Introduction

Of all the potential threats posed by climatic variability and change, those associated with water resources are arguably the most consequential for both society and the environment (Waggoner, 1990). Climatic effects on agriculture, aquatic ecosystems, energy, and industry are strongly influenced by climatic effects on water. Thus, understanding changes in the distribution, quantity and quality of, and demand for water in response to climate variability and change is essential to planning for and adapting to future climatic conditions. A central role of the U.S. Geological Survey (USGS) with respect to climate is to document environmental changes currently underway and to develop improved capabilities to predict future changes. Indeed, a centerpiece of the USGS role is a new Climate Effects Network of monitoring sites. Measuring the climatic effects on water is an essential component of such a network (along with corresponding effects on terrestrial ecosystems).

The USGS needs to be unambiguous in communicating with its customers and stakeholders, and with officials at the Department of the Interior, that although modeling future impacts of climate change is important, there is no more critical role for the USGS in climate change science than that of measuring and describing the changes that are currently underway. One of the best statements of that mission comes from a short paper by Ralph Keeling (2008) that describes the inspiration and the challenges faced by David Keeling in operating the all-important Mauna Loa Observatory over a period of more than four decades. Ralph Keeling stated: “The only way to figure out what is happening to our planet is to measure it, and this means tracking changes decade after decade and poring over the records.”

There are three key ideas that are important to the USGS in the above-mentioned sentence. First, to understand what is happening requires measurement. While models are a tool for learning and testing our understanding, they are not a substitute for observations. The second key idea is that measurement needs to be done over a period of many decades. When viewing hydrologic records over time scales of a few years to a few decades, trends commonly appear. However, when viewed in the context of many decades to centuries, these short-term trends are recognized as being part of much longer term oscillations. Thus, while we might want to initiate monitoring of important aspects of our natural resources, the data that will prove to be most useful in the next few years are those records that already have long-term continuity. USGS streamflow and groundwater level data are excellent examples of such long-term records. These measured data span many decades, follow standard protocols for collection and quality assurance, and are stored in a database that provides access to the full period of record.

The third point from the Keeling quote relates to the notion of “poring over the records.” Important trends will not generally jump off the computer screen at us. Thoughtful analyses are required to get past a number of important but confounding influences in the record, such as the role of seasonal variation, changes in water management, or influences of quasi-periodic phenomena, such as El Niño-Southern Oscillation (ENSO) or the Pacific Decadal Oscillation (PDO). No organization is better situated to pore over the records than the USGS because USGS scientists know the data, quality-assure the data, understand the factors that influence the data, and have the ancillary information on the watersheds within which the data are collected.

To fulfill the USGS role in understanding climatic variability and change, we need to continually improve and strengthen two of our key capabilities: (1) preserving continuity of long-term water data collection and (2) analyzing and interpreting water data to determine how the Nation’s water resources are changing.

Understanding change in water resources to date and predicting change into the future must be done in full recognition of the other factors that influence water availability, including changes in water use, land use, the design and operation of water infrastructure, and the depletion of groundwater. There is widespread debate about the relative importance of nonclimatic factors versus climatic factors in determining water conditions and characteristics over the coming decades (Lins and Stakhiv, 1998). Differentiating climatic from nonclimatic effects is, therefore, a critical component of any effective assessment of how climate has, is, and will affect the Nation’s water resources. Moreover, such assessments are critically dependent upon a quality-assured, spatially and temporally comprehensive water-resources database.

For more than 100 years, the USGS has been monitoring the Nation’s rivers and groundwater resources, and currently maintains the world’s most sophisticated information system of surface-water, groundwater, water-quality, and water-use data. The USGS is uniquely positioned to play a central role in providing both data and analyses to address the recently recognized increment of hydrologic uncertainty associated with climatic variability and change (Moss and Lins, 1989).

The USGS draws upon two decades of climate change research related to hydroclimatology—the study of hydrologic events and conditions within their climatologic context. USGS research provides an empirical basis for integrating the physical sources of variability in a hydrologic time series with the statistical properties of the varying driving force itself. This research also highlights several themes where our incomplete understanding of important hydrologic processes and conditions continues to serve as an impediment to the broader incorporation of climate change information in water-resources planning and design. Thus, expanding our understanding of these processes is key to developing improved capabilities for dealing with climate uncertainty in the field of water-resources planning and management.

First posted April 13, 2010