Hydrology Module 4

What is the difference between saturated and unsaturated zones in soil?

Saturated zones are filled with water, while unsaturated zones contain air and water.

What is porosity in soil?

how much water soil can hold

What is soil moisture (𝜃)?

amount of water held in the soil

What is the maximum value of saturated soil moisture (𝜃𝑠)?

the highest amount of water the soil can hold when fully saturated

What does hydraulic conductivity (𝐾) measure?

measures the ease of water flow through soil or rock.

What is the difference between confined and unconfined aquifers?

Confined aquifers are bounded by impermeable layers above and below, while unconfined aquifers are only bounded below and have a water table that can fluctuate.

What is an aquifer?

An aquifer is a saturated geologic layer that allows water to flow fairly easily through it.

What is a confining layer/aquitard/aquiclude?

a relatively impermeable layer that restricts groundwater movement, such as clay or shale.

What percentage of drinking water in the U.S. comes from groundwater?

Approximately 37% of city/county-delivered water and 90% of rural drinking water comes from groundwater.

What is the importance of groundwater globally?

Globally, about 50% of drinking water and 40% of irrigation water comes from groundwater.

What are the key soil characteristics discussed in Module 4?

grain size, porosity, and soil moisture

What is capillary tension in the context of soil?

suction pressure in the unsaturated zone above the water table.

What does the term 'hydraulic head (𝐻)' refer to?

total potential energy of water in the subsurface, combining elevation and pressure.

What is the significance of the water table in unconfined aquifers?

represents the upper boundary of the saturated zone and can change with water storage

What is the role of groundwater in irrigation?

Groundwater accounts for approximately 40% of irrigation water used globally.

What is the relationship between soil type and hydraulic conductivity?

Different soil types have varying hydraulic conductivities, affecting how easily water can flow through them.

Which soil type is most porous?

Clay

Which soil type is least porous?

Sand

Aquifer constraints

A confining layer/aquitard/aquiclude: a relatively impermeable layer that greatly restricts the movement of groundwater (clay, shale, bedrock).

Confined aquifers

Confined at top AND bottom; Upper boundary of aquifer does not change in time (analogous to pipe flow); Most recharge under pressure.

Unconfined aquifers

Confined at bottom only; Upper boundary: water table; Water table moves up/down as storage in aquifer changes (analogous to flow in streams); Main H2O source: recharge from above.

Hydraulic head (𝐻)

Height that the water reaches in a piezometer or well; Also known as piezometric head.

Piezometer

A tube/pipe that is open at either end, used to measure hydraulic head (𝐻) in the field.

Pressure head

Measured relative to atmospheric pressure (so value = 0 at water table surface).

Equipotential lines

Lines of constant hydraulic head.

Flowline/streamline (FL)

Direction of flow.

Flow net rules

1) FL & EP intersect at right angles; 2) EP is perpendicular to impermeable boundaries; 3) EP is parallel to constant head boundaries; 4) FL is evenly spaced; 5) At water table, H = Z.

Interpreting flow nets

Identifying types of boundaries such as pressure and impermeable.

Hydraulic gradient

The slope of the hydraulic head in an unconfined & confined aquifer.

Flow visualization

The representation of groundwater flow using flow nets.

Flow calculation

The process of determining the flow of groundwater based on hydraulic head and other parameters.

Water table

The upper boundary of an unconfined aquifer, where the pressure is equal to atmospheric pressure.

Total hydraulic head

The sum of elevation head and pressure head.

Elevation head

The height of a point above a reference level, typically the water table.

Recharge

The process by which water enters an aquifer from the surface.

Labeling flow nets

Assigning values to equipotential lines and determining pressure heads.

Darcy's Law

Q = K A * (dH/dL)

Darcy flow in the saturated zone

qx = K0 * (dH/dx)

Flowrate per unit area

q = K0 * (dL)

Volumetric flowrate

Q = K0 * A * (dH/dL)

Rules for determining pressure head

1. If the water drains freely at the bottom, h = 0 at that location

2. the water flows freely in at the top without ponding or added pressure, pressure head = 0 at that location

3. If there is standing water at the top of the soil column, pressure head = depth of water

Hydraulic conductivity

a measure of how easily water can move through a saturated porous material; q = K * dL/dH