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Q: Is there additional rainfall, streamflow and weather data besides what is shown in the Rainfall & Weather section of this Web site?
A: Yes. Historical weather data, including rainfall and streamflow information, which does not appear on this site can be provided directly by the District for a nominal fee to cover the cost of compilation and reproduction. Please call (602) 506-8701 for more information or view the Rainfall & Weather contact page.
Q: Why do most of the rain gages appear to read in increments of 0.04 inches?
A: Rain gages actually read in millimeters. One millimeter is equal to 0.03937 inches, which rounds up to 0.04 inches.
Q: Why are there more rain gages in some areas of the County than in others?
A: The density of rain gages and the placement of individual gages are based on a variety of factors, including flood hazard potential, District projects in the area, the location of channels, permitting requirements, environmental restraints, watershed boundaries and the proximity of existing gages.
Q: How do stream gages work?
A: The majority of the District's stream gages use a sensor called a "pressure transducer." This device senses the pressure exerted by the water above it and calculations are made based on the pressure to determine the depth of the water. Once the depth is known, the flow rate, or discharge (represented in cubic feet per second or cfs), is calculated by using a rating curve. A rating curve is a graph of water depth versus discharge. The curve is created through the application of either a mathematical formula or mathematical model to determine the amount of discharge a stream channel may produce based on a given water depth.
Q: Why do stream gages sometimes show a depth value but no corresponding discharge?
A: In many cases the stream gage is not at the bottom, or zero point, in the stream channel or the pool area behind a dam. It is often necessary to locate the instrument above or below this point. In these cases there will either be some positive or negative depth displayed when discharge numbers begin to register. In any case, streamflow only occurs when the discharge value is above zero.
Q: How is the Contoured Rainfall Map produced?
A: The Contoured Rainfall Map is produced from the ALERT database and ARCview GIS software. A program takes information from the form submitted on the Web, such as the ending date/time and the desired map duration, and produces from the ALERT database a table of rain gage values to be plotted. This is then read into ARCview, and a plug-in called Spatial Analyst interpolates a surface, much like a topographic map, to produce the contoured map. The image is then copied to a paint program where the scale and time period text are added. Finally, the image is sent via FTP to the Rainfall Information page of the Web site.
Q: Why do the wind speed sensors appear to read so low?
A: Wind speed sensors do not transmit the instantaneous wind velocity but rather an average wind speed over a time period.
Q: What is a "water-year"?
A: The water-year is used by the U.S. Geological Survey (USGS) and other federal agencies to define a period of data collection. It runs from October 1 through September 30. For example, water-year 2006 began on October 1, 2005, and ended on September 30, 2006.
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