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Drought: The Palmer Drought Severity Index
Drought is one of the most consequential aspects of variation in precipitation and temperature patterns in terms of its impacts on natural ecosystems and human systems. The production of food and clean water can be strongly affected, as can forest products production, outdoor recreation, ecosystem processes such as wildland fire, and many other processes affecting ecosystem services. Having a grasp on recent ranges of variability in drought conditions can provide a context for understanding ongoing and future climate change and its impacts on ecosystem services. Although predicting future drought conditions is fraught with uncertainties, extreme precipitation highs and lows are both becoming greater in many regions. As a result, some regions may experience more frequent drought and flood conditions, with stronger variation over time becoming a "new normal."
The self-calibrated Palmer Drought Severity Index (scPDSI) is a locally calibrated index that estimates the severity of drought conditions relative to normal conditions measured for the region. This is one of the most frequently used standard measures of drought in the United States. Negative values indicate unusually dry conditions, with moderate drought indicated in the -2 to -3 range.
The West Wide Drought Tracker (WWDT) is a system developed by the Western Regional Climate Center to generate regularly updated drought data for the continental United States, including the scPDSI and other indices. WWDT utilizes the PRISM (Parameter-elevation Regressions on Independent Slopes Model) climate data to calculate drought indices, affording high spatial resolution and an archive of drought data over long time periods. PRISM methods utilize digital elevation data and effects such as rain shadows and coastal influences to generate climate data that are widely used and recognized as among the highest-quality spatial climate data available.
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