Part 5—Investigate the Summer Monsoon

Step 1 –
Discover Monsoon Thunderstorm Development in Southern Arizona

Much of southern Arizona experiences bimodal precipitation, or two major seasons of precipitationa winter precipitation season and a summer precipitation season. The area receives approximately 30 centimeters (about 12 inches) of rain per year! Roughly half of that rain is received in each season. During the summer, precipitation arrives to the area in early July with the summer monsoon. A wet monsoon, or rainy season, is caused by prevailing winds that bring moisture from the Gulf of Mexico and, according to some scientists, from the Gulf of California and the Pacific Ocean, as well.
  1. If necessary, launch My World and re-open the sabino_watershed_part_4.m3vz project file that you saved at the end of Part 4.

  2. Click the Link Tool, then click the orange flag in the lower left-hand corner of the map, and select Thunderstorm Movie.
    Link_tool_EET
    This movie shows the development of a monsoon storm cell near Tombstone, Arizona (approximately 100 km (60 miles) southeast of Tucson), and was recorded during a late August afternoona typical stormy afternoon during the summer monsoon season.
  3. Play the movie several times, as needed.
  4. Click the Link Tool again, and then click on the orange flag in the lower left-hand corner of the map, and choose Monsoon Radar Movie. Be patientit may take a moment for the movie to load.
    The linked movie shows the storm radar in southeastern Arizona, which includes Tucson and Tombstone, on the same August afternoon as the Thunderstorm Movie you viewed. Blues and greens represent less-severe storm conditions (and less rain), whereas yellows, oranges, and reds represent severe storm conditions (and more rain). The date and hour at the right of the movie screen is in Greenwich Mean Time, which is seven hours ahead of Tucson time. The movie begins, therefore, at approximately 5:00 p.m. local time in Tucson.
    Radar Circle

    How radar works

    The blues, greens, reds, and yellows in the radar movie are actually echoes, or electronic signals that have been reflected back to a central radar antenna. These echoes provide information on the location and distance of the precipitation from the antenna. The maximum distance that radar can detect precipitation is approximately 230 kilometers (143 miles) from the radar antenna. This distance represents the outermost edge of the radar area, forming a circumference around the radar source. If you want to know if it is raining beyond the radar circle, data from an adjoining radar source must be used (text adapted from The Weather Underground, Inc.).
  5. Play the movie several times, if needed, to examine the movement and direction of the storms. Use both movies to answer the following questions.
    • What do the storm cells look like on the radar screen at the beginning of the movie?
      The storm cells typically begin as small, individual cells and are distributed across the entire radar region.
    • What happens to the size of the storm cells over several hours?
      The cells become much larger as the evening progresses.
    • In what direction do monsoons tend to migrate during a typical summer afternoon and evening?
      The storm cells migrate from east to west.
  6. Close the Movie windows when you have finished viewing the movies.

Step 2 –
Explore a Monsoon Thunderstorm in Sabino Canyon

  1. Turn on the Radar (8/26/03, 5 p.m.) layer.
    This layer shows the storm radar and total rainfall amount as of 5:00 p.m. on August 26, 2003 in the Santa Catalina Mountains and surrounding area. The radar is broken up into small rectangular grids that you see on your screen. A grid cell indicates that there was rain in that location, and each grid represents a given amount of rainfall received. Refer to the legend at the bottom of the Map window to determine rainfall amounts and the corresponding map colors.



  2. Hide or show the Shaded Relief Map layer as needed to answer the following questions and to see the colors of the radar grids more clearly.
    • What percentage of the Sabino Canyon watershed was rained on as of 5:00 p.m. on August 26, 2003?
      100 percent of the watershed had been rained on, because the entire watershed is covered with radar grids.
    • Where did the majority of this heavy rainfall occur in the watershed (in the higher or lower elevations within the watershed)?
      The heaviest rainfall occurred in the lower elevations of the basin, closer to the watershed outlet.
  3. Hide the Radar (8/26/03, 5 p.m.) layer.

Step 3 –
Compare Streamflow During Monsoon Season to the Long-term Record

  1. Show the Shaded Relief Map layer.
  2. Activate the Sabino Creek Gauging Stations layer.
  3. Using the Link Tool, click the Lower Sabino Creek gauging station symbol (black triangle) near the outlet of the Sabino Creek watershed, then click on the link name in the pop-up window.
    This image shows a hydrograph, or a graph of time versus streamflow, for Lower Sabino Creek during August and September 2003. (Note: No streamflow data are available for the Upper Sabino Creek stream gauge after 1959, when it was abandoned.)
    • Describe the hydrograph for the Lower Sabino Creek stream gauge. Was the storm on August 26, 2003 unusual, in terms of flow at the gauge?
      The flow seems to be a little greater than other storm events, but it doesn't seem to be too unusual, since there are two smaller flow events and one larger event during the same month.
  4. Close the image.
  5. Click the Link Tool, then click on the orange flag in the lower left-hand corner of the map, and choose Sabino Creek Flow, 19882006.
    This image shows the long-term flow record in Sabino Creek, from 1988 to 2006. The August 26, 2003 storm is labeled in the graph for comparison.
    • How does the flow from the August 26, 2003 storm compare to the long-term flow record in Sabino Creek?
      When comparing the flow to the long-term record, it appears normal. There are other outliers (unusual events) in the record showing much higher flow than this storm, particularly those of spring 1991 and summer 1999.
    • Speculate on what kinds of natural or human-caused events or activities might cause the streamflow in a watershed to increase over long or short periods?
      Answers will vary. Natural events that could contribute to higher flow include intense rainfall in a short period, rainfall lasting for several days, fire, and landslides (or debris flows). Human-caused events might include fire, deforestation, forest thinning, or urban development. All of these events ultimately remove vegetation from the landscape and allow more rainfall to run off the land surface to the stream drainage.
    • Speculate on what kinds of natural or human-caused events or activities might cause the stream flow in a watershed to decrease over long or short periods?
      Answers will vary. Activities that may reduce the flow in a watershed may include drought, dam construction along a stream (both human or natural), or planting new vegetation that changes the way that rainfall runs off into a stream.
  6. Close the image when you are finished.
  7. Quit My World GIS and do not save changes.

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