Although groundwater is renewable through natural recharge, the resource can be temporarily depleted by overpumping. If groundwater pumping withdrawals exceed aquifer recharge, then groundwater is being "mined" or overdrafted from the basin. Groundwater overdraft conditions may be evident in groundwater declines that form a large "cone of depression" in the aquifer around a pumped well or in the area of an active well field. If pumping
continues to exceed aquifer recharge over time, the cone of depression will continue to expand outward from the well(s). The rate of water level (or hydraulic head) decline and growth of the cone of depression depend on the aquifer's hydraulic characteristics, the surrounding geohydrology, the rate and volume of water pumped, and the rate of recharge to the aquifer. A cone of depression will continue to grow until equilibrium is achieved and aquifer recharge equals aquifer discharge. In some cases, equilibrium may be achieved when the cone of depression intercepts a source of recharge, such as a surface body of water or a connected aquifer, or it intercepts recharging groundwater by altering the natural groundwater flow direction in the aquifer system. Under these circumstances, the cone of depression is said to "capture" water from the new source.
Pre-1940 groundwater use in the Upper San Pedro Basin was not enough to produce generalized water table declines (Freethey, 1982). Withdrawals from 1940 to 1966 reduced water levels by 0.3 to 0.5 ft/yr over much of the regional aquifer (Putman, et al, 1988). One well in the Huachuca well field showed a local water table decline of 2.9 ft/yr due to heavy pumping from Fort Huachuca and Sierra Vista (Putman, et al, 1988).
One major area of obvious long-term overdraft exists in the Upper San Pedro Basin. In 1974, Harshbarger and Associates, a Tucson consulting firm, reported the existence of a cone of depression approximately 4 miles long and 1.5 miles wide in the Sierra Vista-Fort Huachuca area, with long axis parallel to Huachuca Mountains (NW-SE direction) (Harshbarger and Assoc., 1974). Roeske and Werrell (1973) reported that, in 1968, this cone of depression covered an area of about 5 square miles and was centered around the military well field, extending approximately 3.5 miles in a NW-SE trend. By 1978, the cone's axis had shifted about 90 degrees to a NE-SW trend and was centered about 2 miles east of the 1968 depression, but remained about the same size. The cone's extent in the area north and east of Sierra Vista is ill defined because it falls within the firing range on Fort Huachuca where few wells are accessible (Putman, et al, 1988).
By 1986, the same cone of depression had extended one mile in width and its center had shifted farther eastward. Putman, et al (1988) reported the cone's size as 7.5 square miles in 1986. The water levels in two wells located at the 1986 center of the cone of depression showed a net decline of 50.2 and 48.2 feet, respectively, during the period 1974 to 1986 (Putman, et al, 1980). Other wells within the cone of depression showed smaller declines over the same period. Net decline rates over an area of 25 square miles centered at Sierra Vista ranged from 0.4 to 3.9 ft/yr (averaging 1.4 ft/yr) for the period 1968 to 1986 (Putman, et al, 1988).
Harshbarger and Associates (1974) also reported the existence of a second smaller (3- mile long) cone of depression near Huachuca City along the Babocomari River. It parallels the river in a SW-NE direction and recent field data substantiate its existence (Schwartzman, 1990).
Rates of water level declines in the regional aquifer outside population centers for the period 1968 to 1986 were about same as pre-1966 rates (0.3 to 0.5 ft/yr) except in Sierra Vista area where it increased to an average of 1.4 ft/yr (Putman, et al, 1988). Palominas-Hereford area wells showed declines of 8 to 15 feet for the same period (0.4 to 0.8 ft/yr). These wells are very susceptible to seasonal fluctuations in streamflows, but their proximity to the river prevents water levels from declining more than they have (Vionnet and Maddock, 1992). Brown (1966) attributes some of the groundwater declines to the water table adjusting to the lower level of the
riverbed resulting from downcutting of the San Pedro River in the late 1800's. Except in Sierra Vista, pumping alone could not account for such a wide spread water table decline (Putman, et al, 1988).
Portions of the regional aquifer near the basin margins where little groundwater is used or where recharge is sufficient to offset pumping have shown little water table decline over the last 30 years (1). The water table in the vicinity of the San Pedro River has also changed little over the long term. This stability in the water table near the stream is most likely attributable to groundwater replacement by streamflow infiltration, but evidence indicates that infiltration losses in that area are increasing due to increasing demands on groundwater in the Sierra Vista-Fort Huachuca Area (see Vionnet/Maddock model below).
Some wells in historically artesian areas of the aquifer (where the hydraulic head in the confined regional aquifer exceeds that in the floodplain aquifer) show some decline in artesian head. These head decreases may be a natural result of letting wells run freely over period of many years (Putman, et al, 1988), or they may reflect a more regional depletion in the regional aquifer. Ground-water flow directions remain essentially unchanged in the Upper San Pedro Basin except in the Sierra Vista-Fort Huachuca area where flow is toward the cone of depression (Putman, et al, 1988).
Most groundwater level declines have occurred in upper basin fill, where alluvium is most permeable and the greatest amount of water is stored (Harshbarger and Assoc., 1974). Continued withdrawals may increase the rate of water level decline as the upper basin fill dewaters. Future withdrawals from the lower basin fill may have a more significant impact as a result of lowered specific yield. As pumping continues, the aquifer's specific yield may decline meaning that more aquifer volume is dewatered for the same volume of water withdrawn (Putman, et al, 1988).
The town of Naco (population 5,000) uses about 300 ac-ft/yr, and Cananea (population 30,000) uses about 2,000 ac-ft/yr from wells in Upper San Pedro Basin. About 300 ac-ft/yr are used by the rural population. Stockpond consumptive use is estimated at about 1,000 ac-ft/yr. Consumptive water use by the Cananea copper mine and mill totals about 3,100 ac-ft/yr with about 11,700 ac-ft/yr coming from wells in the Upper San Pedro Basin (ADWR, 1990).
Shallow wells in the inner valley (see Figure 3) draw water from floodplain aquifer, causing groundwater drawdown in the floodplain aquifer. Decreasing water levels around a well form a roughly circular cone of depression, deepest at the well and lessening with distance from the well. As pumping time increases, the volume and radius of the cone increase and its edge may reach the streambed. If streamflow is present, the well will draw water directly from the stream. If no surface water is present in the vicinity of the cone of depression, the cone continues to increase in size in the floodplain aquifer. If the cone reaches the interface between the floodplain aquifer and the regional aquifer, the well will begin drawing some water from the regional aquifer. Because it is much less transmissive and has a lower storativity value than the flood plain aquifer, the regional aquifer usually provides only a small proportion of water to these shallow floodplain aquifer wells (Putman, et al, 1988).
Deeper wells in the inner valley that penetrate the regional aquifer, or both the regional aquifer and floodplain aquifer, draw water from the regional aquifer system and may draw from the floodplain aquifer as well. Wells screened over an interval spanning parts of both aquifers promote leakage from the confined regional aquifer into the lower pressure floodplain aquifer (Putman, et al, 1988).
Wells outside the inner valley that penetrate the basin fill draw water from the regional aquifer. Their cones of depression may intersect the floodplain aquifer and draw water from that source or promote downward leakage from the floodplain aquifer to the regional aquifer as a result of changing pressure gradients due to pumping. Even wells pumping from the regional aquifer outside the inner valley may eventually impact the surface water in the San Pedro River over long time period as cones of depression continue to grow (Putman, et al, 1988).
In many areas along the San Pedro River, phreatophytes have been replaced by irrigated cropland. Although evapotranspiration rates may have changed as a result, Harshbarger and Associates (1974) reported no significant changes in the hydrologic regime by 1972.
While streamflow depletions do occur due to pumping wells near the San Pedro River, water levels in the floodplain aquifer along the river have not yet reflected these depletions. Freethey (1982) found that groundwater elevations along the San Pedro River have remained relatively constant from 1968 to 1978, but his model predicted that 29% of total pumpage derived from streamwater depletion. Putman, et al (1988) also found that heads in the regional aquifer near the river showed little impact by pumping in the Sierra Vista as of 1988, but they did not specify what percentage of total pumpage derived from river depletions. Vionnet and Maddock (1992) used a more sophisticated computer model to calculate stream losses resulting from pumpage. Their model found that 40% of the water pumped came from streamflow.
Groundwater withdrawals in the Huachuca City area may have had a significant impact on groundwater flow along the Babocomari River, where the second cone of depression exists.
