ALERT Hydrologic Data Collection System

ALERT Hydrologic Data Collection System
ALERT is an acronym for Automated Local Evaluation in Real Time. This describes a cooperative program initiated by the National Weather Service's California-Nevada River Forecast Center in the 1970s. In this program, a local agency installs, maintains, and monitors event-reporting field sensors that report current hydrologic conditions, in real time, through radio telemetry. Event-reporting refers to the ability of sensors to transmit their status as hydrologic conditions change; i.e. rainfall occurs or streams and reservoirs rise and/or fall. Such sensor status data transmissions are received by a base station, which decodes the radio signal's site of origin, and data value. These values are logged in a computer database for report generation, analysis, and archiving purposes. Through a system of radio repeater sites, as well as computer networks, the data are received by both the local operator and interested agencies in adjoining areas, including the National Weather Service. NWS uses the real time data to verify their forecasts, and to monitor conditions for issuance of various hydrologic and meterologic statements.

The district began installation of its ALERT system in 1983, and its system currently includes: 44 rain gauges, 38 streamflow gauges, 11 reservoir gauges, and one weather station.

The real time data are used internally at the district to monitor hydrologic conditions in support of maintenance and operations functions, as well as to make flow projections on some of the larger watersheds. For flow forecast purposes, data reports are exported from the ALERT database for input into hydrologic models. Such models are used to forecast a range of possible peak reservoir and stream levels. While the district does not engage in "flood warning" (this function is performed by NWS), the district does act in an advising capacity to the various agencies that may also be monitoring hydrologic conditions. To facilitate such monitoring, external users--including NWS, various cities in Santa Clara County, and agencies in adjacent counties--may have direct phone modem access to the ALERT database. In early 1998, current ALERT data became accessible through the district’s Web site. These data are unedited, and should be considered PRELIMINARY.