Meteorological drought time-scales and hydrological droughts

A recent collaborative paper in Journal of Hydrology analyzes the characteristic meteorological drought time-scales that best explain observed hydrological droughts measured as streamflows anomalies in the conterminous US. Characterizing and predicting hydrological drought is difficult due to the complex processes that propagate changes and anomalies through the hydrologic cycle but critical because of the severe economic, environmental, and social effects a prolonged hydrological drought can have on a community.

To better understand the relationship between meteorological drought time-scales and hydrological drought, Peña-Gallardo et al. (2018) compared the Standardized Precipitation Evapotranspiration Index (SPEI), an index of climatic drought severity, with the Standardized Streamflow Index (SSI), an index of hydrological drought severity, in gauged natural basins across the coterminous United States.

SPEI vs SSI Correlations
Figure 1: Correlation between SPEI and SSI across the CONUS for each month of the year. Between May - January, correlations between SPEI and SSI are almost all greater than 0.5 for all basins across the CONUS. This suggests that there is a strong relationship between climatic and hydrological drought in these basins during these months. In January - April however, there are many correlations in the Rocky Mountains and northeastern CONUS that are notably small. These low correlations highlight the complex relationship between climatic and hydrological drought and show that relationships may not hold true across space and time.

These results suggest that for the most part, hydrological drought is directly related to climatic drought at within the CONUS, generally at small time-scales. However, this relationship is spatially and temporally variable. Factors such as soil characteristics, vegetation cover, and elevation all affect the time-scale at which climatic drought influences hydrologic drought.

In Eastern Montana, for example, these results would suggest that a period of two to four months with below average rainfall could be a big enough climatic drought to cause a major hydrological drought with notably lower than average streamflows. In 2017, this relationship was made clear when a flash drought occurred in eastern Montana and the Dakotas. Despite a wetter than average winter and spring, a warm and dry spell in June and July lead to abnormally low soil moisture and plant productivity across the region.

In western Montana, on the other hand, the effects of the warm and dry summer were much less severe. This is likely due in part to the spatial variability of the climatic-hydrological drought relationship detailed by Pena-Gallardo et al. The western half of Montana is mountainous and rugged whereas the eastern half of the state is relatively flat and unvaried. The mountains in the western half of the state hold snow much later into the summer than the lower elevation, flat terrain of eastern Montana. This snow can melt throughout the summer and provide a stable baseflow to the rivers and streams of the west, regardless of how dry and hot the summer might be. This difference in topography across the state means it may take a prolonged, year-long climatic drought to have a notable impact on western Montana streamflow but a climatic drought of only a month or two in eastern Montana to achieve the same effect.

As the climate of Montana changes and becomes less predictable, it is important to understand the spatial and temporal relationships between climatic and hydrological drought within the state. Just because there is record snowfall in the winter and spring doesn’t mean there will be a great harvest of alfalfa and barley across the state. Farmers and land managers must recognize this and understand the hydrologic conditions that will negatively affect the land and crops where they live.

-- Post by Colin Brust (


For more information, refer to:

Peña-Gallardo, M., Vicente-Serrano, S.M., Hannaford, J., Lorenzo-Lacruz, J., Svoboda, M., Domínguez-Castro, F., Maneta, M., Tomas-Burguera, M., Kenawy, A.E., Complex influences of meteorological drought time-scales on hydrological droughts in natural basins of the contiguous United States, Journal of Hydrology (2018), doi: