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Saguaro National Park, Arizona Water Resources Scoping Report Technical Report NPS/NRWRD/NRTR-97/95 By David N. Mott
The City of Tucson Water Department (Tucson Water) is required by state law to balance withdrawal and recharge by the year 2025, but at present ground water mining continues in both the Tucson Basin and Avra Valley. The majority of the Tucson Basin has experienced ground water declines, and water levels in Avra Valley have also declined. The drawdowns that result from this pumping may have an effect on some of the riparian habitats near park boundaries. Historic accounts and research indicate that the hydrology of the Tucson area has changed significantly in the past 100 years. In the mid-1850s, the entire valley was a forest of mesquite trees, with cottonwoods, willows and walnuts along the major streams. Much of the area was marshy, and malaria was a major problem for the original Fort Lowell along the Santa Cruz River. The introduction of cattle, the cutting of trees for fuel and building materials, and possibly a drought in the late 1800s led to changes in the hydrology such as down cutting of stream channels and dewatering of streambeds. The beavers, fishes and marshes disappeared; deep arroyos only occasionally filled with water replaced the marshes. Some of these changes still occur, and must be considered when evaluating the significance of water resources and riparian habitats within Saguaro National Park. Water is a critical resource at Saguaro National Park, essential to the area's biologic communities in the midst of a harsh, desert environment. Springs, seeps, perennial stream reaches, and riparian areas sustain unique habitats important in maintaining the desert ecosystems the park was established to protect. Past land use activities, ongoing ground water withdrawals, and continued growth in the Tucson area have created major changes in hydrologic processes and riparian landscapes within and adjacent to the park. Basic data describing and quantifying the park's water resources have never been collected; therefore, changes in plant and animal communities can not be correlated with alterations of fundamental hydrologic factors responsible for the existence of these ground and surface water influenced areas. Between 1947 and 1985, the majority of the Tucson area experienced ground water declines ranging from 40 to 80 feet (Bureau of Reclamation, 1986). From 1940 to 1985, water levels declined by more than 100 feet throughout most of the Avra Valley (Cuff and Anderson, 1987) resulting in increased pumping costs. As the aquifers are dewatered, the basin's land surface subsides and surface fissures occur at the basin margins, causing damage to agricultural facilities, roads, utilities and structures. Drawdown also results in decreased transmissivity as permeable sediments are dewatered, and increased vertical affective stress causes compaction of the aquifer system ( Anderson, 1988). Water is a critical resource at Saguaro National Park. It provides an essential element of the area's biologic communities in the midst of a harsh, desert environment. Springs, seeps, perennial stream reaches, and riparian areas sustain unique habitats important in maintaining the desert ecosystems that the park was established to protect. Past land use activities, on-going ground water withdrawals, and continued growth in the Tucson Basin have created major changes in hydrologic processes and riparian landscapes within and adjacent to the park. The Tucson area depends almost entirely on ground water to supply its municipal, agricultural, and industrial water needs. The rate of ground water withdrawal far exceeds aquifer recharge, resulting in ground water mining, which has been taking place for many years. The impacts of ground water withdrawal to the Tucson Basin's perennial streams and associated riparian corridors can only be described as devastating. Perennial streams no longer exist in the basin, and riparian corridors have been diminished in both quantity and quality. The cities' growth and intensive development have expanded to the boundaries of both park units. 1. Loss of phreatophytic vegetation: Water level declines in riparian areas can change the nature and reduce the quality of the habitat provided by phreatophytic vegetation. With the continued loss of riparian habitat in the Tucson Basin, preservation of riparian habitat within the park becomes increasingly critical. Too little is known about the interactions between ongoing ground water withdrawals, future developments, and the physical hydrology and biological processes active within the park's riparian areas. A successful preservation scheme must include some means of quantifying and offsetting potential water level declines if these declines threaten to impact riparian resources. Three riparian areas within RMD are situated in areas susceptible to water level declines. TMD, because of its smaller catchment basins, minor orographic rainfall, and differing geology, does not contain any areas mapped with riparian vegetation. The riparian areas of concern within RMD are shown in Figure 2, and include Rincon Creek near the NPS expansion area, lower Box Canyon, and the lower reaches of the drainages feeding Tanque Verde Wash. Riparian habitat is also present at higher elevations along mountain drainages, but these areas are not considered readily susceptible to water table declines. With the expansion of Tucson, and proposals for major developments near these three areas, new well fields could lower water tables resulting in the loss of phreatophytes and their associated value to the park's wildlife. The ground water reservoirs supporting the riparian areas within RMD are not directly connected to the Tucson Basin aquifer. As shown in Figure 5, the major aquifer of the Tucson Basin lies outside the boundaries of RMD. This is important because the drawdown within the Tucson Basin is, in most areas, already sufficient to place the water table beyond the depth where phreatophytic vegetation can access and utilize this resource. Osterkamp (1973b), reported depth to water in wells in the Tanque Verde Wash and Rincon Creek areas is often less than thirty feet, and this is supported by the riparian vegetation mapped in these areas. The thickness of the saturated alluvium in the three areas varies greatly, but the overall storage capacity is orders of magnitude less than the saturated sediments within the Tucson Basin, and well field development up-gradient from the Pantano Fault could have an immediate and pronounced effect on water table elevations. Burkham (1970) determined that within Tanque Verde Wash and Rincon Creek, the thickness of the saturated alluvium varies from 20 to over 100 feet thick. The depth to ground water below Tanque Verde Wash was five to 30 feet, and from 10 to 100 feet under Rincon Creek's channel. He noted that, in general, alluvial thicknesses and ground water depths are least near the mountains. Seepage measurements performed by Burkham (1970) revealed relatively low infiltration rates along Rincon Creek within the NPS expansion area, indicating small thicknesses of permeable alluvium and interferences with shallow ground water. In lower reaches of Rincon Creek, infiltration rates were extremely high as a result of the increased thickness of alluvial sediments and the spreading out of the flow over a broader channel and flood plain. David N. Mott National Park Service National Park Service |