The general goal of the research is to use bristlecone pine from the Rio Grande Headwaters region to develop a temperature reconstruction spanning 2,000 years, augmented with updated and new hydroclimate reconstructions, to evaluate long-term, regional temperature and hydroclimate relationships. Anomalously warm summer temperatures reduce runoff by increasing the rates of evapotranspiration and by decreasing soil moisture, which has a persistent effect into winter and the following summer. However, there is disagreement to the extent to which recent warming has influenced upper Rio Grande flows. This project will address changes in runoff efficiency (the proportion of precipitation that becomes runoff) over time using instrumental datasets, an updated Rio Grande streamflow reconstruction, and a local precipitation reconstruction to assess the role of temperature in modulating runoff efficiency during droughts and pluvials. The Rio Grande supplies surface water to over 2 million acres of agriculture and over 6 million people. Predominantly supporting agriculture and rural economies, the Rio Grande also serves urban centers including Albuquerque, El Paso, and Ciudad Ju�rez. Ongoing drought has led to severe water shortages, yet the Rio Grande has been understudied compared to the upper Colorado River basin, possibly due to the proportion of poorly-represented communities the river serves. This project moves toward placing paleoclimate information into practice by engaging with upper Rio Grande water stakeholders in order to understand their water supply and management concerns. The potential Broader Impacts include greater understanding of water resources in the southwestern United States, support for a post-doctoral researcher, and substantive interaction with water resource managers and stakeholders in the region. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.