The Pomerene Canal and the St. David Ditch, both used for irrigation in the St. David-Benson area, are the only two permanent direct surface water diversions in the Upper San Pedro Basin. The St. David Ditch was developed in 1881 and currently irrigates about 1050 acres of land, mostly pasture (79%) and alfalfa (7%) (ADWR, 1990). The Ditch carries water from the San Pedro River diversion dam 7 miles south of St. David. A major ditch realignment in 1916 added three miles of ditch and brought 550 more acres into production (ADWR, 1990). The Ditch is now 8 miles long and has an estimated maximum capacity of 25.0 cfs. The Ditch's average annual discharge for the period 1968 to 1972 was 4,600 ac-ft/yr. In beginning and end of irrigation season, the entire flow of San Pedro River is often diverted (ADWR, 1987). Water from two wells operated by the St. David Irrigation District is often pumped into the ditch to supplement surface water during low flow periods. Of 1,100 acres irrigated by the St. David Ditch, only 278 acres are irrigated solely by surface water (Putman, et al, 1988).
The Pomerene Canal, roughly 7.5 miles downstream of the St. David Ditch (between St. David and Benson), was completed in 1912 and later destroyed in 1927 and 1931. The Canal was eventually reconstructed in 1934 0.75 miles upstream from the original dam. The Canal is currently seven miles long and partially lined. For the period 1968 to 1972, the Canal's average annual discharge was 1,400 ac-ft/yr (Putman, et al, 1988). The Canal now irrigates about 1050 acres, most of which are used for growing pasture (67%) and small grains (11%) (ADWR, 1990). Other direct diversions made in past have been washed out or silted in by frequent floods. Continued downcutting of river made construction of diversions works increasingly difficult, leading to the eventual use of wells in inner valley (Putman, et al, 1988). Some minor surface water diversions in the Upper San Pedro Basin carry water from natural springs in mountain areas.
Major changes in environmental conditions in the Upper San Pedro Basin over the last century are credited in part to Spanish, Mexican, and American settlements. As with other major rivers in southern Arizona, the flow of the San Pedro River has been significantly altered by channel entrenchment or downcutting of streams caused by erosion (Hastings and Turner, 1965). This downcutting probably resulted from the combined effects of the commencement of large scale grazing in southeastern Arizona in the 1880's, followed by flooding in the mid-1880's to 1890 which accelerated erosion and channel cutting. Another hypothesis states that change to a drier climate was the primary cause of channel entrenchment, with grazing being a secondary factor. Although the direct cause of the entrenchment is uncertain, a reasonable explanation is that extreme erosion resulted from increased discharge and runoff produced by a combination of changes in climate (aridification) and vegetation (Hastings and Turner, 1965).
Hastings and Turner (1965) describe the San Pedro and Santa Cruz Rivers as changing from streams that "wound sluggishly...through grass-choked valleys dotted with cienegas and pools to steep-banked streams with highly irregular regimes... the beds much of the time..dry, sandy wastes that support little, if any, vegetation. During flash floods, the channel is filled bank to bank with a raging, muddy torrent that carves new incisions into the floodplain" (Hastings and Turner, 1965). The same authors also report accounts as early as 1858 of part of the river going dry at times.
Upstream of the Charleston streamgage, the San Pedro River remains relatively unincised and maintains a dense corridor of riparian vegetation (Putman, et al, 1988).
Naturally elevated levels of fluoride in the St. David-Benson area were reported as early as 1934 by Bryan, et al (1934). Groundwater in the regional aquifer basin fill is predominately of a calcium-bicarbonate type with total dissolved solids (TDS) in the range of 200 to 400 mg/l (Thompson, et al, 1984; Konieczki, 1980). The U.S. Environmental Protection Agency (EPA) rates a TDS of 500 mg/l as the Federal Drinking Water Standard limit (U.S. EPA, 1979). Along the San Pedro River from St. David to the Narrows, the groundwater is primarily a sodium-bicarbonate or sodium-sulfate type with TDS greater than 1000 mg/l in many cases. The same chemistry occurs in an area three miles long south of Palominas and in an area six miles long between Hereford and Lewis Springs. This chemistry is roughly equivalent to that of the confined areas of the regional aquifer underlying the San Pedro River and agricultural areas along the river (Putman, et al, 1988). The floodplain aquifer generally exhibits higher TDS than the regional aquifer, and floodplain aquifer water quality is generally similar to that of the surface water in the San Pedro River (Roeske and Werrell, 1973).
