Increasing soil water drought in response to altered precipitation timing across the western United States

Authors: ZHANG, F. - University Of Arizona; Biederman, Joel; SCHLAEPFER, D.L. - Us Geological Survey (USGS); BRADFORD, J.B. - Us Geological Survey (USGS); REED, S.C. - Us Geological Survey (USGS); SMITH, W.K. - University Of Arizona

Submitted to: Ecohydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/6/2024
Publication Date: 12/4/2024
Citation: Zhang, F., Biederman, J.A., Schlaepfer, D., Bradford, J., Reed, S., Smith, W. 2024. Increasing soil water drought in response to altered precipitation timing across the western United States. Ecohydrology. 18(2). Article e2749. https://doi.org/10.1002/eco.2749.
DOI: https://doi.org/10.1002/eco.2749

Interpretive Summary: Recent research by us and others has documented increasing trends in several types of drought over recent decades across the western United States. Our recent work used 45 years of daily measurements at 337 weather stations to quantify increases in the mean and longest annual dry spells between subsequent rainfall events. However, ecosystems respond to soil moisture, and it is unknown how the documented increases in precipitation drought translate into soil moisture drought. Here we used daily weather records from the same 337 stations to drive a soil moisture model, and we explored the predicted soil moisture variables. We found that since 1975, the duration of the annual longest dry spell in the soil (a dry spell was defined by moisture values low enough to cause significant plant stress) increased by 1.5±0.2 day/decade for grassy and 1.7±0.2 day/decade for woody vegetation. The correlation between precipitation drought trends and soil moisture drought trends was stronger in shallow soils than deep soils, reflecting the storage and buffering capacity of deep soils. Likewise, the rainfall-soil moisture drought trend correlation was stronger in sandy soils and weaker in clay soils, which store moisture and buffer the soil against rainfall changes. Our results underscore the necessity of considering the interactions between vegetation type and soil depth when evaluating ecosystem vulnerability to drought

Technical Abstract: Weather records indicate trends in recent decades toward longer drought events and higher temperatures in the western United States, and the climatic changes are leading to significant impacts on ecosystems. However, although aridification effects on soils determine terrestrial system responses, the consequences of these meteorological trends for edaphic drought remain unexplored. Here, we examined how longer droughts and warmer temperatures influence soil water deficit in different vegetation types, soil depths, and soil textures. We used the SOILWAT2 ecosystem water balance model to quantify long-term trends of soil water deficit at 337 weather stations across the western United States. Most stations exhibited significant edaphic drying trends under both grassy and woody vegetation. The duration of the longest continuous dry soil period increased by 1.5±0.2 day/decade for grassy and 1.7±0.2 day/decade for woody vegetation. These trends are consistent with regional meteorological drought patterns, and edaphic patterns maintained a tight relationship with the trends of meteorological drought across all stations. Correlation between meteorological drought and edaphic drought was higher under woody vegetation (0.45) compared to grassy vegetation (0.34) and higher at surface soil depths (0.46) compared to the deeper depths (0.34). Among the three soil textures, the correlation between meteorological and edaphic drought was highest on sandy soils and lower on finer-textured soils with more clay and silt. Using the regional categories delineated by the eight western NEON regions, we found that the Pacific Northwest, Pacific Southwest, and Desert Southwest exhibited stronger increases in edaphic drought through time, but also lower correlation between meteorological drought and edaphic drought. 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. Our results underscore the necessity of considering the interactions between vegetation type and soil depth and physical structure when evaluating ecosystem vulnerability to drought, with significant implications for land and water resource management strategies across the western United States.