OLD GROUNDWATER SUSTAINS GROUNDWATER-DEPENDENT VEGETATION DURING THE 2011-2017 CALIFORNIA DROUGHT

Hydrologic response times to climatic and anthropogenic perturbations are an area of ongoing investigation. Springs provide a means for monitoring groundwater responses as they represent an integrated aquifer signal. In arid environments, perennial springs are important features that serve critical needs ecologically and domestically. Vibrant, green vegetation is a strong indicator of groundwater discharge by springs in desert landscapes. However, how does the vegetation surrounding these springs respond to significant perturbations such as extended drought? Is groundwater-dependent vegetation response indicative of hydrologic stability? We hypothesize that springs discharging old groundwater will be more hydrologically stable and thus show dampened vegetation dying/drying to extreme drought conditions. In comparison, springs discharging young groundwater will be less hydrologically stable and associated vegetation will rapidly respond to extreme drought conditions

The 2011-2017 California drought was the driest period since records began in 1895 and thus provided an opportunity to investigate spring riparian vegetation response to an exceptional drought and severely decreased recharge conditions. The hypothesis was tested by extracting time series of Landsat 7-derived NDVI from digitized polygons surrounding emergences of snowmelt-recharge driven springs in Owens Valley, CA. Time series metrics include the standard deviation of annual peak NDVI and the slope of the linear regression fitted to annual peak NDVI for the 2011-2016 period after onset of the drought and prior to drought recovery. Calculated metrics were then compared with spring water residence time indicators to assess which springs had more resilient vegetation during the drought. Results indicate that springs discharging a fraction of young (<60 y) water, as indicated by ³H and ³⁶Cl/Cl ratios, were more susceptible to the drought than springs discharging submodern or ancient groundwater. These results provide support for the hypothesis but also offer support for the highly debated conceptual model that old groundwater can buffer the immediate effects of climate change. This work also has implications for the utility of groundwater residence time data for better understanding ecosystem responses to extreme droughts.