There are currently two relatively disparate branches of hydrology, consisting of hydrology as earth science and hydrology as engineering. In the former, hydrology often looks at the land surface as a pristine system, unaffected by humans and governed by our understanding of the system’s physics. In the latter, humans are seen as an integral component of the system, yet these studies tend to be applied and narrowly focused on addressing a specific water problem. Given the extensive imprint of humans on the hydrological cycle in almost every part of the globe and the need for fundamental mechanistic understandings for prediction, these two approaches should move towards and learn from one another.

Hydrologic systems face significant changes, both from climate and humans, and to understand these changes we need tools that are based on the physical understanding of the natural system and incorporate the activities and feedbacks with human systems. Non-stationarity exists due to climate change as well as changes to the watershed structure (e.g. land use change, engineering works, etc.). The hydrology of the system does not exist in a black box, unaffected by other systems, and is interconnected with climatic, ecological, social, and earth surface changes. As such, there can also be feedbacks with society; human settlement patterns and economic activity are often dependent on freshwater availability, and humans alter the natural system to suit their needs.

In order to understand the coupled natural-human system, we need both process-based, universal hydrology laws for human landscapes that are not place-based and studies that focus on a holistic understanding of a particular system. What is critical is a fundamental understanding of human-hydrologic systems that builds upon both the earth science and engineering traditions in hydrology. Additionally, concepts from Earth system science can inform hydrology about the context, approaches to prediction as well as theoretical approaches to better understand and predict the hydrologic cycle and its human modifications within the setting of the whole Earth system.

This special issue will explore this research space, with papers that address both our fundamental understanding of these coupled systems as well as case studies that focus on place-based issues. It will serve as a common ground for researchers interested in addressing coupled natural-human hydrology questions and problems to share their research. More broadly, it is our hope that this special issue will serve as a catalyst for dialog in the hydrologic and the broader earth sciences community about how to address the water problems facing different regions while viewing the hydrologic system as a system that is governed by universal laws, modified by humans, constantly in a state of change, and intertwined with many other systems, particularly the earth system at large.