Seasonal precipitation variability controls shallow soil water drought events across the southwestern United States

The semi-arid climate of the Southwestern United States (‘Southwest’) presents unique challenges for quantifying drought conditions due to annual potential evapotranspiration being significantly greater than annual precipitation. Southwestern vegetation is adapted to seasonal soil water recharge for primary productivity, with recharge delays potentially resulting in drought impacts. Understanding how delays in seasonal precipitation timing and magnitude create soil water anomalies is key for characterizing drought dynamics in Southwestern soils; however, the lack of long-term, reliable soil water datasets have restricted this effort to a local scale. Here, we couple sophisticated soil water modeling, site-specific soil information, and spatially continuous, high resolution meteorological data to create a soil water dataset for the purpose of characterizing shallow drought onset and cessation patterns in Southwestern soils. Daily matric potential at 10 cm and 30 cm was simulated from 1979 to 2020 at 240 locations throughout 4 Major Land Resource Areas (MLRA). Historical matric potential anomaly time series were percent ranked from 0 to 100 %, with consecutive days below the 15th percentile quantified as drought events. Drought events were categorized by duration and analyzed by onset and cessation season. Results showed that short-term droughts (60 – 270 days) were frequent, and typically resulted from delayed or slowed starts to the MLRAs major modal precipitation season. Long-term droughts (>270 days) were infrequent and occurred only during specific years, requiring below average anomalies in one or more consecutive rainy seasons. Long-term droughts were more likely to occur in MLRAs with unimodal precipitation distributions, due to soil water anomalies likely remaining unresolved until the following rainy season. MLRAs with bimodal precipitation distributions made long-term drought development difficult as consecutive below average rainy seasons were needed. With expected changes in Southwestern climate over the coming decades, understanding how changing precipitation patterns will impact shallow soil drought development is key for future impact assessment and mitigation.