Assessing summertime hydrological cycle acceleration through drought indices

Abstract. The rate (or velocity) of the hydrological cycle affects water availability for agriculture, energy production, and planning for droughts or floods. Therefore, acceleration in the velocity of the hydrological cycle is likely to impact multiple hydrological domains and management practices. Previous work has primarily studied hydrological cycle velocity and acceleration through the lens of flux magnitudes and their change. Motivated to expand this definition to characterize temporal coupling between stages in the hydrological cycle, we introduce a novel definition of hydrological cycle velocity and acceleration derived from the concept of drought propagation. We define the hydrological cycle velocity as the response time between the 1-month Standardized Precipitation Index (SPI) and the 1-month Standardized Soil Moisture Index (SSI) and define acceleration as the change in response time between an early time period and a late period. Using gridded reanalysis data over the conterminous United States (CONUS) from 1951–2020, we analyzed the summer period (June, July, August) to minimize the lagging-effects of cold-season processes. Response times exceeded 100 days in southwestern CONUS (indicating a slower hydrological cycle velocity), but were substantially shorter elsewhere, 10–20 days, indicating a faster hydrological cycle velocity. A Random Forest variable importance analysis revealed strong negative associations of response time and mean annual flux magnitudes, with higher precipitation and evaporation associated with shorter response times. Regarding potential acceleration between an earlier (1951–1985) and later (1986–2020) period, 48.47 % of reanalysis grid cells experienced a deceleration (lengthening of response time) of their local summer hydrological cycle, while 39.32 % of grid cells experienced an acceleration (shortening of response time). However, a false detection rate correction found a lack of robust field significance (a_FDR=0.05), a finding reinforced by a regional decadal trend analysis. By framing hydrological cycle acceleration in terms of propagation of meteorological anomalies to land surface anomalies, we expand the conceptual basis for diagnosing changes in the hydrological cycle.