Topic 8: Water Cycle and Climate
What is the water cycle?
~The water cycle is also called the
hydrologic cycle. It is a model used to
illustrate the movement and the phase
changes of water at and near the Earth's
surface.
~It is fueled by the sun.
What happens when precipitation falls on
the ground?
~It can be stored or retained on the
surface as ice or snow (water retention).
~It can infiltrate or seep (sink into) the
Earth’s surface.
~It can flow over the Earth’s surface as
runoff.
~It can be evaporated or transpired
(both of these together is called
evapotranspiration).
Factors Affecting Infiltration
** Remember: Infiltration means to sink into. **
Ex: Watering a plant
•
Slope of the Land
•
Sorting
•
Degree of Saturation
•
Shape
•
Porosity
•
Capillarity
•
•
Permeability
Packing
•
•
Vegetation
Land Use
Slope of the Land
The steeper the slope (gradient), the less
the infiltration or seepage
--- inverse or indirect relationship
Degree of Saturation
The more saturated the loose Earth
materials are, the less the infiltration
--- inverse or indirect relationship
 Water
infiltrates into the ground until it meets the interface
between the zone of saturation and the zone of aeration
This interface is the WATER TABLE
 The depth of the water table below the surface varies with
the amount of infiltration
 Subsurface water below the water table is called
GROUND WATER
Porosity
Porosity is the percentage of open space (pores
and cracks) in a material compared to its total
volume
Generally: the greater the porosity, the
greater the amount of infiltration that can occur
---Direct Relationship
Dependent upon shape
Shape
 Well rounded particles have a greater porosity.
 Round particles = more pore
space, higher porosity, and more
infiltration
 Angular particles = less
pore space, less porosity,
and less infiltration
Packing
 The closer the particles are packed
together, the lower the porosity
 Therefore if you have a lower
porosity, infiltration will also be lower.
Sorting
 If all the particles in a material are about the same size,
they are said to be sorted
 If the particles are of mixed sizes, they are said to be
unsorted
Sorted = higher porosity
or pore space
Unsorted = lower porosity
because the smaller particles fill
in the pore space
Permeability
The ability of a material to allow fluids such
as water to pass through it. Larger particles
will increase permeability, because pore
space is larger.
**Impermeability may be due to tight packing
or cementing of particles, which seals off the
pores from one another.
ex: ice in the winter
Capillarity
The process by which water is drawn into
openings due to the attractive force
between water molecules and the
surrounding earth materials. As the
particles decrease, capillarity increases.
When water moves upward against gravity,
it is called capillary action.
ex: How plants take water from the soil
Vegetation
Grasses, trees and other plant types capture
falling precipitation on leaves and branches,
keeping that water from being absorbed into the
Earth
If any water gets through the vegetation, the
velocity of the water will be reduced and this will
give the ground more time to absorb the water
Ground without vegetation usually has high runoff
and low infiltration rates
Land Use
Roads, parking lots, and buildings create
surfaces that are not longer permeable.
These impermeable surfaces often channel
runoff.
Farming, cutting down trees and grazing
animals will reduce vegetation and
therefore decrease permeability.
Factors Affecting Runoff and Stream
Discharge
Runoff can occur when:
1) The rate of precipitation exceeds the permeability rate
2) The pore space of loose material or rock is saturated
with water
3) The slope of the surface is too great to allow infiltration
4) The water on the surface has not evaporated
**The greater the runoff, the greater the amount of stream
discharge (volume of water flowing past a certain spot
in a stream in a specific amount of time.)**
UNITS FOR STREAM DISCHARGE = meters/sec
**Flooding occurs when a stream overflows its normal
channel**