The four chapters in Part 1 provide an overview of water as a resource for human use, addressing basic questions about supply—how much water is available and how that varies over space and time; demand—how much water we are using and for what purposes; and scarcity—the gap between supply and demand. A central theme of these chapters is that water supply is not a fixed quantity, but rather is affected by both natural variability and anthropogenic factors, including land-use change, climate change, and water use itself.
We start in Chapter 2 with an analysis of water stocks and flows, paying special attention to the spatial and temporal variability in flows, and to the hydrologic tools—largely rooted in the concept of “stationarity”—that are used to quantify that variability. Chapter 2 also includes a discussion of the megadroughts that have been discovered in the paleoclimate record, a discussion that ultimately leads us to question the stationarity model. Chapter 3 further undermines the stationarity model by discussing the two contemporary large-scale drivers of changing water availability—climate change and land-use change—and their implications for water management and planning. Chapter 4 turns to the demand side, with a brief history of how people have used water over the last 10,000 years, a more detailed discussion of the last 100 years or so, and a quantitative look at how much water we use for what purposes; this involves delving into some important definitional issues, including the concept of water footprints. Chapter 5 defines scarcity, introduces various scarcity indicators, identifies where scarcity is (and isn’t) a problem, and examines the phenomenon of depletion: rivers, lakes, and aquifers where human water use has changed the amount and timing of water availability, with significant impacts on both communities and ecosystems.