Earth Science
ademola
Sep21_hydrology.pptxHydrology
PSC 121 Prince George's Community College
Four agents of change:
Air
Ice
Water
gravity
Which is dominant and why?
Background: making sedimentary rock
1. Weathering
Erosion
Sediment deposition
On Earth, its water
PSC 121 Prince George's Community College
Hydrology:
the study of the movement, distribution, and
quality of water on Earth
Hydrosphere: the total amount of water on a planet;
either solid, liquid or gas
either on surface, underground, or in atmosphere
Sections 22.1 and 22.2 of text
Another cycle: Hydrological Cycle (ch 22.1 text)
The hydrological cycle recycles our water
This is our 2nd recycling system: which was the first?
3
Where is all the Earth’s water?
PSC 121 Prince George's Community College
More underground than in
lakes and rivers!
What is the H2O?
PSC 121 Prince George's Community College
Fresh water does the cycling
Most (3/4) is locked up as ice
Almost all the rest is groundwater
(lakes and rivers are tiny fraction, but are
critical for erosion and sediment formation)
Most of the liquid fresh water: Underground (either soil moisture or groundwater) !
PSC 121 Prince George's Community College
Water table
Saturation: means holds maximum amount of water
PSC 121 Prince George's Community College
Groundwater
Any water that is below the water table
May be in soils or porous rocks- all air spaces filled saturation
Can be tapped for wells or supply artesian springs
key point: must dig below the water table
A flood is simply the water table rising from runoff.
Click on image to view
a groundwater animation
http://earthguide.ucsd.edu/earthguide/diagrams/groundwater/index.html
PSC 121 Prince George's Community College
Key three factors which control how water flows underground.
1. Porosity
2. Water Retention
3. Permeability (text calls this “hydraulic conductivity”)
Physical Properties of Sediments and Water Flow – Pore Space
Unoccupied or void space between sediment particles
Can be filled with air or water
Measured by packing solid in a container to measure the total volume; water is added just to the surface filling the empty space added water = pore space
Do demo
PSC 121 Prince George's Community College
How do we measure porosity?
We measure volume of water to saturate. That tells you the volume of the pores
Total volume: rock + water added = 150 ml
Water added = initial – final in beaker = pore space = 150 – 75
Here it was: 75 ml = pore space
From Demo:
Volume of pore space
% Porosity = 100 X -------------------------- = 100 x 75/150 = 100 x .5 = 50%
Total volume (rock + air)
PSC 121 Prince George's Community College
Physical Properties of Sediments and Water Flow – Pore Space
Pore space depends upon size and shape.
Question: if you have uniform, rounded particles, will you have greater pore space with bigger or smaller particles???
PSC 121 Prince George's Community College
A trick question: mixed particles pack more tightly together
More but smaller spaces
Larger but fewer spaces
Mixed – less
empty space
http://techalive.mtu.edu/meec/module06/Packing.htm
Same porosity
PSC 121 Prince George's Community College
Physical Properties of Sediments and Water Flow – Pore Space
Larger but fewer spaces
Mixed – less
empty space
Most sediments are not regular in shape and pack more
tightly. This reduces the pore space even more and reduces
the connectivity of the space.
PSC 121 Prince George's Community College
Physical Properties of Sediments and Water Flow – Water Retention
Amount of water that is retained as water flows through sediments
Measure by draining saturated soil- how much
Water do you recover?
Water is retained on the surface so the greater the surface area, the higher the water retention
Which will have more retention for the same volume - gravel or sand?
PSC 121 Prince George's Community College
Physical Properties of Sediments and Water Flow – Water Retention
For the same volume, smaller particles will have more surface area, and therefore, greater water retention.
Sand will have higher water retention.
PSC 121 Prince George's Community College
Physical Properties of Sediments and Water Flow- Permeability
Measure of the flow of water through ground material
Depends on the size of sediment particles because it is controlled by the connectivity of pore space and surface area
http://techalive.mtu.edu/meec/module06/Permeability.htm
PSC 121 Prince George's Community College
Different permeabilities- different flow times
Different permeabilities and soils
PSC 121 Prince George's Community College
Sand: High
Reason is lots rounded particles leave lots of air space
These pores are well connected.
Why its hard to grow things in sand- the water flows out
too quickly. But… water retention is higher than for
gravel because the particles are small
Clay: Very low
small particles are flattened and they don’t have a lot of pore space
What pore space they have are not connected
Also hard to grow things in clay- water doesn’t seep down to
roots easily. It has too much water retention!
Large Gravel: Very high
High porosity- like sand in this regard. Well connected pore space.
But with bigger particles, even less water retention than sand
So highest permeability of them all
PSC 121 Prince George's Community College
Permeability: how to measure
Either:
flow time: time for a set amount of water to flow through the sediment (minutes/100 mL)
short times means high permeability
or
flow rate: volume of water that flows through a given amount of sediment in a set amount of time (mL/minute)
Fast rate means high permeability
PSC 121 Prince George's Community College
Permeability
High Permeability is either:
short flow time
fast flow rate
Low Permeability is either:
long flow time
slow flow rate
PSC 121 Prince George's Community College
Porosity
2. Water Retention
3. Permeability
How does particle size (or type of soil) affect the three variables?
We study water underground because that is where most of the liquid fresh
water is a critical part of the water cycle
Above ground water (rivers/lakes etc) is however important for erosion/deposition.
We will look at this in the next class
What do these three terms mean, how they’re measured and how to interpret
Summary
Water and soil. How groundwater flows
