# 1 Nature and Purpose of this Book

This book introduces the most basic and essential concepts in groundwater science. In particular, the fundamentals of Darcy’s law, hydraulic head, hydraulic gradient, and potentiometric contours are presented in a manner with minimal mathematic detail but rather in a conceptual and visual manner that transforms intuition into knowledge. This book pairs well with the work by Woessner and Poeter (2020), which presents the fundamentals with mathematical underpinnings. The book introduces the Darcy experiment and Darcy’s law in the context of hydraulic head gradients to establish head as the key parameter along with hydraulic conductivity for understanding flow. Conceptually, it shows that water wells are piezometers used to measure hydraulic head. Emphasis is on steady, saturated flow, first in one dimension and then in two dimensions to show how head distributions and gradients within the saturated zone of the subsurface are influenced by variability of hydraulic conductivity and the boundaries of the flow domain. An emphasis is placed on illustrating how head profiles (vertical head distribution) and potentiometric contours are used to infer groundwater flow directions in aquifers where flow is generally horizontal, and in aquitards where it generally is vertical. This book does not discuss flow nets in a formal way, but some figures show flow nets where the water table, equipotential lines (in map view and cross section) and flow lines are presented, because these relationships are so fundamental to groundwater science. The Groundwater Project book by Poeter and Hsieh (2020) explains flow nets and how to construct them. Overall, this book will provide students with the foundation needed to visualize groundwater flow patterns based on water elevation measured in wells and with consideration of basic geologic properties.

Since hydraulic head measurements can be obtained in the field relatively easily with minimal uncertainty, understanding the spatial nature of head data is key to the development of hydrogeological thinking. This book includes of a set of simple schematic figures with embedded narrative that illustrate specific points. In addition, example problems are presented throughout the book; each is a sketch figure with questions asked about what the figure shows and/or requiring sketches by the student. Answers to the questions are provided at the end of the book, along with an explanation of the rationale for the correct answer. For multiple-choice questions, the rationale for why the other choices are incorrect is presented, which provides an opportunity for deeper understanding. The example problems require contemplation and integration of concepts, not solving equations.