30 Sep 96
detected. An event can be defined as the occurrence of a
storms)? How do users know if the system is operating
given incremental change. For example, consider a tipping
during quiet periods? Normally, event systems are designed to
bucket raingage that "tips" after receiving one millimeter of
report automatically at least twice per day with a message
rainfall. Each tip generates a report that is immediately
indicating its operational status. A central computer is often
transmitted to the central station. As the storm intensity
programmed to sound an alarm when the expected status
increases, the tips occur more frequently and more reports are
signals are not received.
generated. As the storm intensity diminishes, the reporting
rate decreases. The rise and fall of the data reporting rates
(4) Mixed reporting. Technical advances have pro-
exactly match the ever changing storm intensity.
duced more intelligent and capable remote units that combine
interrogation, timed, and event reporting modes. In many
(a) Data reporting rates that directly mimic the rate of
cases, new technology allows users to take advantage of the
change of environmental conditions make event reporting
best that each technique has to offer, all in one package. For
systems ideal for applications where response time is limited.
example, with mixed reporting, a user could define a routine
In small watersheds, the time between the occurrence of
interrogation schedule to ensure receiving data at fixed
intense rainfall and the onset of flooding is short, often a few
intervals. At the same time, the remote unit could be pro-
minutes or hours. Event reporting systems provide immedi-
grammed to transmit a data report spontaneously when an
ate indications of intensity changes that maximize potential
event has occurred, achieving the responsiveness of an event
system. The Geostationary Orbiting Environmental Satellite
(GOES) satellite system is an example of a mixed system.
(b) Event systems require very little power. Most of the
Routine data reporting is done through timed reporting.
time the remote site is quiet or off. When an event occurs,
However, an event channel is available for emergency use if
the remote unit is briefly turned on to transmit a short data
rapidly changing field conditions are detected by the data
message (nominally 0.25 sec). The unit turns off again and
waits for the next "event" to occur before transmitting again.
For raingages with 1-mm tipping buckets in climates receiv-
ing 1,000 mm (40 in.) of rain per year, the total amount of
data transmission time is about 7 or 8 min per year.
to and from remote locations. Satellite, telephone, meteor-
burst, and VHF/UHF have all been used successfully for
(c) Each remote data site in an event reporting system
automated data collection.
acts independently. Whenever an event is detected, the
remote site immediately transmits its data. Such indepen-
(1) Telephone. Telephone systems are frequently used in
dence creates opportunities for messages from two or more
automated data collection systems. The initial costs are low,
remote sites to collide with a resulting loss of data. To protect
and the data transfer rates can be quite high (2,400 baud or
against unacceptable data losses, two measures are normally
greater on voice-grade lines). Two types of telephone
employed. First, data messages are kept as short as possible.
systems are in common use, dial-up and leased-line. Leased-
Short messages decrease the potential for message collisions.
line systems have dedicated telephone lines (leased by the
Secondly, data messages are composed with information that
user) that are continuously connected to a remote site. The
enables data recovery from future messages. For example,
user can request data every few seconds if necessary.
raingage data is reported as an accumulated value. If a data
Leased-line is a direct "hard-wire" connection to the remote
report is lost, the next successful message contains the new
site. It can be expensive, of course, since the user has total
total. Thus, no volume information is lost. Only the timing
and continuous control of the telephone line.
information for the "missing" millimeter of rainfall is lost.
(a) Dial-up systems are a less expensive telephone alter-
(d) In well designed event reporting radio systems, data
native. The telephone lines are not continuously connected to
loss due to collisions is minimal. Far more data are lost
the remote site. Whenever data reports are required, the
remote site is dialed up just like one person calling another.
Relatively large numbers of gages can be accommodated.
When data reporting is finished, normally in a minute or less,
Individual event reporting systems receiving data from more
the line is disconnected. Actual line usage is quite short.
than 500 gages are in successful operation.
(b) Telephone systems have their advantages but are not
generally recommended for flood warning - preparedness
(e) Event reporting systems generate reports when an
programs. Reliability during storm conditions is always a
"event" or "change" occurs. What happens if nothing
concern. Telephones often fail under the very conditions that
changes for a long time (e.g., long periods between rain
require maximum data system integrity. In addition,