Vegetation zones in the Upper San Pedro Basin range from Sonoran Desert to coniferous (subalpine and montane) forests on mountains higher than 7400 feet above MSL, transitioning to chaparral and evergreen woodlands at lower elevations (Putman, et al, 1988; Brown, 1982). Vegetation below 3,500 feet in elevation is classified as desert. The southern portion of the watershed is Chihuahuan desertscrub, consisting of creosotebush, tarbush, whitethorn, acacia, ocotillos, and mesquite.
Much of valley floor is grassland. The semi-desert grasslands contain several species of grasses including Grammas and Threeawns. These grasslands (3,500 to 5,000 foot elevations) have been significantly altered since Anglo settlement. Mesquite and juniper have invaded large areas of grasslands in the San Pedro River watershed, and continue to be replaced by scrub species, including Mormon Tea and ocotillo.
About 33 square miles along the Upper San Pedro River are covered by agriculture and riparian vegetation. "Riparian" vegetation is defined by its occurrence along stream channels, and these vegetation zones are more diverse in both flora and fauna populations than surrounding drier regions. Because riparian vegetation depends on instream flows and groundwater from the regional groundwater aquifer, riparian zones play very important roles in the hydrology of a watershed (ADWR, 1990).
In addition to many grasses, the riparian corridor surrounding the San Pedro River supports several tree species including cottonwood (Populus fremontii), Gooding Willow (Salix gooddingii), Seep Willow (Baccaris glutinosa), mesquite and salt cedar (Tamarix chimensis). Most of these are phreatophytes-- deep-rooted plants which obtain water from the underlying aquifer (ADWR, 1990; Jackson, et al, 1987). The riparian corridor also hosts the widest variety of animal species of all the different environments in the Upper San Pedro Basin: about 47 species of amphibians and reptiles, 52 species of mammals, and more than 275 species of birds of which 45 are considered riparian obligates (Jackson, et al, 1987).
The perennial river floodplains typically occupied by riparian stands consist of deep, well-drained soils which are potentially some of the most agriculturally productive in the San Pedro River watershed. Consequently, much of the irrigated acreage in the watershed occurs on cleared lands which were formerly occupied by riparian vegetation. If unmaintained, much of this agricultural land would return to riparian stands provided sufficient water is available. Research on water salvage through phreatophyte eradication shows that short term increases in flows are possible, although replacement vegetation must be established in order to provide bank stability. In some cases, irrigated lands cleared of phreatophytes use less water than the natural vegetation, providing a net savings to the surface water system (ADWR, 1990).
The United States Congress established The San Pedro Riparian National Conservation Area in 1988 in an effort to protect the rare riparian habitat from damage due to increasing water demands in the surrounding area (Nat'l. Res. Council, 1992). The SPRNCA is the Nation's first riparian preserve (ADWR, 1990). The reserve constitutes a narrow band of 56,000 (USC, 1988) acres of riparian habitat along the San Pedro River managed by the BLM. All flora and fauna in this ecosystem depend on instream flows. Wildlife use the riparian zone for habitat, food and water (Jackson, et al, 1987). Consumptive water use by phreatophytes is discussed in more detail under the section entitled "Natural Depletions."
Figure 3 shows a schematic cross-section of the Upper San Pedro River Valley. Groundwater from the floodplain aquifer provides most of perennial streamflow in the San Pedro River. Some flow derives from the regional aquifer at shallow bedrock constrictions (ADWR, 1990). Valley fill sediments (up to about 1500 feet deep) constitute the major water bearing unit: the "regional aquifer" (RA) (ADWR, 1990). The regional aquifer underlies most of basin, with areal extent of about 1200 square miles in Upper San Pedro Basin (Ariz. Water Comm., 1974).
