Drought intensity is commonly characterized using meteorologicly-based
metrics that struggle to provide insight into water deficits within
deeper hydrologic systems. In contrast, Global Positioning System (GPS)
displacements are sensitive to both local and regional
hydrologic-storage fluctuations. While a few studies have leveraged this
sensitivity to produce geodetic drought indices, hydrologic drought
characterization using GPS is not commonly accounted for in drought
assessment and management. To motivate this application, we produce a
new geodetic drought index (GDI) and quantify its ability to
characterize hydrologic drought conditions in key surface and
sub-surface hydrologic reservoirs across California. In northern
California, the GDI exhibits a strong regional association with
reservoir storage at the 1-month time scale (correlation coefficient:
0.83) and groundwater levels at the 3-month time scale (correlation
coefficient: 0.87), along with moderate associations with stream
discharge at the daily (instantaneous) time scale (correlation
coefficient: 0.50). Groundwater in southern California is best
characterized with a 12-month GDI (correlation coefficient: 0.77), and
reservoir storage is optimized with the 3-month GDI (correlation
coefficient: 0.72). Differences between northern and southern California
reveal that the GDI is sensitive to unique aquifer and drainage basin
characteristics. In addition to capturing long-term hydrologic trends,
rapid changes in the GDI initiate during clusters of large atmospheric
river events that closely mirror fluctuations in traditional hydrologic
and meteorological observations. We show that GPS-based hydrologic
drought indices provide a significant opportunity to improve drought
assessment, in California and beyond, by improving our understanding of
the hydrologic cycle.