Space-time Trends in U.S. Meteorological Droughts

Proceedings of World Environmental and Water Resources Congress 2009 - World Environmental and Water Resources Congress 2009: Great Rivers, 2009

Droughts are prolonged abnormalities of moisture deficits that vary widely across temporal and spatial scales. Many hydrometeorologic variables are used to monitor the status of a drought. However, because of the dependence structure between all affecting variables under various temporal windows, an integrated spatiotemporal analysis of droughts cannot be easily achieved. In this study, a copula-based drought analysis was performed by using long-term monthly precipitation dataset for the upper Midwest United States. The spatio-temporal dependence relationships between various drought variables were investigated, and their joint probability distribution was constructed by combining drought marginals and the dependence structure. A copula-based joint deficit index (JDI) was adopted for an objective (probability-based) description of the overall drought status and compared to the Palmer drought severity index results. Results from the copula-based JDI provide information for drought identification, and further allow a month-by-month assessment for future drought recovery.

Water, 2019

The Northeast United States is a generally wet region that has had substantial increases in mean precipitation over the past decades, but also experiences damaging droughts. We evaluated drought frequency, intensity, and duration trends in the region over the period 1901–2015. We used a dataset of Standardized Precipitation Evapotranspiration Index (SPEI), a measure of water balance based on meteorology that is computed at multiple timescales. It was found that the frequency of droughts decreased over this period, but their average intensity and duration did not show consistent changes. There was an increase in mean SPEI, indicating mostly wetter conditions, but also in an increase in SPEI variance, which kept the likelihood of extremely dry conditions from decreasing as much as would be expected from the wetter mean state. The changes in the SPEI mean and variance, as well as the decrease in drought frequency, were most pronounced for longer timescales. These results are consistent...

Natural Hazards, 2016

Drought is among the most insidious types of natural disasters and can have devastating economic and human health impacts. This research analyzes the relationship between two readily accessible drought indices-the Palmer Drought Severity Index (PDSI) and Palmer Hydrologic Drought Index (PHDI)-and the damage incurred by such droughts in terms of monetary loss, over the 1975-2010 time period on monthly basis, for five states in the south-central USA. Because drought damage in the Spatial Hazards Events and Losses Database for the United States (SHELDUS TM ) is reported at the county level, statistical downscaling techniques were used to estimate the county-level PDSI and PHDI. Correlation analysis using the downscaled indices suggests that although relatively few county-months contain drought damage reports, drought indices can be useful predictors of drought damage at the monthly temporal scale extended to 12 months and at the county-level spatial scale. The varying time lags between occurrence of drought and reporting of damage, perhaps due to varying resilience to drought intensity and duration by crop types across space, along with differing irrigation schedules and adaptation measures of the community to drought over space and time, may contribute to weakened correlations. These results present a reminder of the complexities of anticipating the effects of drought, but they contribute to the effort to improve our ability to mitigate the effects of incipient drought.

Journal of Hydrometeorology, 2017

Probabilistic forecasts of U.S. Drought Monitor (USDM) intensification over 2-, 4-, and 8-week time periods are developed based on recent anomalies in precipitation, evapotranspiration, and soil moisture. These statistical forecasts are computed using logistic regression with cross validation. While recent precipitation, evapotranspiration, and soil moisture do provide skillful forecasts, it is found that additional information on the current state of the USDM adds significant skill to the forecasts. The USDM state information takes the form of a metric that quantifies the ''distance'' from the next-higher drought category using a nondiscrete estimate of the current USDM state. This adds skill because USDM states that are close to the next-higher drought category are more likely to intensify than states that are farther from this threshold. The method shows skill over most of the United States but is most skillful over the north-central United States, where the cross-validated Brier skill score averages 0.20 for both 2-and 4-week forecasts. The 8-week forecasts are less skillful in most locations. The 2-and 4-week probabilities have very good reliability. The 8-week probabilities, on the other hand, are noticeably overconfident. For individual drought events, the method shows the most skill when forecasting high-amplitude flash droughts and when large regions of the United States are experiencing intensifying drought.

2015

This thesis proposes a multi-scale study of U.S. drought risk and predictability in order to better understand 1) how land, atmosphere, and oceans interact during the onset of drought, 2) how the processes involved in generating mechanisms of drought can be better identified by a multi-scale study, rather than a single dimensional study, and 3) how reliable known drought mechanisms are in explaining and predicting recent and future drought events. In this thesis, there are seven chapters. Chapter 1 summarizes the objectives and goals of the thesis research. Chapter 2 investigates changes in the low flow regime over the eastern U.S. region due to climate change and variability. It also assesses the association between low flows and large-scale atmospheric circulations. In Chapter 3, the multi-scale driving mechanisms of droughts and floods over the southeastern U.S. are studied using a recently published regional reanalysis dataset. It establishes favorable conditions of the southeas...