Roeske and Werrell (1973) report that artesian conditions generally exist for wells penetrating to depths greater than 200 feet. In the Palominas-Hereford and Saint David areas, artesian pressures are high enough to bring water to the surface. Artesian pressures have been declining in both areas due to prolonged pumping (Vionnet and Maddock, 1992). The majority of recharge to the regional aquifer occurs along mountain fronts and a small portion via stream channel infiltration. Recharge along the Huachuca Mountains is estimated between 5.5 cfs (Harshbarger and Assoc., 1974) to 6.9 cfs (Freethey, 1982).
Direct infiltration over the valley floor is considered negligible because of high evaporation and low precipitation rates (Freethey, 1982). The ADWR (1990) estimates mountain front recharge equal to 25,620 ac-ft/yr. Recharge from underflow across the International Border is estimated at 700-3400 ac-ft/yr (Freethey, 1982; ADWR, 1990). In general, no groundwater communication exists between adjacent basins (Freethey, 1982), but there are two exceptions to this rule: west of Elgin on the headwaters of the Babocomari River and an area on the southwest slopes of the Huachuca Mountains. In both of these areas, surface water flows toward the San Pedro River, but groundwater flows in the opposite direction, out of the San Pedro Basin. These two non-tributary groundwater basins are known as Cienega Creek groundwater basin (near Elgin) and the San Rafael groundwater basin (on the southwest slope of the Huachuca Mountains) (ADWR, 1990).
An aquifer's capacity to transmit water, or hydraulic conductivity, and the volume of water an aquifer can release or take into storage, described by the storativity value, are the primary scientific descriptors of an aquifer. These two values can be used to estimate regional groundwater flow rates, groundwater storage volumes, and the effect of a pumping well on the aquifer and other nearby wells. The transmissivity of the aquifer is calculated by multiplying its hydraulic conductivity by the saturated thickness of the aquifer at the same location. Transmissivity estimates for the regional aquifer vary from 500 ft2/day (Harshbarger and Assoc., 1974) to 27,000 ft2/day (Schwartzman, 1990). Putman, et al (1988) suggest a transmissivity of 4,000 - 8,000 ft2/day for the whole Upper San Pedro Basin. The general direction of groundwater flow is toward San Pedro River except near the river, where the flow direction turns parallel to river (Vionnet and Maddock, 1992). Unconfined aquifer specific yields range from 0.02 to 0.15 (Harshbarger and Assoc., 1974). Values of storativity in confined portions of the regional aquifer are on the order of 10E-5 (Freethey, 1982).
The rate of groundwater flow is governed by the hydraulic conductivity and the hydraulic gradient (slope of the water table or piezometric surface) in the regional aquifer. The average groundwater flow rate is about 22.6 ft/yr in the Upper San Pedro Basin. Thus, a particular particle of water recharged at the mountain fronts may take years or even centuries to travel through the regional aquifer to the center of the valley (ADWR, 1990).
The total volume of recoverable groundwater in an aquifer is determined by the geology and chemistry of the aquifer material, which in turn determine the number and size of open pore spaces that exist. Water held in storage in an aquifer resides in these pores, but not all water held in aquifer storage can be released from storage. Some water remains in the voids due to molecular surface attraction. Table 3 lists the total volume of recoverable groundwater held in storage in the regional aquifer of the Upper San Pedro Basin.
| Sierra Vista | ||
| Benson | ||
| total in Upper San Pedro Basin |
A secondary aquifer exists in shallower "floodplain aquifers" (FPA), deposited above the valley fill sediments by the San Pedro River and its tributaries. These aquifers are much smaller and narrower than the regional aquifer (200 yards to several miles wide), and only 40 to 150 feet deep (Roeske and Werrell, 1974). The floodplain area is estimated at 35,055 acres for the Upper San Pedro Basin (Putman, et al, 1980). The alluvium forming the long, narrow, and shallow aquifer beneath and alongside the San Pedro River expresses itself at the surface in what is called the "inner valley," also referred to as the younger alluvium (see Figure 3).
