EESA01H3 Lecture Notes - Lecture 5: Vadose Zone, Dew Point, Evapotranspiration

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EESA01H3 F: Lecture 5 - Water Resources and Hydrology
The Water Balance: Why are wet places wet and why are dry places dry?
Inputs - Outputs = Change in Storage.
Inputs: Hail, dew, precipitation (rain and snow), runoff, groundwater flow.
Outputs: Evapotranspiration, runoff, groundwater flow.
Storage: Volume; groundwater, soil moisture, lakes and reservoirs, rivers and
streams.
How wet something is, or the volume.
Inputs can also be outputs.
Runoff and groundwater.
Everything must be in the same units when calculating a change in storage.
Precipitation: mm - depth
Evapotranspiration: mm - depth
Groundwater flow and runoff: volume per time
Watershed: 10,000 x Area of watershed in hectares
Runoff:
Runoff is often measured in relation to streamflow.
Due to impervious surfaces which block the absorption of water in urban areas, we can
see runoff on sidewalks.
Water accumulates in depressions (low topographical points) such as drainage
grates.
Evapotranspiration:
Key Controls -
1. Energy:
No evapotranspiration on the winter as plants are dormant
2. Gradient in vapor pressure:
The amount of pressure depending on the moisture on the atmosphere
Too saturated pressure means you've reached dew point
100% relative humidity above a lake looks like fog.
3. Wind:
It keeps the gradient in vapor pressure in check.
4. Resistance from plants:
A biotic control, which is the hardest to predict
Groundwater:
There is moisture at the surface of the earth.
It enters groundwater through infiltration and percolation.
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The earth is made of stratigraphic layers.
Water table: the upper boundary of groundwater.
Smaller pores such as in clay and silt hold water more tightly.
This results in the formation of aquitards and aquicludes.
Aquitards: Low conductivity.
Aquicludes: Very low conductivity.
Unconfined and confined aquifers have low aquifer conductivity.
Unconfined: Aquifers are above a vadose zone and under atmospheric
pressure.
Confined: a sandwich. These aquifers are below an aquiclude, which prevents
water flow. They are usually under a high amount of pressure.
Water is unable to percolate and recharge the aquifer from the surface.
It tends to have a small zone where it is recharged.
A spring: is an area where groundwater comes to the surface.
When groundwater reaches rivers, lakes or wetland the direction of the flow is:
Localized recharge: is from surface water to groundwater.
The localized recharge of surface water to groundwater.
Discharge: is from groundwater to surface water.
“The discharge of groundwater to surface water.”
Artesian well: comes from a confined aquifer where there is pressure that released the
water. No pumping is required.
Overuse of Groundwater:
It is a renewable resource but is being used faster than its recharge rate.
If you pull too much groundwater:
The water table lowers due to a change in storage higher than input.
Cone of depression: when the water table lowers beneath a certain area.
Can be a geotechnical structural issue
Soil can compress, resulting in the collapsing of earth (sinkhole). This is because water
is no longer present to hold water together.
Precipitation is the Most important Hydrological Input:
1. Air needs to cool enough such that water will condense. It needs to reach dew point at
the very least.
Dew point: is the temperature at which a given parcel of air becomes saturated.
This is 100% relative humidity.
Look at
2. Condensation nuclei must be present in the atmosphere.
Condensation nuclei: are particles in the atmosphere on which water molecules
can condense.
3. Water droplets must grow to a sufficient size such that they don’t evaporate entirely
before hitting the ground.
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