Groundwater throughout much of the Upper San Pedro Basin shows a trend of calcium-bicarbonate type shallow groundwater changing to sodium-bicarbonate type in deeper groundwater (Bryan, et al, 1934). Heindl (1952) noted that shallow artesian waters are higher in TDS than artesian water from wells more than 600 feet deep, and that the shallow waters were usually of a calcium-sulfate or sodium-sulfate type. Roeske and Werrell (1973) and Thompson, et al (1984) also noted high sulfate contents in shallow waters of the St. David-Pomerene area.
Nitrate contamination exceeding state safe drinking water standards has been found in the groundwater and surface water near St. David (Reents, 1985). Possible sources for this contamination include an explosives and chemical manufacturing firm as well as sewage effluent (Self, 1987). The site was nominated to the Federal Superfund's National Priority List for toxic waste cleanup in 1986 (Putman, et al, 1988). Cyanide contamination in the Tombstone area has resulted from Contention Mine's use of cyanide leaching of silver ore. The contamination most likely resulted from seepage from two holding ponds containing cyanide-rich solutions (Kennet, 1985).
San Pedro River water is predominately calcium-bicarbonate type with a median TDS of 250 mg/l at the International Border to about 400 mg/l at the Narrows. No major differences in water quality occur along river (DeCook, et al, 1977).
Surface water quality problems in the Upper San Pedro Basin include nitrate contamination in the St. David area, bacterial contamination from grazing, and acid mine drainage from Cananea, Mexico. Bacterial contamination in the form of high coliform counts, measured immediately after precipitation events, probably results from runoff from grazed ranchlands (DeCook, et al, 1977; SouthEastern Ariz. Gov'ts. Org., 1981). Periodic heavy metal contamination of the San Pedro River occurs through wastewater spills from Cananea Copper Mine in Sonora, Mexico. The largest spills occurred in two consecutive winters from December 1977 to March 1979 (Eberhardt, 1981). A more recent episode occurred in 1986 (Self, 1987).
Past threats to surface water included acid mine drainage to Greenbush Draw (U.S. EPA, 1985), cyanide leaching solution spilled into Walnut Gulch (Kennet, 1985), and raw sewage releases to Greenbush Draw (Self, 1987). General sewage effluent contamination had also occurred in several urban areas (DeCook, et al, 1977). All of these problems have reportedly been corrected (Self, 1987).
The Arizona Water Commission (AWC) completed the first reported modeling effort for the Upper San Pedro Basin in 1974. The model was part of a project undertaken to analyze the water conditions in the Fort Huachuca area. The model area extended from the International Boundary with Mexico to Saint David, Arizona. The model geometry consisted of one layer with a regular grid of 1-square-mile cells. The conceptual model characterized the aquifer system as being unconfined and recharged by mountain front recharge. The AWC treated the San Pedro River as a constant head boundary.
Under the employ of the USGS, Freethey developed conceptual and numerical groundwater models of the Upper San Pedro Basin. His study aimed to evaluate the existing definition of the hydrologic system and the "relative sensitivity of the model to changes in major effects." Freethey's model area extended from International Boundary to Fairbank, Arizona about 27 miles north of the Border, and set the foundation for all subsequent modeling efforts of this area. Freethey used Trescott's 1975 numerical model, predecessor to the USGS model MODFLOW, with the original river package (constant stream leakance). His model included two aquifer layers: one confined and below 1000 feet deep where no data were available, and the other above that, where data were available, and unconfined. He introduced an irregular grid, with cells ranging from 0.6 to 1 mile on a side. Freethey subtracted 30% of pumping for irrigation from his input values to account for irrigation return flow. All pumping was divided into 10 periods. Freethey reported problems simulating steep mountain-front gradients. He found that the model required unreasonably low hydraulic conductivity and transmissivity values to match the steep hydraulic gradients observed along the mountain fronts. Considering this problem, Freethey acknowledged that these values would affect transient model results when the effects of pumping reached the model boundaries.
