PHYSICAL AND GEOCHEMICAL SIGNATURES OF SPRINGS RESPONSE TO RECENT CLIMATE CHANGE IN A SEMI-ARID REGION

Environmental responses to climate change have been observed in a variety of sources from tree rings to species distributions to stream flow. Likewise, monitoring of spring response can assist in understanding how climate change has affected the hydrologic system in the past and also provide insight into how water supplies may react to future stresses including prolonged drought conditions. Physical and geochemical data were collected from 2002 to 2006 from a suite of 16 springs in the Middle Verde River watershed of central Arizona. Spring locations represent different geologic units, regional and local aquifer systems, and elevations within the watershed. While central Arizona has experienced drought conditions for over a decade, recent fluctuations in climate have afforded the opportunity to observe spring response to a wide range of climatic conditions. One of the most severe drought years in the last century (2002) was followed two years later by an anomalously wet winter (2004-2005), and 2006 likely will be another historically dry year. Many springs which were dry during a spring survey in 2002 had reactivated after the wet winter of 2004-2005, but have dried since then. Springs monitoring during these climatic fluctuations and correlation of spring responses to climate data can provide valuable insight into the behavior of the aquifers supplying the springs. Analysis of discharge rate trends and geochemical constituent and isotope (³⁴S, ³H, δ ¹⁸O/ δ ²H) concentrations and ratios in spring water allow for determinations of aquifer size and volume, recharge area, hydraulic properties, residence time within the aquifer, and an aquifer system's sensitivity to drought conditions. Initial δ ¹⁸O/ δ ²H data show that springs sourced from regional aquifers or high elevations are more depleted (-11/-80 average δ ¹⁸O/ δ ²H ratio) indicating a predominantly winter/high elevation recharge event. Low elevation and alluvium-sourced springs are much less depleted and have evaporative signatures and evidence of a more humid water vapor source, most likely from summer monsoon events. These and other data were used in conjunction with preexisting hydrologic measures (stream flow, reservoir levels, well levels) of climate change to better understand the response of the watershed as a whole.