Hydrological and Ecological Impacts of Increasingly Similar Simulated Analogs of the Contemporary Drought in the Western United States

The 2011-2016 drought in the western United States is among the most severe and persistent droughts in recorded history, including 2011-2014 having the second lowest observed winter precipitation of any 3-year period in California, leading to significant hydrological and ecological impacts, and economic and environmental losses (Vose et al., 2014, Goulden & Bales, 2019). In this study we identify meteorological droughts simultaneously in space and time in historical simulations from the CMIP5 and CMIP6 multi-model ensembles, with approximately 49,000 simulated drought events identified. We gradually filtered the simulated droughts by how well their location, severity, and spatial and temporal characteristics match those of the 2011-2016 drought in the western U.S. As we narrow the matching constraints, we quantify if the local-scale hydrological and ecological impacts (evaporation, leaf area index, soil moisture, and runoff) of the simulated droughts also narrow. Our findings suggest that there is a wide range of ecological and hydrological impacts associated with the simulated droughts, even when the matching constraints are exceptionally strict (and thus the spatiotemporal characteristics of the simulated droughts closely match the observed drought). Across all models, there is widespread precipitation and runoff decreases of varying magnitudes. Soil moisture, evaporation, and LAI showed a much less consistent response, with evaporation and LAI being particularly inconsistent with both increases and decreases in these variables found across simulations. This range cannot be easily explained by structural uncertainty in the multi-model ensemble, as results hold even among individual models. Our finding suggests that, at least in models, there is a range of magnitudes of decreases in hydrological variables and highly variable ecological impacts that are consistent with the contemporary drought in the western United States.

Vose, RS., Applequist, S., Squires, M., Durre, I., Menne, MJ., Williams, CN., Jr., Fenimore, C., Gleason, K., Arndt, D. (2014). Improved Historical Temperature and Precipitation Time Series for U.S. Climate Divisions. JAMC.

Goulden, ML., Bales, RC. (2019). California forest die-off linked to multi-year deep soil drying in 2012-2015 drought. Nature Geoscience.