Although the transient simulation did not include streamflow diversions, the simulation for the period 1940 to 1977 showed increased recharge across the International Boundary and decreased discharge to the San Pedro River throughout its length as a result of pumping. Freethey's model showed that in losing reaches of the San Pedro, losses increased by more than 80%. In gaining reaches, discharge to the stream decreased by about 30%. Loss of groundwater to evapotranspiration dropped about 20% over the period due to small water table declines in the vicinity of river resulting from pumping. The model calculated that 5,600 ac-ft (53%) of the total 10,500 ac-ft pumped from system in 1977 were withdrawn from aquifer storage. The remaining 47% came from evapotranspiration (15%), streamflow (29%), or changes in underflow (3%).
Freethey derived the riverbed leakance value used in the steady-state calibration from the assumption that the ratio of vertical to horizontal conductivity in the riverbed is 1:100. His subsequent sensitivity analysis suggested that leakance could be higher (ratio of vertical to horizontal conductivities up to ten times larger) but not lower. Head changes were fairly insensitive to changes in riverbed leakance values. The model water budget was fairly insensitive to upward changes in riverbed leakage, but it was highly sensitive to decreases in leakage of more than a factor of ten. Simulated head levels were found to be insensitive to changes in evapotranspiration discharge, but the model water budget was very sensitive to these changes (i.e., decreased evapotranspiration produces increased discharge to the stream and increased underflow). Because the model water budget and heads were insensitive to changes in vertical conductance between aquifer layers, Freethey suggests that the system could be modeled as two-dimensional.
Putman, et al conducted a study to examine the hydrology and water use of the Upper San Pedro Basin, and to provide information for a decision on whether the ADWR should designate Upper San Pedro Basin as an Active Management Area under the 1980 Groundwater Management Act (Ariz. Revised Statutes '45-412). The 1980 Act allows the director of the ADWR to form an Active Management Area if any of the following conditions exists: 1) active management practices are necessary to preserve the existing supply of groundwater for future needs, 2) land subsidence or fissuring is endangering property or potential groundwater storage capacity, or 3) use of groundwater is resulting in actual or threatened water quality degradation. No known land subsidence problems exist in the Upper San Pedro Basin (Strange, 1984), and no known water quality problems of regional significance have resulted from the use of groundwater.
Putman, et al, developed a water budget for the period 1968 to 1976 for the inner valley (corresponding to the width of the floodplain alluvium) using only measured values from USGS streamgages at Palominas and the Narrows. Their model assumes no long term change in storage has occurred (equilibrium condition supported by inner valley well hydrographs and groundwater contours), and does not include the Sierra Vista-Fort Huachuca area. The results of their water budget model are listed in Table 8.
The ADWR 1988 model by Putman, et al updated Freethey's 1982 effort, maintaining the same areal extent. This area is expected to be impacted the most by pumping in the Sierra Vista-Fort Huachuca area. Putman, et al used the same data as Freethey except they added pumpage for periods after 1978 and estimated future irrigation acreage. The model was updated to 1985 with an average domestic pumping figure for the period 1978 to 1985 with data from domestic water suppliers to the Sierra Vista-Fort Huachuca area. An average irrigation pumpage figure for 1978-1985 was also used.
The ADWR model simulated 1985 water levels at the 4150-foot elevation without showing a cone of depression around the Sierra Vista-Fort Huachuca area. Although the cone's existence was projected by interpretation of 1986 data, Putman, et al (1988) stated that, "this contour is poorly controlled by field data and the alternate interpretation of this contour line simulated by the model is entirely feasible." In spite of the model's prediction, the "ADWR followed the practice of previous reports on this area in showing a cone of depression in the Sierra Vista-Fort Huachuca area" in their groundwater level contour map. Simulated water levels decreased a maximum of about 20 feet from 1977 to 1985 around Sierra Vista and a maximum of about 18 feet near Palominas.