The most important floodplain aquifer in the Upper San Pedro Basin bounds the San Pedro River. Table 3 lists the total recoverable groundwater in storage in the floodplain aquifer as estimated by the ADWR (1990) based on an average saturated thickness of 60 feet. Putman, et al (1988) estimated the volume of water held in storage by the Upper San Pedro Basin floodplain aquifer at 0.421 ac-ft by using an average saturated thickness of 100 ft. Recharge to the floodplain aquifer derives from streamflow, upward leakage and lateral flows from the regional aquifer, and agricultural return flows (deep percolation.), run-off water percolation, and underflow across International Border (ADWR, 1990). Transmissivities and porosities are generally high in this aquifer. Representative well yields range from 200 to 1,800 gallons per minute (gpm), averaging 700 gpm. Estimates of transmissivity range from 600 to 18,000 ft2/day depending on method of estimation. Estimates of specific yield range from 0.05 to 0.25, with values 0.1 to 0.12 commonly used (Roeske and Werrell, 1973).
The floodplain aquifer water table exhibits seasonal fluctuations, but so far it shows no net long term declines because of rapid recharge from the San Pedro River (ADWR, 1990). In addition to serving as the main conduit for surface water/groundwater interaction, the floodplain aquifer provides necessary water for phreatophytes within the riparian zone of the inner valley and is the major supplier for irrigation wells along San Pedro River and its principal tributaries (Putman, et al, 1988).
Groundwater communication between the regional aquifer and floodplain aquifer is generally from the regional aquifer to the central floodplain aquifer, but this may reverse in times of low river flow. Flow between the floodplain aquifer and the San Pedro River is toward the San Pedro River in perennial reaches (baseflows). The ADWR (1990) estimates underflow entering the U.S. potion of watershed through the floodplain aquifer under 1989 development conditions at 900 ac-ft/yr (ADWR, 1990). Freethey (1982) estimated the floodplain aquifer underflow from the International Border at 700-3,400 ac-ft/yr. Groundwater velocities in the floodplain aquifer are generally higher than in the regional aquifer (i.e., greater than 22.6 ft/yr).
Prehistoric mammoth bones and spear points in Aravaipa Canyon (north of the Narrows) indicate human habitation as early as 11,000 B.C. The Cochise culture occupied the eastern watershed area from about 5,000 BC to 1,000 A.D. The Hohokam, a culture based on irrigation agriculture, diverted water from the middle San Pedro River from about 500 A.D. until their disappearance around 1400 A.D. The Sobaipuri Indians, ancestors to the Pima and Papago, lived in the region until 1762 when the Spanish relocated them to the Santa Cruz Valley (Tucson area) to protect the missions of that area from Apache raids. By the 17th century, the Athabascan speaking Apache Indians had moved south into the river valleys of central and southern Arizona. The Apache were primarily hunters and gatherers, but also depended heavily on frequent raids of other area inhabitants to supplement their subsistence. Apaches were forced to move to reservations outside the area in the late 1800's, and no Native Americans have occupied the San Pedro River region since (ADWR, 1990).
Spaniards first explored the area in 1539, and in 1540 Francisco Vasquez de Coronado led a large exploration party up the San Pedro in search of the fabled Seven Cities of Cibola and rumored gold riches. Father Eusebio Kino traveled and mapped the region between 1691 and 1702 (ADWR, 1990).
Anglo beaver trappers moved into the lower San Pedro River Basin in 1826, marking the initial Anglo penetration of the area. The Mormon Battalion moved north along the San Pedro River on its way to Tucson during the Mexican War (1846-1848), but did not remain in the area at that time. The Treaty of Guadalupe Hidalgo (1848) brought many Anglos into the area as U.S. Government surveyors (ADWR, 1990).
In general, pre-1865 water use was limited to Indians and Mexicans for stock and irrigation of small plots along San Pedro River (beans, maize, and squash) (Putman, et al, 1988).
