IMPLICATIONS FOR REGIONAL HYDROGEOLOGY OF AN ARIZONA RURAL WATERSHED BASED ON SPRING DISCHARGE VARIABILITY AND RECHARGE RESPONSE

The many springs discharging in the 7,000km2 Middle Verde River Watershed of Central Arizona reflect groundwater conditions and surface-water contributions to the watershed. An inventory of over 160 springs in this region was conducted in 2002 to better understand the physical, chemical, and ecological characteristics of these springs in order to develop a comprehensive physical, biological, and cultural classification system for springs. From this inventory, 16 springs located in different geologic formations were selected for a pilot monitoring study to understand seasonal spring discharge fluctuations and their relationship with regional hydrogeology. Out of these springs, 3 springs were located in Precambrian Granitic rocks, 2 in regional Mississippian/Devonian Limestone aquifers, 2 in regional Permian Sandstone aquifers, 2 in a shallow Permian Karst Limestone aquifer, 4 in Tertiary Basalt flows, and 3 in Quaternary deposits and alluvium.

Monitoring consisted of regular site visits with hand measurements of discharge in order to determine diurnal, seasonal, and climatic variations in spring discharge for springs in different aquifers or rock types. The main recharge events for the aquifers in this region are from winter and spring snowmelt. Spring hydrographs show that the springs located in the shallow Permian Karst Limestone aquifer are the most variable. Discharge from one of these springs was 0 gpm in January, before snowmelt occurred, 600 gpm in March during peak snowmelt, and 46 gpm in May after recharge from snowmelt ceased. Of the remaining springs, roughly half have shown small but significant increases in discharge during peak snowmelt or shortly after, while half had little or no change in response to recharge events.

Variability in spring discharge was used to estimate total annual discharge and characterize trends in baseflow. At some springs, discharge measurements were used to study regional hydrogeologic processes, and determine hydraulic properties of aquifers. The seasonal discharge fluctuations along with stable isotope analyses before and after these recharge events were indicators of the response these aquifers have to recharge and flow through the aquifer. Information from this study will be useful for future management of the water resources in the region.