Pumping estimates for the future were based on population projections and consumptive use figures based on the 1980 Census (228 gal/day/ person). Putman, et al projected a pumpage of 14,370 ac-ft for the year 2000 in the model area. Outside the model area, the projected pumpage is 4,220 ac-ft in the year 2000 based on the projected population. The projected pumpage in the Sierra Vista subbasin also accounted recent BLM land exchange (establishment of the SPRNCA in 1988) which included the San Rafael Del Valle and San Juan de Las Boquillas Y Nogales Land Grants along the San Pedro River and additional farmland between Palominas and the International Border. Part of the purpose for the land exchange included the eventual retirement of irrigated acreage on the BLM land, and therefore, the cessation of pumping on these lands. Projected agricultural pumpage was set at 3,500 ac-ft/yr for the model.
| Inflows | |
| San Pedro River at Palominas | |
| Underflow | |
| Tributary Inflow | |
| Direct Precipitation | |
| Groundwater Discharge | |
| Total Inflow |
|
|
| San Pedro River at the Narrows | |
| Underflow | |
| Streambed Evaporation. | |
| Evapotranspiration by Phreatophytes | |
| Agricultural Consumptive Use | |
| Total Outflow |
Putman, et al concluded that groundwater declines in Sierra Vista area are due to increased municipal and industrial pumping. Their study found that retirement of agricultural land will produce groundwater level increases in Palominas area. Their model simulated groundwater level declines of up to 0.7 ft/yr for the period 1977 to 1985 over much of the Sierra Vista subbasin. Assuming no pumping on BLM land exchange lands, the simulated declines for 1985-2000 average 1 to 2 ft/yr throughout the subbasin. Pumpage in the Sierra Vista-Fort Huachuca area is projected to double from 1986 to 2000, with a maximum annual rate of groundwater level decline projected at 6 ft/yr by the year 2000. This increased pumping would produce an additional water level decline of up to 80 feet in this area by the year 2000. Putman, et al predict that the cone of depression in the Sierra Vista-Fort Huachuca area will continue to develop, but will not affect water levels near Charleston on the San Pedro River by the year 2000. The model simulated a decline of 4 feet about 1 mile west of the river, but none closer to the river. This finding, as well as simulated increased leakage from the river suggest that river flow may be sufficient to offset effects near the river of pumpage in Sierra Vista-Fort Huachuca area, at least until the year 2000. The ADWR model projects increased water levels of 5 to 20 feet between Hereford and the International Border from 1985 to 2000 due to discontinued agricultural pumpage.
Outside the model area, urban and agriculture growth are expected to be small. Populations of Bisbee, Tombstone, and Benson are projected to grow by a total of 2,980 people by the year 2000, producing a projected increase in groundwater withdrawals by 760 ac-ft in the subbasin. Putman, et al expect the impact from this pumping on the hydrologic system to be minimal because of the geographic distributions of these communities.
Putman, et al also examined the effects of Sierra Vista area pumping on water levels outside the model boundaries. Assuming pumpage continues at its present rate and hydrologic conditions remain the same, these effects could show up as declines of 0.0 to 0.7 ft/yr. This means a projected decline of up to 10 feet by the year 2000 in the regional aquifer outside model boundaries.
Pumping in the Mexican portion of Upper San Pedro Basin is projected to increase from 28,000 ac-ft in 1985 to 40,000 ac-ft by the year 2000. Putman, et al found that the impact of this pumping on the Arizona portion of the basin could not be predicted with the current information.
Groundwater withdrawn from aquifer storage comprised 53% of total pumpage (within the model boundaries) in Freethey's model and at 59% in the updated model for 1985. This result shows increased withdrawal from storage with increased pumping. Most water pumped from storage is presumed to come from the upper basin fill, since most wells penetrate that unit.
The ADWR model showed underflow into Upper San Pedro Basin at International Border increasing from 3,400 ac-ft in 1977 to 3,700 ac-ft/yr in 1985. Simulated underflow exiting the model boundary near Fairbank remained at about 300 ac-ft/yr up to the year 2000. Underflow exiting the basin at the Narrows was assumed to continue at the same rate of 120 ac-ft/yr into the near future.
Simulated groundwater level rises in the Palominas area between 1985 and 2000 would bring groundwater levels near the elevation of the bed of the San Pedro River and suggest increased discharge from the groundwater to the river above 1985 conditions. The ADWR model showed this discharge to the river stabilizing at about 12% of total river flow between 1985 and 2000. Freethey's 1982 model simulated groundwater discharge to the San Pedro River at 13% of total river flow in 1977, and 19% in 1942. While Freethey's model showed no change in heads in cells representing the river at Lewis Springs and Charleston, simulated heads did decline west of the river. Although the model used by Freethey (1982) and Putman, et al (1988) cannot simulate changes in river stage due to increased infiltration to the groundwater system (or decreased discharge to the river system), the ADWR simulation indicates increased infiltration from the river to the groundwater system, which would likely "reduce low flows in the river to slightly lower levels" (Putman, et al, 1988, p. 139).
