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.

Porosity

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?

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 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

Groundwater resources are assessed using what satellite missions?

GRACE

What does GRACE stand for?

Gravity Recovery and Climate Experiment.

How does GRACE measure groundwater changes?

By detecting small changes in Earth's gravity field related to water mass movement.

What are two main categories of groundwater threats?

Overpumping and contamination (including saltwater intrusion and pollution).

What happens when groundwater withdrawals exceed recharge?

The water table lowers

What is the “cone of depression”?

A drop in the water table around a well due to excessive pumping.

How does climate change worsen groundwater depletion?

Higher temperatures increase evaporation and irrigation needs; intense rain events don’t recharge groundwater.

What is land subsidence?

The gradual sinking of land due to groundwater removal.

Where has land subsidence been a major problem in the U.S.?

San Joaquin Valley and Mojave Desert in California

What is saltwater intrusion?

The movement of seawater into freshwater aquifers due to lowered water tables or rising sea levels.

What conditions increase saltwater intrusion risk?

Lowered freshwater levels and rising sea levels.

What are the health impacts of consuming saline groundwater?

High blood pressure

How does saltwater intrusion affect agriculture?

It reduces freshwater availability for crops and livestock

What are common mitigation strategies for saltwater intrusion?

Injection of freshwater or reduction of groundwater pumping.

What are the two main types of groundwater pollution sources?

Point sources and nonpoint sources.

Give an example of a point source of groundwater contamination.

Leaking underground storage tanks or pipes.

Give an example of a nonpoint source of contamination.

Fertilizer runoff or widespread chemical application.

Why is nonpoint source pollution harder to manage?

It is diffuse and lacks a single identifiable origin.

How does groundwater contamination spread?

Contaminants move with groundwater flow and disperse slowly.

What major law protects surface waters from pollution?

The Clean Water Act (CWA).

Does the Clean Water Act protect groundwater directly?

Not explicitly

Which case clarified that groundwater-connected discharges can fall under the CWA?

County of Maui v. Hawaii Wildlife Fund (U.S. Supreme Court, April 2020)

What is the difference between federal and state groundwater regulation?

Federal law covers pollutants reaching surface waters

Why is groundwater protection important for sustainability?

It ensures long-term access to drinking water

What happens when groundwater is pumped faster than it can recharge?

The water table drops

Why does overpumping increase during droughts?

Surface water becomes scarce

Why does intense rainfall not always help recharge groundwater?

Heavy precipitation often runs off quickly into rivers and oceans instead of infiltrating the ground.

What happens to water wells as the water table lowers?

They produce less water

Why does land subsidence occur?

Removing too much groundwater causes soil and rock layers to compact and sink.

Why does groundwater depletion threaten food production?

Lower water availability reduces irrigation for crops and livestock.

What happens when saltwater intrudes into freshwater aquifers?

Freshwater supplies become too saline for drinking or agriculture.

Why does lowering the water table increase saltwater intrusion?

It reduces the pressure of freshwater pushing against seawater

What are health risks of drinking saline groundwater?

High blood pressure, preeclampsia, increased infant mortality

What are economic impacts of groundwater overuse?

Increased pumping costs, reduced agricultural productivity, infrastructure damage from land subsidence

Why is groundwater contamination difficult to clean up?

It moves slowly

What happens to contaminants once they enter an aquifer?

They travel with groundwater flow and disperse

Why are nonpoint source pollutants harder to regulate?

They come from many diffuse sources rather than one identifiable point.

Main goal/purpose of the Clean Water Act?

To protect the quality of navigable surface waters in the United States.

Why was the Maui Hawai’i case significant for groundwater protection?

because the Court ruled that the Clean Water Act (CWA) applies to pollutants that travel through groundwater to reach navigable surface waters. This meant that discharging treated wastewater into underground wells requires a permit if it has a “functional equivalent” of a direct discharge into surface water. The decision closed a major loophole, confirming that groundwater pathways can fall under CWA jurisdiction when they affect rivers, lakes, or oceans.

Why is groundwater regulation inconsistent across the U.S.?

Because states differ in how they define and protect groundwater under their laws.

Why is groundwater vital to the hydrologic cycle?

It supports rivers

Why does climate change increase groundwater demand?

Higher temperatures raise evaporation and irrigation needs

Why can lowering groundwater levels lead to economic inequality?

Richer users can afford deeper wells

Why is managing groundwater sustainably important?

To ensure long-term water availability for people

Why is groundwater management challenging?

It’s invisible

How much of total water withdrawals come from groundwater?

About one-third globally.

What fraction of irrigation water comes from groundwater?

Roughly half worldwide.

Why is groundwater considered a “strategic reserve”?

It provides water during droughts when surface water is scarce.

What does SW-GW interaction mean?

The exchange of water between surface water bodies and the groundwater beneath or adjacent to them.

What are gaining streams?

Streams that receive groundwater inflow.

What are losing streams?

Streams that lose water to the groundwater system.

What can excessive groundwater pumping cause?

It can turn gaining streams into losing streams or disconnect them entirely.

What happens when surface and groundwater interactions are underestimated?

Water resources can be overcounted or mismanaged

What is the goal of water regulation?

To preserve water resources over time and adjust to changes like drought or surplus.

What is the goal of water allocation?

To reduce uncertainty

What are riparian rights?

Rights based on land ownership along water bodies

What are appropriative (prior appropriation) rights?

water rights based on the principle of “first in time, first in right.” Allow individuals to divert and use water for a beneficial purpose regardless of land ownership, and the seniority of the right determines priority during shortages

What are the main differences between riparian and prior appropriation systems?

Riparian ties rights to land ownership