Climate variability impacts forests and woodlands in the southwestern United States (SWUS) through altered soil moisture availability. Seasonal soil moisture patterns differ locally across elevation, soil depth, and dominant plant species, as well as regionally due to differences in seasonal climate forcings including winter snowfall and summer rainfall. This diversity makes it challenging to anticipate the impacts of change to seasonal precipitation across the SWUS. We sought to understand common characteristics and relationships of seasonal precipitation (daily rainfall and regional snow water equivalent) and soil moisture dynamics at shallow (0-10 cm, 10-20 cm) and intermediate (50 cm) soil depths across 9 diverse forest and woodland sites comprising 3 elevation gradients in the SWUS. Our primary objective was to determine how the development of time-varying measures of soil moisture conditions enhances understanding of commonalities in climate influences on ecosystem moisture dynamics across these gradients. Building on this insight, we also focused on quantifying similarity and divergence in moisture patterns between seasons, soil depths, and wet and dry conditions using information theory.
Results/Conclusions
Development of seasonal soil moisture metrics provided enhanced spring and summer moisture surplus and deficit. Compared to month-based seasons, these soil moisture-based seasons allowed for more precise attribution of seasonal precipitation to low, average, and high soil moisture patterns at different depths in the soil profile. Specifically, seasons defined by winter snow accumulation, spring soil moisture recharge and subsequent depletion, and summer rainfall-derived soil moisture may improve understanding of temporal climate and moisture patterns in these ecosystems. Lower elevation sites experienced earlier snowmelt recharge and soil moisture depletion. Peak spring snowmelt input was positively correlated with the regional snowfall across elevations (p < 0.05). Across all sites, snow-derived moisture informed moisture in deeper soil layers (50 cm), whereas later-season rainfall informed moisture in shallower soil layers (0-10, 10-20 cm). Yet, information flow from shallow to deeper soil layers was enhanced in wet years, indicating that coinciding high snowfall and precipitation initiates an important change to the hydrology of these ecosystems. Together, these results provide new insight on common linkages between seasonal precipitation, rainfall-dependent shallow soil layers, and recharge-influenced deeper soil layers in SWUS forests and woodlands.