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# Groundwater Cycle

### Overview

Groundwater is the water beneath the ground surface. It is a vast freshwater reservoir often overlooked because invisible, yet 1000 times greater than all lakes and rivers. The Earth is blue for its oceans, but it is green for the blankets of freshwater under our feet. Half of the worldâ€™s population relies on groundwater for drinking and almost half of the irrigated land now depends on groundwater, a ten-fold increase in the past 50 years.

This course explores the water cycle from an underground perspective. We start with the description of groundwater as a resource: How much is there? Where is it? How do we use it? How much groundwater do plants and trees use every year? How much water do aquifers lose during droughts? How much do they gain during rain events or in a typical year?

In the second objective of this course, we describe underground waters and the properties and classification of aquifers. What is the difference between a confined and unconfined aquifer? What is porosity and does it influence groundwater resources? We then explain and apply Darcyâ€™s law. Darcy is a 19th century hydraulics engineer who famously worked on bringing clean fresh water to the public fountains of Dijon, France. His law describes flow in porous media and is the cornerstone of subsurface hydrology. We will review his experiments and show how he arrived at his law empirically. We even will show in a lab video how to calculate the hydraulic properties of porous media following Darcyâ€™s steps. After this empirical overview, we will demonstrate how we can derive the same law from first principles using Newtonâ€™s force balances.

In our third objective, we will build up from Darcyâ€™s law to derive the other principles of groundwater motion. What are the differences between confined and unconfined flows? What happens when it rains and the aquifers recharge? What happens in soils with inhomogeneous properties? What is a water divide? Finally, we will introduce modeling principles to translate these foundations into real-life engineering. How can we solve the equations of motion in excel using finite difference? How can we design tile drains to lower the water table in a cornfield?

This first course of our three-course series introduces groundwater cycling on Earth, from the description of stocks to fluxes and to the basics of modeling. We will use these fundamental principles in our remaining two topical courses: Wells Hydraulics and Groundwater Contamination.

### Syllabus

Week 1: The Underground Water Cycle

We start with the description of groundwater as a resource: How much is there? Where is it? How do we use it?

• Importance of Groundwater
• Groundwater Use
• Water Cycle, Groundwater Availability
• Global Water Cycle
• Water Distribution
• Watershed Water Balance
• Groundwater Recharge from Hydrographs Analysis
• Recession Curve Displacement Method
• Seasonal Recession Method

Week 2: Aquifers

We describe underground waters and the properties and classification of aquifers. What is the difference between a confined and an unconfined aquifer? What is porosity and does it influence groundwater resources?

• Underground Water and Aquifer Types
• Lab Water in a Sand Box
• Underground Water and Aquifers
• WaterTtable, Piezometric Head and Potentiometric Surface Maps
• Review of Piezometric Head
• Potentiometric Surface Maps
• Porosity and Underground Matrix
• What is Porosity?
• Measuring Porosity
• Specific Yield, Specific Retention, Specific Storage, Storativity
• Specific Yield, Specific Retention
• Specific Storage and Storativity

Week 3: Darcyâ€™s Law

We explain and apply Darcyâ€™s law. This law describes flow in porous media and is the cornerstone of subsurface hydrology.

• Darcy the Engineer
• Darcyâ€™s Law: An Empirical Law
• Permeameters
• Darcy Data and Hydraulic Conductivity
• Darcyâ€™s Law: A Fundamental Law
• Physics Basics: Energy, Forces and Pressures = Einstein, Newton and
• Stokes and Poiseuille Law

Week 4: Groundwater Flow

We build up from Darcyâ€™s law to derive the other principles of groundwater motion. What are the differences between confined and unconfined flows? What happens when it rains and aquifers recharge?

• Principles of Groundwater Flow
• Confined Flow
• Dupuit: Unconfined Flow
• Recharge and Divide
• Boussinesq Equation
• Leaky Confined

Week 5: Modeling and Applications

We introduce modeling principles to translate mathematical models into real-life engineering. How can we solve the equations of motion in excel? How can we design tile drains to lower the water table in a cornfield?

• K Charge
• Finite Difference Principles
• Finite Difference Application

Antoine Aubeneau

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