In 1877, Mormon church members moved into the area and established the first Anglo settlement, St. David. In 1881, townspeople built the St. David Ditch to transport irrigation water from the San Pedro River to users east of the river. At that time, settlers fell ill to malaria because of the marshy environment around the San Pedro River. An earthquake in 1887 produced earth fissures which formed washes that drained the area of excess water (ADWR, 1990).
Fort Huachuca and the Tombstone mine were also established in 1877, and a smelter near Hereford was built in 1878. Rapid development of mining, farming, ranching, and railroads occurred in the 1880's. Hereford developed around the smelter, ranching and the railroad and became a prominent railroad stop and shipping station for cattle during World War I (ADWR, 1990). The town of Fry (named for Oliver Fry, who moved his family to the area in 1912 from Texas, and later named Sierra Vista in 1956) grew outside Fort Huachuca.
Miner Ed Schiefflin discovered a very rich silver strike in 1878 near present day Tombstone. He named his mine Tombstone in honor of warnings that he would fall prey to Indian raiders in the area. The town of Tombstone grew around his claim and was the largest in Arizona at the time, with a population of 15,000. The Tombstone boom began to wane in 1881, when miners struck water at 520 feet in the main shaft of the Grand Central mine. Surface pumps were installed to remove the water, but a fire in 1886 destroyed one of the pump houses. The remaining pumps were inadequate to serve the existing mines and a major flood in 1890 followed by a fire in the remaining pump house in 1892 caused residents to virtually abandon Tombstone (ADWR, 1990).
Benson, founded in 1880, served as an important Southern Pacific Railroad station for the Arizona territory and remains important to the community's economy. The Benson Canal Company (later replaced by the Pomerene Water Users Association) opened a canal in 1912 to supplement the community's use of well water for irrigation (ADWR, 1990).
St. David, Pomerene, and Benson developed around irrigated agriculture and a Railroad station at Benson. From 1900 to 1940, the mining industry declined and Fort Huachuca reduced to minor post. After World War II began, both Fort Huachuca and Sierra Vista began to grow and municipal and industrial water use increased.
Large scale mining in Bisbee stopped in 1975, although some mining still continues in Tombstone and Bisbee. Agriculture expanded through the 1970's, declining to current levels in the 1980's. Urban areas continue to grow. Sierra Vista (east of the Huachuca Mountains) is the largest city (population 35,000 in 1991) with the main employers being manufacturing, services, public administration, agriculture and national defense. Huachuca City (15 miles north of Sierra Vista) is the next largest city in the area. Retirees and tourists have recently become more attracted to area. Table 4 lists populations for several communities in the Upper San Pedro Basin.
Fort Huachuca, the area's largest employer, has approximately 11,700 military and civilian employees plus 11,200 military family members. In total, Fort Huachuca directly supports 66% of Sierra Vista's population (Arizona Dept. of Econ Sec., 1988). Benson experienced a population loss from 1980 to 1990. The nature of Benson's population, many "winter visitors" and retirees (25% of population over age 65), may contribute to the volatility of its population (ADWR,1990).
Agriculture and mining are currently the predominant resource-oriented activities in the San Pedro River watershed. In addition to these two industries, manufacturing, services and public administration form the economic base of the watershed. Most land in the San Pedro River watershed is used for agriculture and livestock ranching. Apache Nitrogen Products, Inc. (ANPI), located south of Benson, is the largest manufacturing plant in the watershed. ANPI employs 116 people and manufactures mining explosives and nitrogen fertilizers. Tourism is very important to the service sector. The mild climate plus attractions like historical Tombstone, Ft. Huachuca, Coronado National Memorial (south of Sierra Vista), and other recreational activities like birdwatching and camping attract a significant number of tourists.
