Abstract
This study examines wet season droughts using eight products from the Frequent Rainfall Observations on GridS database. The study begins by evaluating wet season precipitation totals and wet day counts at seasonal and decadal time scales. While we find a high level of agreement among the products at a seasonal time scale, evaluations of 10 year variability indicate substantial non-stationary inter-product differences that make the assessment of low-frequency changes difficult, especially in data-sparse regions. Some products, however, appear more reliable than others on decadal time scales. Global time series of dry, middle, and wet region standardized precipitation index time series indicate little coherent change. There is substantial coherence in year-to-year variations in these time series for the better-performing products, likely indicative of skill for monitoring variations at large spatial scales. During the wet season, the data do not appear to indicate widespread global changes in precipitation, reference evapotranspiration (RefET) or Standardized Precipitation Evapotranspiration Index (SPEI) values. These data also do not indicate a global shift towards increasing aridity. Focusing on SPEI values for dry regions during droughts, however, we find modest increases in RefET and decreases in SPEI when wet season precipitation is below normal. Dry region SPEI values during droughts have decreased by −0.2 since the 1990s. The cause of these RefET increases is unclear, and more detailed analysis will be needed to confirm these results. For wet regions, however, the majority of products appear to indicate increases in wet season precipitation, although many products perform poorly in these regions due to limited observation networks, and estimated increases vary substantially. Synopsis: Our analysis indicates a lack of increasing aridity at global scales, issues associated with non-stationary systematic errors, and concerns associated with increases in reference evapotranspiration in global dry regions during droughts.
Highlights
While droughts typically involve a lack of available water, they are complex, slow onset disasters that vary by social and/or environmental context
In this analysis, we examine a subset of precipitation indices as defined by the Expert Team on Climate Change Detection and Indices, as well as the Standardized Precipitation Index (SPI) and Standardized Precipitation Evapotranspiration Index (SPEI)
Our study looks at changes in SPEI in dry regions when SPI values are less than −0.7, i.e. when dry regions are experiencing seasonal droughts
Summary
While droughts typically involve a lack of available water, they are complex, slow onset disasters that vary by social and/or environmental context. According to the Emergency Database (EM-Dat) (EM-Dat 2019), between 2015 and 2018, droughts affected nearly 430 million people, resulting in reported economic losses of $33 billion dollars (USD) This impact helped fuel a recent increase in global hunger (FAO, IFAD, WFP, and WHO 2018) and a ∼70% increase in extreme food insecurity (Funk et al 2019). Many assessments of changes in drought frequency, intensity, or duration use either statistical indices, such as the Standardized Precipitation Index (SPI) (McKee et al 1993), the Standardized Precipitation Evapotranspiration Index (SPEI) (Vicente-Serrano et al 2010), or process-based land surface models. It is broadly accepted that temperaturebased estimates of atmospheric water demand tend to over-estimate anthropogenic increases (Sheffield et al 2012, Trenberth et al 2014) Simple approaches such as the Thornthwaite (Thornthwaite 1948) or Hargreaves (Hargreaves and Samani 1985) estimation procedures are based solely on temperature. New work is demonstrating that coupled atmosphere-land-ocean model simulations tend to indicate much lower tendencies towards increasing aridity than those found when climate change simulations are analyzed using offline models (Milly and Dunne 2016, Greve et al 2017)
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