Satellite-based assessment of hydrologic synchronization across Western U.S. basins: Implications for water security

Study Region This study examines the Western United States including five HUC2 watersheds (Upper Colorado, Lower Colorado, Great Basin, California, Pacific Northwest) and 44 HUC4 sub-basins. This region supports over 60 million people and extensive agriculture with water stress from climate change and multi-year droughts. Study Focus Hydrologic synchronization is quantified using satellite datasets (GLDAS runoff, PRISM precipitation, SMAP soil moisture) and analytical methods including cross-correlation, Principal Component Analysis, co-occurrence analysis, lagged correlation, and whiplash analysis to identify spatial patterns, synchronized extremes, and lead-lag relationships. New Hydrological Insights for the Region The California-Great Basin pair shows 65% runoff synchronization and 67% soil moisture synchronization, with six major basin pairs exceeding 50% synchronization, demonstrating that hydrologic variability is organized into co‑varying regional clusters rather than independent basins. Synchronized extremes occur every 3–4 years (~25% of months), suggesting that multi basin droughts are an intrinsic feature of western hydroclimate. PCA identifies four spatially coherent regions (coastal Pacific Northwest, arid Southwest, coastal California, northern Rockies) explaining 75% of variance. Lagged correlations show that Pacific Northwest anomalies precede those in the Lower Colorado River basin by about five months and that Upper Colorado anomalies lead California, Lower Colorado, and Great Basin by approximately one month. Hydrologic whiplash analysis identifies coastal Pacific Northwest and arid Southwest as hotspots of rapid wet–dry transitions (whiplash in 20% of years), distinguishing these as abrupt shifts and providing a new regional perspective on hydroclimatic volatility.