Hydromet Frequently Asked Questions

Does LCRA use the Hydromet information to issue flood warnings?

LCRA supplies Hydromet data to the National Weather Service's River Forecast Center in Fort Worth to help forecasters decide whether to issue flood and weather warnings. The National Weather Service issues flood watches and warnings.

How many gauges are in the Hydromet network?

The Hydromet network includes more than 380 gauges, including 244 gauges maintained by LCRA, 78 gauges maintained by City of Austin and more than 60 gauges maintained by USGS in cooperation with LCRA, City of Austin and others. LCRA and City of Austin periodically add new gauges to meet specific operational needs.

What do the gauges measure?

The gauges measure hydrological data, including river stage, lake level and streamflow, and meteorological data such as rainfall, air temperature and humidity.

What is streamflow?

Streamflow is the rate at which water flows, measured in cubic feet per second (cfs). One cfs is equal to about 450 gallons per minute.

What is a river stage?

Stage is the height of water in the river channel compared to a nearby benchmark. River channels change over time. The zero elevation of the benchmark, or local datum, is not referenced to the bottom of the channel, but is set at an arbitrary elevation usually below the elevation of the riverbed. Stage can be useful to see how the river level at one location varies over time, for example to compare a predicted flood to previous floods at the same location.

How can I use river stage to find the height of the river above mean sea level?

Add the river stage to the zero elevation of the gauge benchmark, or local datum, to find the height of the river above mean sea level.

For example, the local datum of the Colorado River gauge at Wharton is 52.42 feet above mean sea level (feet msl). If the river stage at Wharton is 40 feet, then the height of the river at the gauge is 92.42 feet msl.

The local datum for river gauges can be found on web pages maintained by the USGS and the National Weather Service. Links to these pages can be found on the LCRA Flood Operations Report. On USGS current conditions at each gauge pages, look for the link to “Summary of all available data for this site”. The local datum of the gauge is shown as “Datum of gage” in the stream site description. (“NGVD29” is the technical reference for elevation above mean sea level used by USGS.) On National Weather Service river forecast pages, the local datum of the gauge is in the lower left hand corner of the graphical forecast. Look for the text “Gage 0 Datum:” followed by the elevation of the gauge datum in feet msl.

What is flood stage?

Flood stage refers to the height (or stage) of a river or stream at which the water begins to create a hazard to lives, property or commerce. Flood stage levels are determined by the National Weather Service and local communities.

How do I find the flood stage at a given location?

Find the flood stage by selecting a Hydromet site on the map while looking at river stage or streamflow data. Flood stage for the site is displayed on the pop-up graph of stage. Not all locations have a predetermined flood stage level.

How often is rainfall measured?

LCRA and Austin Hydromet gauges typically collect and transmit data by radio every 15 minutes. Data collected by USGS for Austin at gauges in the Austin area may be transmitted every 5 minutes to 15 minutes, especially during storms and flash floods. Data collected by USGS or LCRA at more distant gauges beyond the range of LCRA’s radio system may be transmitted by satellite each hour.

How is rainfall measured?

Hydromet gauges catch rain in a funnel and measure the volume of rain with a small device (a “tipping bucket”) at the bottom of the funnel. The volume of water measured by the tipping bucket is converted to an equivalent depth of rain falling over the surface of the funnel. The City of Austin rain gauges report rainfall in increments of 0.04 inches (1 millimeter). LCRA rain gauges report rainfall in increments of 0.01 inches.

On the rainfall pages, what does "count" refer to?

A "count" is an increment of rainfall. For City of Austin gauges, a count refers to 0.04 inches (1 millimeter) of rain. For LCRA gauges, a count refers to 0.01 inches of rain.

Counts accumulate over time at each gauge location until the count is reset. The Hydromet database uses the counts to calculate the amount of rainfall over various periods of time.

What is "conductivity data" and "micromhos"?

Conductivity is the water’s ability to pass an electrical current, measured in micromhos or microsiemens per centimeter (µs/cm). It is an indirect measure of water salinity and an important indicator of water quality. High conductivity readings may indicate high levels of salt and other inorganic dissolved solids, such as chloride, sulfate, sodium and calcium. Conductivity at sites upstream of Lake Buchanan is typically about 700 to 1,000 µs/cm, which is higher than in most of the lower Colorado River watershed.

Select Gauge Data on the Hydromet page, then select “Conductivity” to view conductivity data at select locations. LCRA also displays conductivity and other water quality data on the LCRA Water Quality Data page.

Have more questions about the Hydromet?

For questions about LCRA gauges or programs, contact LCRA through Ask LCRA.

For questions about City of Austin gauges or programs, call the City of Austin Watershed Protection Department’s Floodplain Office at 512-974-2843.

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