The Corps of Engineers used the USGS model (Freethey, 1982) to investigate potential impacts of various water use scenarios on future water levels in the Fort Huachuca area. Changes in evapotranspiration and recharge rates were introduced.
Rovey expanded the model area to cover the entire San Pedro Basin, covering 4,920 square miles, and spanning from the International Border with Mexico to Winkleman, Arizona (roughly 150 river miles). Rovey introduced a new stream-aquifer interaction model. Rovey's model uses seepage rates in calculating river discharges and thus, in calculating the impact of groundwater on surface flows (Rovey, 1975).
Vionnet and Maddock also updated the model originally developed by Freethey (1982). They applied a newer version of the same numerical modeling program, the finite-difference program MODFLOW, to the Upper San Pedro Basin area from Cananea, Sonora, Mexico to Fairbank, Arizona. Vionnet and Maddock modified Freethey's model by updating data sets, applying the Prudic (1989) stream-aquifer interaction module to the model, and adding another layer to provide for bank storage. The streamflow-routing (Prudic) package tracks streamflows that interact with the ground-water system. It permits portions of streams to go dry and then to resume flowing as well as to be diverted and then merged with other streams or tributaries. This hydrologic accounting system limits aquifer recharge through the streambed to streamflow availability. The Prudic package requires that streamflow at the first reach of each modeled stream segment be specified. Vionnet and Maddock used the mean annual baseflow at Palominas as input to the model, then used baseflows at the Charleston gaging station to calibrate the model streamflows. As in the Freethey model, no direct streamflow diversions were modeled. Steady-state values for boundary fluxes, mountain front recharge, and evapotranspiration rates were obtained largely from Freethey (1982) and ADWR (1990). Pumping periods ranged from 1 to 13 years.
The transient simulation period spans from 1940 to 1988. The simulated water levels and streamflows from the transient model calibration attempted to match observed field values. The match between simulated groundwater levels and field values was adequate, but the similarity in simulated baseflows and baseflows estimated from field data was less satisfactory. Vionnet and Maddock attributed this effect to the model's failure to account for the runoff component of streamflows. The authors found significant drops in simulated water levels in the Sierra Vista-Fort Huachuca area after 20 years of development (pumping) from 1968 to 1988. The model did not simulate the secondary cone of depression in the Huachuca City area documented by Schwartzman (1990).
Vionnet and Maddock evaluated the effects of pumping on the San Pedro River via a losing- versus gaining-stream reaches analysis. The authors found that the rate of flow between the stream and the underlying alluvial aquifer has changed over time as a consequence of increased pumping. The model also showed progressively higher losses of streamwater to the aquifer in reaches around the Palominas-Hereford area and downstream of the Charleston Bridge. Some reaches that were previously gaining later began to lose water to the aquifer. Vionnet and Maddock concede that this trend, which may be tied to simulated decreases in flow from the regional aquifer and the alluvial aquifer as a result of pumping, incorporates a good deal of uncertainty in model parameters.
The authors compensated for poor matches in streamflows by reducing the maximum evapotranspiration rate (a highly uncertain parameter) by less than one order of magnitude. Transient simulations indicate that evapotranspiration consumption (primarily by riparian vegetation) declined by about 20% by 1988 compared with predevelopment conditions. The amount of water captured from evapotranspiration averaged 10.17% of the total groundwater withdrawn per pumping period. Over the whole simulation period, aquifer storage supplied 48% of total groundwater withdrawals. The model calculated that approximately 40% of groundwater withdrawn was captured from streams. The net flux to the streams showed a declining trend since early development, with the minimum flux coinciding with the peak pumping period.
Table 9 compares the three models of the Upper San Pedro Basin which addressed stream-aquifer interactions from the International Boundary with Mexico to Fairbank, Arizona.
| method of calculating stream/aquifer interaction | |||
| time period modeled | |||
| % of total pumpage derived from aquifer storage | |||
| % total pumpage captured from reduced evapotranspiration losses | |||
| % total pumpage captured from streamflow | |||
| % total river flow derived from groundwater discharge in Palominas area |
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