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Hydrologic Conditions in Arizona
During 1999–2004:
A Historical Perspective

U.S. Geological Survey Fact Sheet 2005-3081

Figure 1. Locations of 10 long-term streamflow-gaging stations, 10 medium- to long-term gaging station, and 5 long-term precipitation stations.

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Introduction

Arizona’s climate is prone to extreme changes that range from persistent droughts to frequent local and regional flooding. These changes are evident in hydrologic data collected. Streamflow records indicate that a drought in Arizona during 1999–2004 was the worst drought since the early 1940s and possibly earlier. Droughts result from a decrease in the number of already infrequent storms that bring moisture to Arizona. The drought conditions in the Southwestern United States over the last several years, and especially in Arizona, have resulted in several large summer fires, a decrease in potable water for some smaller communities, and depleted water available for surface water as well as ground-water recharge. An unusually wet December 2004 and January 2005 in Arizona has interrupted the multiyear drought. Dry conditions, however, still prevail in parts of Arizona. It is difficult to conclude, therefore, whether the drought is over or if it will persist.

Historical and Current Hydrologic Conditions

Although the spatial and temporal extent of droughts is somewhat difficult to determine, three severe droughts during the 20th century were recognized in a 1989 U.S. Geological Survey National Water Summary (Paulson and others, 1991). The periods of significant statewide droughts, as indicated by records from several streamflow-gaging stations, were recognized as 1932–36, 1942–64, and 1974–77 (figs. 1 and 2; table 1).

This document utilizes long-term data from streamflow-gaging stations to compare the severity of the current drought to those indicated by Paulson and others (1991; fig. 2).

STREAMFLOW-GAGING STATIONS IN NORTHERN AND CENTRAL ARIZONA, 1939–2004—Drought periods shown in gold; horizontal line indicates long-term average. Record for the Paria River begins in 1923; data for 1923–38 not shown.

STREAMFLOW-GAGING STATIONS IN CENTRAL AND SOUTHERN ARIZONA, 1914–2004—Drought periods shown in gold; horizontal line indicates long-term average. Record for the Salt River begins in 1913; data for 1913 not shown. Gaps in plots after 1914 indicate data are not available.

Figure 2. Annual mean streamflow at 10 long-term streamflow-gaging stations in Arizona.

Climatology in Arizona

Precipitation in Arizona is biseasonal, having both winter and summer regimes (Hereford and others, 2002). The moisture comes from three major sources: (1) Pacific winter frontal storms that can produce significant snowpack in northern Arizona as well as flooding in the central and southern parts of Arizona, (2) subtropical Pacific moisture (dissipating hurricanes or tropical storms) that is generally warmer and can produce regional flooding of large magnitude, and (3) convective storms that occur throughout the State during the summer months. The location and intensity of convective storms are difficult to predict as the storms can form quickly and produce large amounts of precipitation in localized areas. They also generally result in flooding in smaller basins and urban areas, but are not significant for production of higher flows in main-stem river systems (Paulson and others, 1991).

Additional Information on Hydrologic Conditions in Arizona, 1999–2004

Figure 3. Annual precipitation at five long-term stations in Arizona. Drought periods shown in gold; horizontal line rejpresents long-term average. Gaps in plot after 1910 indicate data are not available.

Precipitation data acquired by the National Weather Service can serve as an indicator of drought and flood conditions. Data acquired at five long-term precip­itation stations in different parts of Arizona indicate that precipitation during the last 6 years was below the long-term average (fig. 3).

Streamflow in water years (WY) 1999–2004 was compared to historical streamflow data for 20 streamflow-gaging stations in Arizona for this report (table 2 and fig. 1). Included in the 20 stations are 10 long-term stations, as well as an additional 10 medium- to long-term stations (fig. 1). These 20 stations are considered index stations because they have medium to long periods of record and are little affected by flow diversions. The data for the period 1999–2004 for these 20 stations, when compared to historical data, indicate WY 2000 and WY 2002 were two of the driest years during the period of record. Annual discharge for almost all sites for individual years from 1999 to 2004 was well below long-term average conditions, indicating statewide drought conditions since the beginning of WY 1999 (October 1998). Annual discharge exceeded the long-term average only 6 times at the 20 sites from WY 1999 through WY 2004 (table 2). Annual discharge for the 20 sites over this same period of time was less than 10 percent of the average annual discharge 17 times and less than 50 percent 83 times. The drought of 1999–2004 is considered the most severe drought in Arizona since the early 1940s and possibly earlier (table 1). The average streamflow during three drought periods—1942–64, 1974–77, and 1999–2004—was compared to average streamflow for the 10 long-term index stations. Streamflow at the stations was 45 percent of the long-term flow during 1999–2004, 53 percent during 1974–77, and 68 percent during 1942–64 (fig. 2). Data for 1932–36 were insufficient for comparisons with data for 1999–2004.

Although streamflows generally were low in Arizona during 1999–2004, floods during the winter of 2005 were substantial—to the point of filling reservoirs in central Arizona. The climate of Arizona, however, naturally tends to extremes—large floods and severe drought conditions are common. Determining whether this was an interruption to a longer drought, or the beginning of wetter years in Arizona, therefore, is difficult. Data acquired at 20 medium- to long-term streamflow-gaging stations, however, indicate the period 1999–2004 was the driest since the early 1940s and possibly earlier.

—Jeff V. Phillips and Blakemore E. Thomas

References

Other Information

Current streamflow conditions in Arizona can be obtained from
http://waterdata.usgs.gov/az/nwis/rt

Historical streamflow conditions can be obtained from
http://waterdata.usgs.gov/az/nwis/sw

For more information contact:
U.S. Geological Survey
Arizona Water Science Center
520 N. Park Ave., Suite 221
Tucson, Arizona 85719
Telephone: (520) 670-6671 x261
Fax: (520) 670-5592
E-mail: jvphill@usgs.gov

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For more information about USGS activities in Arizona, visit the USGS Arizona District home page.

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U.S. Department of the Interior, U.S. Geological Survey Persistent URL: http://pubsdata.usgs.gov/pubs/fs/2005/3081/ Page Contact Information: USGS Publishing Network Last modified: Tuesday, November 29 2016, 06:16:37 PM

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