Consistent, Long-Term Hydrologic Databases are Valuable Resources

Nick Melcher, director of the U.S. Geological Survey’s Arizona Water Science Center, and Jeff Phillips, USGS supervisory hydrologist, contributed this Guest View

Hydrologic data are needed for a diverse set of basic societal needs that include urban planning, problem assessments, land management, economic development, and environmental monitoring. All of us that work in the hydrologic sciences hear complaints regarding the lack of basic descriptive hydrologic data. In reality, the volume of hydrologic data collected by government, corporate, and non-governmental interests is staggering and tabulating the data or even accurately identifying its sources can be difficult. The problem is not so much the amount of hydrologic data that is collected, but more the usability of the data.

Typically, hydrologic data are collected during an economically driven short-term assignment to assess a specific condition or problem. Too often the purpose and methods for collection are not fully documented. Consequently, collateral use of the data is limited and the likelihood that uninformed use of the data will result in inaccurate conclusions is increased. In many instances, the data set never leaves the original collector and over time loses its identity and purpose. There are far more hydrologic data collected than are being used or that are readily accessible. A few minor adjustments to the data-collection process can result in usable data that are accessible for multiple purposes.

Figure 1. Trends in annual streamflow of San Pedro River at Charleston, Arizona.

The U.S. Geological Survey hydrologic database is populated largely with data collected for specific purposes. Over time, however, procedures and standards have been developed that ensure the data are consistent and reproducible, documented and archived, and easily accessible to the public. First, the data are collected using nationally consistent methods. These methods are tested and documented in widely distributed publications. Periodic reviews are conducted by both local and national experts to ensure compliance with methods and standards. Second, the data are compiled and permanently archived in formatted databases. These databases contain fields for documenting collection methods, data purpose, and special considerations. All data collected for all projects must be archived in permanent databases. All USGS hydrologic data are public domain, and most databases are Internet accessible. This process enables the data to serve many purposes and provides for future access and comparison.

One of the most important benefits of consistent, long-term hydrologic databases is that they can be used to determine long-term, as well as short-term, trends. The ability to assess the sustainability of present or planned water-management plans is a critical need throughout the Southwest and the Nation. Long-term data and the ability to analyze trends are fundamental to the resolution of many of the most complex hydrologic problems. Hydrologists are only now beginning to understand the effects of climatic variability and the importance of considering that variability in management decisions. The most important water adjudication in the Southwest, the Lower Colorado River apportionment, was based on flow records from 1895 to 1922, resulting in an estimated mean annual flow of 16.1 million acre-feet per year at the Lees Ferry compact point. Consequently, a total of 15 million acre-feet per year was apportioned between the upper and lower States. Subsequent data indicate that the long-term mean annual flow for 1895-2003 is only 12.2 million acre-feet, creating a serious social, economic, and legal dilemma for us and for future generations (Webb and others, in press).

The use of streamflow records for the San Pedro River Basin is another excellent example of the need for long-term hydrologic data. The USGS has been operating the San Pedro River at Charleston gaging station (09471000) since 1904, and growing concerns about the water needs for basin development verses riparian habitat illustrate the need for determination of long-term trends in streamflow. Utilization of trend-analysis methods, along with streamflow records for this station, has allowed USGS hydrologists to determine that annual flow diminished by more than 50 percent during the past 100 years (fig. 1; Thomas and Pool, in press). On the evening of July 6, 2005 zero flow was recorded in this reach of the river for the first time during the 101 year period of record.

Analysis of long-term records is enabling scientists and observers to assess substantial changes in streamflow and the factors that can contribute to those changes.

–Webb, R.H., Hereford, Richard, and McCabe, G.J., in press, Climatic fluctuations, drought, and flow in the Colorado River, in Gloss, S.P., Lovich, J.E., and Melis, T.S. eds., The state of the Colorado River ecosystem in Grand Canyon: U.S. Geological Survey Circular 1282.
–Thomas, B.E., Pool, D.R., in press, Trends in streamflow of the San Pedro River, southeastern Arizona, and regional trends in precipitation and streamflow in southeastern Arizona and southwestern New Mexico: U.S. Geological Survey Professional Paper 1712.