# 9 Conclusion

This book presented the principles and concepts that form the physical foundation of groundwater science and underpins the information in other Groundwater Project books. Basic terms and tools used to describe the hydrogeologic properties of earth materials, compute groundwater flow directions, rates, fluxes and velocities were presented. Generalized mathematical descriptions that govern groundwater flow under standard flow conditions and their use in describing groundwater systems were outlined. This information serves as the foundation for further study of groundwater.

One critical concept to take away from this book is the relationship described by Darcy’s law: that groundwater discharge is directly porportional to hydraulic conductivity, hydraulic gradient and the cross-sectional area perpendicular to the flow direction. An increase in any of those parameter values increases the groundwater discharge. When confronted with an obstacle while attempting to complete a hydrogeologic analysis, it is useful to go back to the basics of Darcy’s law, *Q* = –*KiA*. Usually, it reminds the investigator of how basic hydrogeologic concepts can be applied and leads to a solution.

Another important concept is that flow systems are controlled by aquifer properties and boundary conditions, as well as the locations and magnitudes of sources and sinks. When confronting challenges in conceptualizing the system, it is useful to sketch cartoons representing where water enters the system and where it exits; and to build a water budget to evaluate if the system components are sufficiently known. Answer the question: Where is the water coming from and when is it going? Envision how you could represent the field system on your desk as a miniaturized version in a sand box with recharge sprinkled on the surface and tanks representing water bodies where water enters and exits. Such a simplification can help you focus on the most important aspects of the system so as to develop a better conceptual model of how the system works.

Physical principles and concepts cover a wide range of subjects. The Groundwater Project books that follow this one rely on principles and concepts presented here, and present further, more advanced, principles and concepts.