Journal of Applied Meteorology and Climatology

Monitoring drought conditions in arid and semiarid regions characterized by high levels of intra- and interannual hydroclimatic variability is a challenging task. Typical drought-monitoring indices that are based on monthly-scale data lack sufficient temporal resolution to detect hydroclimatic extremes and, when used operationally, may not provide adequate indication of drought status. In a case study focused on the Four Corners region of the southwestern United States, the authors used recently standardized World Meteorological Organization climate extremes indices to discern intra-annual hydroclimatic extremes and diagnose potential drought status in conjunction with the simple metric of annual total precipitation. By applying data-reduction methods to a suite of metrics calculated using daily data for 1950–2014, the authors identified five extremes indices that provided additional insight into interannual hydroclimatic variability. Annual time series of these indices revealed ano...

2020

The term "flash drought" is frequently invoked to describe droughts that develop rapidly over a relatively short timescale. Despite extensive and growing research on flash drought processes, predictability, and trends, there is still no standard quantitative definition that encompasses all flash drought characteristics and pathways. Instead, diverse definitions have been proposed, supporting wide-ranging studies of flash drought but creating the potential for confusion as to what the term means and how to characterize it. Use of different definitions might also lead to different conclusions regarding flash drought frequency, predictability, and trends under climate change. In this study, we compared five previously published definitions, a newly proposed definition, and an operational satellite-based drought monitoring product to clarify conceptual differences and to investigate the sensitivity of flash drought inventories and trends to the choice of definition. Our analyses indicate that the newly introduced Soil Moisture Volatility Index definition effectively captures flash drought onset in both humid and semi-arid regions. Analyses also showed that estimates of flash drought frequency, spatial distribution, and seasonality vary across the contiguous United States depending upon which definition is used. Definitions differ in their representation of some of the largest and most widely studied flash droughts of recent years. Trend analysis indicates that definitions that include air temperature show significant increases in flash droughts over the past 40 years, but few trends are evident for definitions based on other surface conditions or fluxes. These results indicate that "flash drought" is a composite term that includes several types of events and that clarity in definition is critical when monitoring, forecasting, or projecting the drought phenomenon.

2010

This paper examines the change in precipitation from 1900 to 2006 on a regional scale over a portion of the Central United States. Monthly precipitation data is kriged over the Kansas River Basin region. The Standardized Precipitation Index (SPI) is calculated at several time scales ranging from 1 to 12 months. The linear trend of SPI values over the time period is calculated and analyzed, showing many areas of increasing wetness throughout the area, with drying in isolated regions of the West and North.

Environmental Research Letters, 2010

The effect of climate change on the frequency and intensity of droughts across the contiguous United States over the next century is assessed by applying Standardized Precipitation Indices and the Palmer Drought Severity Index to the full suite of 22 Intergovernmental Panel on Climate Change General Circulation Models for three IPCC-SRES emissions scenarios (B1, A1B, and A2 from the Special Report on Emissions Scenarios (SRES) listed in order of their emissions through 2100 from high to low). The frequency of meteorological drought based on precipitation alone is projected to increase in some parts of the US, for example the southwestern states, and decrease in others. Hydrological drought frequencies based on precipitation and temperature are projected to increase across most of the country, however, with very substantial and almost universally experienced increases in drought risk by 2050. For both measures, the southwestern US and the Rocky Mountain states are projected to experience the largest increases in drought frequency, but these areas may be able to exploit existing excess storage capacity. Drought frequencies and uncertainties in their projection tend to increase considerably over time and show a strong worsening trend along higher greenhouse gas emissions scenarios, suggesting substantial benefits for greenhouse gas emissions reductions.

Geophysical Research Letters, 1993

A basic question in drought climatology is whether there has been a shift toward drier or wetter conditions within the period of operational climatic records. This question is examined for the contiguous United States by analyzing mean values of the Palmer Hydrologic Drought index over the 90-year period 1900-1989. Using isoline maps and difference of means tests, moisture conditions in the 1900-44 period are compared to conditions in the 1945-89 period, and multiple comparisons are made among the three discrete climatologically standard 30-year periods (1900-29; 1930-59; 1960-1989). For each mapped time series there are distinct core regions with above-and below-normal moisture conditions. Comparisons of the 45year means reveal significant differences at 39% of the nations' climatic divisions, with clear inverse relationships in the most abnormally wet or dry regions. Comparisons among the 30-year mean periods show that the largest changes are in the interior, stretching from the northcentral Rocky Mountains into the northern Great Plains. A transition from a wetter to a drier moisture regime occurs between the early and middle 30-year periods in this region, with drier conditions persisting through the last 30year period. The degree of regional heterogeneity in mean moisture conditions, coupled with the changing moisture status among the discrete time periods, suggests that the perception of trends in hydrologic drought conditions is subject to both spatial and temporal scaling.