Drought frequency, intensity, and duration shifts over the U.S. during the last century
The impact of climate change on the hydrological process has long been stated in the literature based on different indicators, datasets, and statistical approaches. Considering drought events, many of the efforts to understand the trend shifts have considered the standardized precipitation index (SPI) over the last 30 to 60 years. SPI, while adequately represents the wet and dryness, does not consider the main impact of increased temperatures on water demand. Recent evidence of accelerating the water cycle has not been analogous with the changes of the water vapor in the atmosphere and evaporative demand.
Therefore, it is critical to consider the role of evapotranspiration (ET) on the frequency, intensity, and duration of drought events based on historical observations. In this context, the methodology used in calculating ET in historical datasets is an important aspect.
The Penman-Monteith (P-M) approach considers both atmospheric and vegetation parameters for developing ET; and thus, is a superior method for agri-climatological analysis compared to other temperature or radiation-based indices. With the availability of CRU-TS historical data that satisfies the parameters used in the P-M approach, long-term SPEI data have become available (from 1901 to 2018).
The Penman-Monteith (P-M) approach considers both atmospheric and vegetation parameters for developing ET; and thus, is a superior method for agri-climatological analysis compared to other temperature or radiation-based indices. With the availability of CRU-TS historical data that satisfies the parameters used in the P-M approach, long-term SPEI data have become available (from 1901 to 2018). In this study, we take this opportunity to use this historical SPEI dataset and analyze the changes in drought events over the United States. We divided the historical SPEI into ten rolling climates normal (i.e., 1901-1930, 1910-1940, ..., 1990-2018) and three smaller periods (2000-2018, 2005-2018, 2010-2018) and evaluated the shifts in intensity, duration, and frequency of drought events based on the probability density functions over the U.S. with respect to the long-term average. Given the climate dependency of the drought event, we divided the U.S. into 7 major climate types and performed the analysis for each climate type.
The analysis and processing are underway and examples of the results as spatial plots and probability density functions (PDFs) are provided here. The outcome will be published as a peer-review publication (it has also been submitted to AMS-2022). The results will provide important insights to understand how the drought has been evolved historically and how the changes in climate are linked with the changes in drought events.