Evaluation of the Stream-Gaging Network in Maryland, Delaware and Washington, D.C.
By Stephen D. Preston
Excerpt From Introduction
The surface-water discharge data provided by the stream-gaging network in Maryland and surrounding areas (fig.1) are critical to many types of hydrologic and water-quality evaluations. Typical uses of surface-water discharge data in Maryland include (1) regional flood prediction and warning, (2) water-supply evaluation and protection, (3) water-quality evaluation, (4) contaminant-load estimation, and (5) assessment of ecosystem stability and diversity. For all of these purposes, surface-water discharge data must be accurate and available in sufficient detail for the type of analysis being performed.
In order to provide the surface-water discharge data to meet the needs of most types of hydrologic evaluations currently and for the future, a stream-gaging network must be stable for an extended period of time. For most evaluations, discharge data must be available for a period of time that is long enough to account for temporal hydrologic variations. Typically, stream discharge records should be at least 5 years in length (to encompass most variations), and the accuracy of evaluations can be significantly enhanced by record lengths greater than 20 years.
In Maryland, long-term surface-water discharge records are particularly important because of continued changes in the landscape that are caused by man's activities. Ongoing conversion of pervious forest and agricultural land to relatively impervious developed land will cause changes in the hydrologic response of watersheds. These changes can cause the degradation of water quality and stream habitat. To detect the hydrologic effects of land-use changes, long-term discharge records are necessary. If stream-gaging stations with long-term discharge records are discontinued, the State's ability to detect and manage the adverse effects of land-use change will be limited.
To meet the need for surface-water discharge information in Maryland, the stream-gaging network should also represent the full range of geographic conditions. Many current hydrologic studies have been designed to encompass a specific geographic area simply because of the availability of discharge information. For example, sediment or chemical constituent load-estimation studies are often designed where stream-gaging stations already exist to take advantage of historical discharge information (Preston, 1996). Furthermore, retrospective studies of contaminant loading are limited to locations where water-quality data have been collected in association with a stream-gaging station (Langland and others, 1995). These examples illustrate the fact that the stream-gaging network serves as a foundation for many types of data collection and evaluation. In order to facilitate and enhance such studies in the future, the stream-gaging network needs to be designed to be stable for the long term and to be representative of the entire State.
This report describes the historical and current stream-gaging network in Maryland and provides some initial results of an evaluation of its adequacy. In particular, the report describes temporal trends in the characteristics of the network and the results of an initial evaluation of its representativeness for the State and surrounding areas. This is not a comprehensive network evaluation, but rather is intended to provide a description of potential network problems and to document the need for more detailed evaluation and planning.
The primary emphasis of this evaluation is the use of stream-gaging stations as a network for evaluating regional hydrologic conditions in Maryland and surrounding areas. Individual stream-gaging stations can be supported to (1) provide site-specific discharge information, or (2) represent large multistate regions. In both of these cases, the stations may have limited value as a component of a statewide network for making evaluations in Maryland. The evaluation was based on the data from a group of stream-gaging stations that represent specific conditions in the State and that can be used to perform statewide or regional evaluations. All work described in this report has been performed by the U.S. Geological Survey in cooperation with the Maryland Geological Survey.
