About ALERT System Sensor Data
ALERT sensors are programmed to report in every 12 hours regardless if a change has occurred. Data is sent to the base station using radio telemetry.
Precipitation
All precipitation gauges measure rainfall depth in 1mm increments. The base station computer which collects and reports the data uses the relation 1 mm=0.03937 inches to convert millimeters to inches. Reported values are in inches. Gauges used are tipping buckets and are programmed to report each tip.
Stream Gauges
Stream gauges measure depth (stage). Reported values may not represent the actual depth of water at a stream gauge. Caution should be used in making decisions based on reported stage information.
Sensors are surveyed to a local datum. The majority of the sensors used to measure stage are fixed in place. Initial stage reported is the elevation of the sensor above the most recently surveyed channel bed. Sensors cannot measure the presence or absence of water below them.
Most stream channels have sand beds that can aggrade and/or degrade over time, depending on how much scour or deposition has taken place in any particular flow event. Distance between the sensor and the channel bed may have changed since the most recent survey.
Stream gauge sensors are interrogated every 5 minutes and programmed to report on a change of 0.2 feet or 2.4 inches. Reported values are in feet.
Data Errors
Automatic rain gauges are set to tip (measure) at 1mm increments. If precipitation less that 1mm falls into the gauge, evaporation may dry the tipping bucket between storms, causing a loss in measurement.
Wind can reduce precipitation catchment by creating turbulence around a rain gauge and catchment error increases as wind velocity increases. A common observation during thunderstorms is that the rain appeared to be blowing sideways. This can reduce the amount of precipitation that falls into a gauge.
Other errors may be introduced by the effects of vandalism, gauge height, calibration errors, blockages caused by bugs, and the physical environment (trees and buildings) near the gauge.
