Recognizing Climate Signals in Groundwater

Climate signals recorded in groundwater are difficult to recognize in basins where there has been human impact to the resource. Signals are better recorded in undeveloped groundwater basins where long-term monitoring has occurred. In northeastern California there lies 22,000 km(2) Quaternary-aged volcanic aquifer system that is largely undeveloped. Based on isotope analyses, groundwater is recharged at volcanic centers of Mt. Shasta, Medicine Lake Volcano, and Lassen Peak, which act as focal points of high elevation snowmelt infiltration that flows 20-50 km in these aquifers before emerging as similar to 85 m(3)/s of spring discharge. Groundwater residence times were measure at <30 years. During extended droughts base flow shows 20-50% decrease in volumetric discharge. Furthermore, changes in aerial groundwater storage is captured by inter-annual differences in late summer base flow and records intra- and inter-decadal climate cycling. No net long-term change in storage is observed over the past 100 years.