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Droughts are difficult to detect and monitor. Drought indices, most commonly the Palmer Drought Severity Index (PDSI), have been used with limited success as operational drought monitoring tools and triggers for policy responses. Recently, a new index, the Standardized Precipitation Index (SPI), was developed to improve drought detection and monitoring capabilities. The SPI has several characteristics that are an improvement over previous indices, including its simplicity and temporal flexibility, that allow its application for water resources on all timescales. In this article, the 1996 drought in the southern plains and southwestern United States is examined using the SPI. A series of maps are used to illustrate how the SPI would have assisted in being able to detect the onset of the drought and monitor its progression. A case study investigating the drought in greater detail for Texas is also given. The SPI demonstrated that it is a tool that should be used operationally as part of a state, regional, or national drought watch system in the United States. During the 1996 drought, the SPI detected the onset of the drought at least 1 month in advance of the PDSI. This timeliness will be invaluable for improving mitigation and response actions of state and federal government to drought-affected regions in the future.

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The Standardized Precipitation Evapotranspiration Index (SPEI) was computed based on the monthly precipitation and air temperature values at 609 locations over China during the period 1951-2010.Various characteristics of drought across China were examined including: long-term trends, percentage of area affected, intensity, duration, and drought frequency. The results revealed that severe and extreme droughts have become more serious since late 1990s for all of China (with dry area increasing by ∼3.72% per decade); and persistent multi-year severe droughts were more frequent in North China, Northeast China, and western Northwest China; significant drying trends occurred over North China, the southwest region of Northeast China, central and eastern regions of Northwest China, the central and southwestern parts of Southwest China and southwestern and northeastern parts of western Northwest mainly due to a decrease in precipitation coupled with a general increase in temperature. In addition, North China, the western Northwest China, and the Southwest China had their longest drought durations during the 1990s and 2000s. Droughts also affected western Northwest, eastern Northwest, North, and Northeast regions of China more frequently during the recent three decades. The results of this article could provide certain references and triggers for establishing a drought early warning system in China.

27Given the increasing use of the term "flash drought" by the media and scientific 28 community, it is prudent to develop a consistent definition that can be used to identify 29 these events and to understand their salient characteristics. It is generally accepted that 30 flash droughts occur more often during the summer due to increased evaporative demand; 31 however, two distinct approaches have been used to identify them. The first approach 32focuses on their rate of intensification, whereas the second approach implicitly focuses on 33 their duration. These conflicting notions for what constitutes a flash drought (i.e., 34 unusually fast intensification versus short duration) introduce ambiguity that affects our 35 ability to detect their onset, monitor their development, and understand the mechanisms 36 that control their evolution. Here, we propose that the definition for flash drought should 37 explicitly focus on its rate of intensification rather than its duration, with droughts that 38 develop much more rapidly than normal identified as flash droughts. There are two 39 primary reasons for favoring the intensification approach over the duration approach. 40

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Recent droughts in the United States have highlighted the nation's current and increasing vulnerability to this natural hazard. Drought-related impacts are also becoming more complex, as illustrated by the rapidly rising impacts in sectors such as recreation and tourism, energy, and transportation. Environmental and social consequences are also of increasing importance. Conflicts between water users and disputes between political entities on transboundary water issues are a reflection of the need for improved documentation of the consequences of extended periods of water shortage. Unfortunately, no national drought impact database exists and drought impact statistics are not routinely compiled at the state, regional, or national level. Without this information, it is an arduous task to convince policy and other decision makers of the need for additional investments in drought monitoring and prediction, mitigation, and preparedness. The National Drought Mitigation Center at the University of Nebraska-Lincoln is addressing this problem by creating a web-based Drought Impact Reporter (DIR) that has the following primary functions: (1) to create a database archive of drought impacts information; (2) to provide an interactive map delivery system that is efficient and user-oriented; (3) to build links with governmental agencies, nongovernmental organizations, university research groups and extension programs, and others, including the public, in order to provide timely impact reports to ensure a comprehensive collection of drought impacts across all potential sectors and scales; and (4) to foster a continual process of user feedback, evaluation, assessment, and

Abstract:The Standardized Precipitation Index (SPI) is now widely used throughout the world in both a research and an operational mode. For arid climates, or those with a distinct dry season where zero values are common, the SPI at short time scales is lower bounded, referring to non-normally distributed in this study. In these cases, the SPI is always greater than a certain value and fails to indicate a drought occurrence. The nationwide statistics based on our study suggest that the non-normality rates are closely related to local precipitation climates. In the eastern United States, SPI values at short time scales can be used in drought/flood monitoring and research in any season, while in the western United States, because of its distinct seasonal precipitation distribution, the appropriate usage and interpretation of this index becomes complicated. This would also be the case for all arid climates. From a mathematical point of view, the non-normally distributed SPI is caused by a high probability of no-rain cases represented in the mixed distribution that is employed in the SPI construction. From a statistical point of view, the 2-parameter gamma model used to estimate the precipitation probability density function and the limited sample size in dry areas and times would also reduce the confidence of the SPI values.On the basis of the results identified within this study, we recommend that the SPI user be cautious when applying short-time-scale SPIs in arid climatic regimes, and interpret the SPI values appropriately. In dry climates, the user should focus on the duration of the drought rather than on just its severity. It is also worth noting that the SPI results from a statistical product of the input data. This character makes it difficult to link the SPI data to the physical functioning of the Earth system.