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 world’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 Darcy’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 Darcy’s steps. After this empirical overview, we will demonstrate how we can derive the same law from first principles using Newton’s force balances.

In our third objective, we will build up from Darcy’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.