Are drought indices and climate data good indicators of ecologically relevant soil moisture dynamics in drylands?

Authors: Barnard, David; GERMINO, M - Us Geological Survey (USGS); BRADFORD, J - Us Geological Survey (USGS); O'Connor, Rory; ANDREWS, C - Us Geological Survey (USGS); SHRIVER, R - University Of Nevada

Submitted to: Ecological Indicators
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/11/2021
Publication Date: 11/16/2021
Citation: Barnard, D.M., Germino, M.J., Bradford, J.B., O'Connor, R.C., Andrews, C.M., Shriver, R.K. 2021. Are climate data and drought indices good indicators of ecologically relevant soil moisture dynamics in drylands? Ecological Indicators. 133. Article e108379. https://doi.org/10.1016/j.ecolind.2021.108379.
DOI: https://doi.org/10.1016/j.ecolind.2021.108379

Interpretive Summary: Droughts are disproportionately impacting global dryland regions where ecosystem health and function are tightly coupled to moisture availability. Drought severity is commonly estimated using climatic water-balance algorithms, like the standardized precipitation-evapotranspiration index (SPEI), but how well these indices predict soil moisture dynamics at ecologically relevant scales and across soil depths is unknown. We combined 36 years of variable-length SPEI, weather data, and modeled soil-water availability at 866 sites across the western United States in a machine learning framework to test for a surrogate of depth-resolved soil moisture dynamics. Overall, SPEI successfully recreated multi-year trends in depth-resolved monthly soil moisture but was poorly predictive of key soil moisture dynamics important to dryland ecosystems in winter-precipitation dominant systems, e.g. annual recharge of deep-soil moisture and spring surface-soil moisture availability. Our findings suggest meteorological drought indices should be used cautiously for resource management and policy making, particularly in dryland regions, and that ecohydrological impacts of droughts should be systematically tested against process-based models.

Technical Abstract: Droughts are disproportionately impacting global dryland regions where ecosystem health and function are tightly coupled to moisture availability. Drought severity is commonly estimated using algorithms such as the standardized precipitation-evapotranspiration index (SPEI), which can estimate climatic water balance impacts at various hydrologic scales by varying computational length. However, the performance of these metrics as indicators of soil moisture dynamics at ecologically relevant scales, across soil depths, and in consideration of broader scale ecohydrological processes, requires more attention. In this study, we tested components of climatic water balance, including SPEI, to recreate multi-decadal and periodic soil-moisture patterns across soil profiles at 866 sites in the western United States. Model results show that SPEI could recreate changes in moisture availability within the soil profile over longer periods of time and for annual recharge of deeper soil moisture stores, but were slightly less successful with recreating spring surface-soil moisture availability which is key to dryland ecosystems dominated by winter precipitation. Meteorological drought indices like SPEI are intended to be convenient and generalized indicators of meteorological water deficit. However, the inconsistent ability of SPEI to recreate ecologically relevant patterns of soil moisture at regional scales suggests that process-based models, and the larger data requirements they involve, remain an important tool for dryland ecohydrology.