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Hydrology of the Willcox Basin

Groundwater Hydrology

Other Basins

Two basins, Cienega Creek and Willcox, have hydrogeologic conditions that are unique in the planning area.  The Cienega Creek Basin has three groundwater sections based on the presence of distinctive aquifers and groundwater flows to the north and to the southwest. Groundwater in the Willcox Basin is generally isolated from surrounding basins, with groundwater flow primarily to the center of the basin, the Willcox Playa.

Willcox Basin

The Willcox Basin occupies the northern part of the Sulphur Springs Valley and is hydrologically separate from the southern part of the valley, the Douglas Basin. Groundwater in the Willcox Basin is found in alluvial deposits consisting of stream and lake-bed deposits.  The stream deposits are the most productive water-bearing unit. The clay-rich lake bed deposits outcrop in the Willcox Playa. There they create localized artesian conditions.  Where the coarse-grained stream deposits are underlain by the lake-bed deposits, perched groundwater conditions may occur.  A playa is a nearly level area at the bottom of a closed desert basin, sometimes temporarily covered by water. 

The Willcox Basin has internal surface water drainage and groundwater flow is thought to have mirrored surface drainage under predevelopment conditions; moving from the outer margins toward the Willcox Playa (Oram, 1993).  However, groundwater flow conditions have been altered significantly due to groundwater pumping for agriculture. Several relatively large cones of depression have developed in the basin including one southeast of the Willcox Playa and another north of the City of Willcox (Figure 3.14-6). Groundwater recharge has been estimated at 15,000 to 47,000 AFA primarily from mountain front recharge and also from agricultural irrigation and stream channel runoff (USGS, 2006b).  Groundwater discharge is primarily from groundwater pumping of more than 176,000 AFA. Estimates of groundwater in storage range from 42 to 59 maf (Table 3.14-6).

Declines in groundwater levels (in excess of 200 feet measured in nine wells between 1954 and 1975), may have caused land subsidence in the basin (USGS, 2006b).  Figure 3.14-6 shows groundwater level changes between 1990-1991 and 2003-2004. A number of declines of greater than 30 feet were measured in wells in the central part of the basin during this period.

Click to view Table 3.14-6

Click to view Table 3.14-6 Groundwater Data for the

Willcox Basin

Click to view Figure 3.14-6

Click to view Figure 3.14-6 Willcox Basin Groundwater Conditions

Concerns about groundwater level declines and future availability of water for all uses has led to an investigation of the geology and hydrology of the Willcox and Douglas basins (USGS, 2006b).  As part of this effort, the Department released a Water Level Change Map Series Report (No. 1) in 2008 summarizing depth to water measurements taken at 578 wells in the Willcox Basin in November/December 1999 and November/December 2005.  Most of the wells (549 of 578 or 95%) showed a water level decline. Forty had declines of more than 40 feet and most of these were located in the area southeast of the Willcox Playa in a predominantly agricultural area (Jacobson and others, 2008).  A summary of the water level changes and a water level change contour map from the map series report are shown in the graphic below.  As shown, most water levels declined between 0.5 and 20.4 feet.  Well yields are relatively high in the basin with A median well yield of 750 gpm was reported from over 1,000 large (>10 inch) diameter wells (Table 3.14-6).

Elevated TDS concentrations exist in some areas and fluoride and arsenic concentrations above drinking water standards have been reported in a number of wells (Table 3.14-7).



Click to view Figure 3.0-5

Click to view Figure 3.0-5 USGS 6-Digit Hydrologic Unit Code Boundaries in the Southeastern Arizona Planning Area

Surface Water Hydrology

The U.S. Geological Survey (USGS) divides and subdivides the United States into successively smaller hydrologic units based on hydrologic features.  These units are classified into four levels. From largest to smallest these are: regions, subregions, accounting units and cataloging units.  A hydrologic unit code (HUC) consisting of two digits for each level in the system is used to identify any hydrologic area (Seaber et al., 1987).  A 6-digit code corresponds to accounting units, which are used by the USGS for designing and managing the National Water Data Network.  There are portions of five watersheds in the planning area at the accounting unit level: Lower Colorado River below Lake Mead; Middle Gila River; Rio Bavispe; San Pedro River; Santa Cruz River; and the Upper Gila River (Figure 3.0-5). 

San Pedro-Willcox Watershed

The Arizona portion of the San Pedro River Watershed is contained entirely within the planning area.  Approximately 696 square miles of the Watershed extends into Mexico. In Arizona, the Watershed includes all of the Aravaipa Canyon and Upper San Pedro basins, most of the Lower San Pedro and Willcox basins and relatively small portions of the Cienega Creek, Douglas and San Rafael basins.  A few tributaries to the San Pedro River begin on the southwest slopes of the Huachuca Mountains in the San Rafael Basin and drain into Mexico. (ADWR, 2005a) The San Pedro River enters the U.S. from Mexico near Palominas (see Figure 3.13-1) and flows north to its confluence with the Gila River. Major tributaries are the Babocomari River and Aravaipa Creek.

With the exception of Whitewater Draw in the extreme southern end of the basin that drains into the Douglas Basin, most of the surface water drainage in the Willcox Basin is to the Willcox Playa.  The playa occupies about 50 square miles in the center of the basin and is a remnant of Pleistocene-age Lake Cochise. (Oram, 1993) 

Some stretches of the San Pedro River are perennial, although recent drought and delay of the summer monsoon has affected some previously perennial stretches for short periods of time, most notably at Charleston in the Upper San Pedro Basin. The Babocomari River, in the Upper San Pedro Basin, is perennial in its upper reach. Aravaipa Creek is perennial within Aravaipa Canyon above its confluence with the San Pedro River as are three of its tributaries in the Aravaipa Canyon Basin (see Figures 3.1-5 and 3.8-5).  Other perennial streams are found in the Lower San Pedro, Upper San Pedro and Willcox basins (Figures 3.8-5, 3.13-5 and 3.14-5).

There are 12 active streamgages in the watershed; two in the Lower San Pedro Basin and 10 in the Upper San Pedro Basin. The gage on the San Pedro River at Charleston has been in operation since 1904. The largest annual flow ever measured in the watershed, (152,798 acre-feet), was recorded at this gage in 1914.  More recently, in 1984, a maximum annual flow of 102,107 acre-feet was measured at the gage on the San Pedro River near Tombstone.  Median annual flow at these gages is 33,203 acre-feet and 29,654 acre-feet, respectively.

The only major springs in the watershed are found in the Lower San Pedro and Upper San Pedro basins. There are 14 major springs in the Lower San Pedro Basin. The largest, Cooks Lake Spring, had a discharge rate of 1,000 gpm when last measured in 1951.  Twelve major springs have been identified in the Upper San Pedro Basin. The largest is Garden Canyon No.1 with a discharge of 134 gpm measured in 1963. Most of the spring measurements in both basins date from before 1980 and may not be indicative of current conditions (see Tables 3.8-5 and 3.13-5).

Fifteen miles of the San Pedro River in the Lower San Pedro Basin from Aravaipa Creek to the Gila River are impaired due to elevated concentrations of E. coli and selenium (Table 3.8-7). In the Upper San Pedro Basin, water quality standards were exceeded in three reaches of the San Pedro River for a total of 53 miles. These reaches are impaired due to elevated levels of E. coli, nitrate and copper (Table 3.13-7).



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