The economy of the San Pedro River watershed is predicted to become more urban based on primary growth in public administration (government) and secondary growth in manufacturing, tourism and the expansion of retirement-type communities. Agriculture and mining are expected to remain important industries (ADWR, 1990).
| Urban Pop. | |||
| Sierra Vista-Fort Huachuca | |||
| Bisbee | |||
| Benson | |||
| Tombstone | |||
| Huachuca City | |||
| subtotal: |
| Rural Pop. | |||
| Cochise Co. | |||
| Pima, Santa Cruz and Graham Co's | |||
| subtotal: |
| Total: |
Channel evaporation from perennial streams is assumed to be minimal. Evaporation from bare soils in and along stream courses is more significant. Water that evaporates from alluvial soil depletes near-surface water storage and provides a sink for infiltrating water from streams, thereby decreasing subsequent surface flows and recharge rates to the regional aquifer.
Evapotranspiration by phreatophytes comprises the single largest natural consumptive use of water in the Upper San Pedro Basin (Table 5). The effect of phreatophyte consumption of groundwater is similar to that of bare soil evaporation. Water that is used by the plants is replaced by surface water infiltration. Hence, that volume of infiltrated water is no longer available in streamflow or for recharge to the aquifer system.
| Riparian | 33,105 |
| Perennial Stream Evaporation | 265 |
| Barren Soil Evaporation | 1,314 |
| Total | 34,684 |
Phreatophytes comprise most of the riparian vegetation and depend largely on the floodplain alluvium for their water supply. Their roots either extend to the water table or saturated capillary fringe just above the water table (Brown and Lowe, 1978). Two distinct biotic communities occur in the Upper San Pedro Basin: Desert Grassland (from the International Border to Hereford) and Chihuahuan Desert scrub (north of Hereford to north of Benson) (Putman, et al, 1988).
Consumptive use is determined by riparian acreage and densities estimated from aerial photographs, field investigations, and published figures. Water consumption by riparian flora is generally high. Riparian habitats are divided into "dense" (75% areal density) and "light" (about 25% areal density) for purposes of estimating consumptive water use by riparian vegetation (Putman, et al, 1980). Table 6 outlines the consumptive water use by phreatophytes in the Upper San Pedro Valley for three periods where aerial photographs were available. For the upstream reach (International Boundary to Tombstone), the total riparian acreage increased from 1955-1977, but then decreased in 1983/85 to below the 1955 level. Figures for 1977 may be less accurate due to the scale of the aerial photographs (1:62,500 in 1977 versus 1:20,000 in other years). Natural events (floods, etc.) can only partially account for reductions in riparian acreage from 1977-1985 (Putman, et al, 1988).
For the downstream reach, the total riparian habitat is declining due to clearing of mesquite bosques (stands of mesquite trees) for agriculture fields and urbanization. The increase in dense phreatophyte coverage over light coverage is probably attributable to an increase in the percentage of vegetation made up by mesquite and tamarisk (Putman, et al, 1988). Hastings and Turner (1965) reported a dramatic increase in mesquite populations along the floodplains and an invasion of tamarisk (introduced over 100 years ago) (Hastings and Turner, 1965).
Summary of Phreatophyte Water Use by Year:
Total Volume Year
25,366 ac-ft 1955
30,830 ac-ft 1977
17,552 ac-ft 1983/85
The Sierra Vista subwatershed contains approximately 6,490 acres of land irrigated almost exclusively from groundwater wells. About 4,610 of these acres (71%) are cultivated as pasture, and the remaining lands are either inactive croplands or crops which include grapes, pecan, and fruit tree orchards as well as vineyards. The Benson subwatershed has about 6,200 acres of irrigated lands. About 3,188 of these acres (55%) are cultivated as pasture. The next biggest crop is alfalfa, covering 1,294 acres (23% of total) (ADWR, 1990).
The Mexican part of the Upper San Pedro Basin contains about 5,100 acres developed for irrigation, but only about 3,460 acres are irrigated on average per year (Int'l. Bound. and Water Comm., 1981 to 1989). Most irrigation is supplied solely by surface water, which limits irrigation water use to about 1.45 ac-ft/ac or 5,000 ac-ft/yr total. Consumptive water uses in Cienega Creek non-tributary groundwater basin and in the San Rafael Basin is very minor.
