# Preface

Groundwater scientists need to have a solid understanding of the occurrence and behavior of groundwater in a variety of conditions and settings. This requires knowledge of how porous media store, yield, and transmit water. Darcy’s Law defines the discharge of groundwater as directly proportional to the area perpendicular to the direction of flow, hydraulic gradient and transmission capacity of the earth material. The forces on water within pore spaces and fractures control groundwater flow rates and directions, so we describe them in detail. Then, we explain mathematical representations of groundwater flow and provide examples of their application to large and small groundwater systems.