Key natural influences on groundwater storage changes in central and southern Arizona

Groundwater resources in central and southern Arizona are increasingly threatened by prolonged drought and rising temperature, curtailed surface water supply, and pumping overdraft. Recent assessments indicate that groundwater storage in the region is rapidly declining. While human influences are well-understood, natural hydroclimatic controls on groundwater storage changes are examined in less detail. Using NASA's Gravity Recovery and Climate Experiment (GRACE) and its successor GRACE-Follow On (GRACE-FO) Groundwater Storage Anomalies (GWSA) and land-surface variables from Western Land Data Assimilation System (WLDAS), we quantified standardized trends, subbasin-scale correlations, and dominant modes via Principal Component Analysis (PCA) from 2004 to 2021 in central and southern Arizona. Using key components, we grouped groundwater subbasins using k-means clustering to simplify assessments. Results show substantial spatial heterogeneity arising from recharge-responsive northern and central subbasins linked to precipitation and subsurface runoff, and southern loss-dominated subbasins linked to weaker recharge coupling and more substantial influence from atmospheric demand. PCA analysis shows that natural modes variability explains about 16% of the spatial variance displayed in GRACE/FO GWSA in the study area, with approximately 72% of that variance explained by total evapotranspiration (≈ 29%), precipitation (≈ 23%), and subsurface runoff (≈ 20%). The remaining variance reflects a combination of anthropogenic influences (pumping, MAR), geologic heterogeneity, and residual model/measurement errors. Our diagnostic framework presents groundwater subbasin scale clusters driven by shared hydroclimatic modes of variability. It has potential as a transferable tool for focused recharge, groundwater sustainability plans, and post-2026 groundwater planning in the Lower Colorado River Basin.