In the late 1800's, the majority of the population in the Upper San Pedro Basin was scattered along the San Pedro River. Groundwater development consisted of the use of artesian supplies, springs and shallow wells (Putman, et al, 1988). Groundwater was obtained from shallow, dug wells in Tombstone area (near mining camps in Walnut Gulch) starting in 1881. Spring water was also piped from Miller Canyon in the Huachuca Mountains to supply growth in Tombstone (Bryan, et al, 1934).
In 1892, the first artesian well was drilled in the north end of basin. By 1903, over 200 wells had been established between the St. David and Benson areas. These wells consisted mostly of low flow artesian wells for domestic and stock uses. Most irrigation water was diverted from the San Pedro River by the St. David Ditch. Artesian wells were also developed for irrigation in the Palominas area, but low discharges precluded extensive agriculture development (Bryan, et al, 1934).
By 1900, artesian water in the Benson area was also being used for locomotive supply by Southern Pacific Railroad and for mine dewatering near Bisbee (Bryan, et al, 1934). Farmland irrigation by groundwater near Benson, Fairbank, Lewis Springs, and Hereford expanded rapidly after 1930. Most wells were shallow, dug, artesian wells equipped with windmills. Springs in Huachuca and Garden Canyons in the Huachuca Mountains provided the main water supply for Fort Huachuca during 1930's (Putman, et al, 1988).
Large scale groundwater use began in WWII, when Fort Huachuca increased its military activity. By 1942, six deep wells were drilled to augment the Fort's water supply. Groundwater pumpage at the Fort for the years between 1942 and 1944 is estimated at 5,440 ac-ft/yr. After WWII, water use decreased greatly until the mid-1950's when the Fort was reactivated and Sierra Vista began to grow (Harshbarger and Assoc., 1974).
Most currently operating wells were drilled in past 40 to 50 years. Domestic well use increased rapidly from 1950's, when high volume pumps became widely available. Most urban and industrial wells tap the regional aquifer. Most irrigation wells pump from the floodplain aquifer near the San Pedro River, although some do pump from the regional aquifer. Agriculture constitutes the major water use at about 70% of all cultural depletions. Municipal and domestic uses account for most of the remaining water withdrawals (22%). Both Sierra Vista and Huachuca City currently depend entirely on groundwater (ADWR, 1990).
Water use in the Upper San Pedro Basin is concentrated in two zones and serves two primary purposes: 1) the Sierra Vista-Fort Huachuca area well field supplies water for municipal, military, and industrial uses. These wells in the regional aquifer are up 1500 feet deep; 2) the Palominas- Hereford area well-field supplies water for agriculture. Some of these wells penetrate the regional aquifer, but most are shallow and only penetrate the floodplain aquifer. Small, isolated wells outside these two areas pump just enough water for domestic and livestock use (Vionnet and Maddock, 1992).
Figure 4 illustrates the trend in total pumping for the entire United States portion of the Upper San Pedro Basin. In 1952, an estimated total pumpage of 21,000 ac-ft was reported, but the actual year the pumping took place is unknown (Heindl, 1952). Estimates of stock and domestic well pumpage are on the order of 300 ac-ft/yr in Sierra Vista subwatershed (Heindl, 1952). Table 7 reports consumptive use values for 1989-90 development conditions in the two subwatersheds in the Upper San Pedro Basin.
| Cultural Depletions | ||
| Irrigation | ||
| Domestic | ||
| Municipal | ||
| Stockponds | ||
| Reservoirs | ||
| Mining | ||
| Industrial | ||
| Total Cultural | ||
| Natural Depletions | ||
| Channel Evaporation. | ||
| Phreatophytes | ||
| Total Natural | ||
| Total Use |
homepage