• below which all available pore spaces in soil/rock 

Aquifers

rock layers that store and transport water “easily” (slowly)

Recharge area

Wherever water enters the aquifer

Discharge area

where water leaves the aquifer

Contribute to increased flow of velocity 

Direction and rate of flow matter. 

Steeper gradient and higher discharge 

Cone of Depression 

the conical-shaped drop in groundwater levels around a well that is being pumped faster than the aquaifer can naturally replenish. 

Overpumping

• lowers the cone of depression

• dries up the stream 

If extraction > recharge 

Causes overdraft 

• water table lowers 

• Where? Arid areas supporting agriculture, industry 

Consequences of overdraft

reduce water supply, land subsidence, and increased pumping cost.

• Subsidence—typically irreversible in long-term decline in the water table.

Subsidence

The sinking of land elevation due to excessive grundwater extraction.

Watershed

a land area that drains water from all streams and rianfall to a common outlet, sich as river mouth, lake, or reservoir.

Aquitard (Confining Bed)

A layer of rock or sediment with low permeability (like dense clay or shale) that restricts the flow of ground water.

Potentiometric Surface 

surface that represents the pressure level to which water would rise in a well drilled into a confined aquifer.