Increasing Soil Water Drought in Response to Altered Precipitation Timing Across the Western United States

Abstract

Recent trends of rising temperatures and longer droughts between precipitation events are impacting water-limited dryland ecosystems in the western United States. Although ecosystem drought response depends directly on soil moisture, trends in soil moisture (e.g., edaphic drought) remain more poorly explored than precipitation (e.g., meteorological drought), representing an important knowledge gap. Here, we applied the SOILWAT2 ecosystem water balance model to quantify long-term trends of soil moisture and edaphic drought using observed daily weather from 1976 to 2019 at 337 stations across the western United States. We assessed edaphic drought for different plant community types (grass dominated vs. shrub dominated), and explored variations with soil depth and texture. The duration of the longest edaphic drought in a given year increased by 1.5 ± 0.2 days/decade for grassy and 1.7 ± 0.2 days/decade for woody vegetation. Importantly, these trends in edaphic drought were consistent with but greater in magnitude compared with meteorological drought indicating more severe water stress for both plants and ecosystems. The correlation between meteorological drought and edaphic drought was greater under woody vegetation (0.45) compared with grass (0.34) and greater at surface soil depths (0–20 cm; 0.46) compared with the deeper soil (20–100 cm; 0.34). Among soil textures, the correlation between meteorological and edaphic drought was highest on sandy soils and lowest on finer textured silty soils. Using the biogeographic domains (eight western NEON domains), we found that the Pacific Northwest, Pacific Southwest, and Desert Southwest exhibited the strongest increases in edaphic drought through time, but lower correlation between meteorological and edaphic droughts. These findings characterize strong but variable connections between edaphic drought and meteorological drought across the western United States and demonstrate the critical influences of vegetation type, soil depth, and soil properties in mediating the magnitude and spatial distribution of edaphic drought.