Palmer drought index

{{Short description|Regional drought index}}

{{Use mdy dates|date=April 2015}}

thumb

The Palmer drought index, sometimes called the Palmer drought severity index (PDSI), or the Palmer Z index, is a regional drought index commonly used for monitoring drought events and studying areal extent and severity of drought episodes.{{Cite journal|last1=Mishra|first1=Ashok K.|last2=Singh|first2=Vijay P.|date=September 2010|title=A review of drought concepts|url=https://linkinghub.elsevier.com/retrieve/pii/S0022169410004257|journal=Journal of Hydrology|language=en|volume=391|issue=1–2|pages=202–216|doi=10.1016/j.jhydrol.2010.07.012|bibcode=2010JHyd..391..202M |url-access=subscription}} The index uses precipitation and temperature data to study moisture supply and demand using a simple water balance model.{{Cite journal|last=Van Loon|first=Anne F.|date=July 2015|title=Hydrological drought explained: Hydrological drought explained|journal=Wiley Interdisciplinary Reviews: Water|language=en|volume=2|issue=4|pages=359–392|doi=10.1002/wat2.1085|doi-access=free}}{{Cite journal |last1=Liu |first1=Yi |last2=Ren |first2=Liliang |last3=Ma |first3=Mingwei |last4=Yang |first4=Xiaoli |last5=Yuan |first5=Fei |last6=Jiang |first6=Shanhu |date=January 2016 |title=An insight into the Palmer drought mechanism based indices: comprehensive comparison of their strengths and limitations |url=http://link.springer.com/10.1007/s00477-015-1042-4 |journal=Stochastic Environmental Research and Risk Assessment |language=en |volume=30 |issue=1 |pages=119–136 |doi=10.1007/s00477-015-1042-4 |bibcode=2016SERRA..30..119L |issn=1436-3240|url-access=subscription }} It was developed by meteorologist Wayne Palmer, who first published his method in the 1965 paper Meteorological DroughtWayne Palmer, "Meteorological Drought". Research paper no.45, U.S. Department of Commerce Weather Bureau, February 1965 (58 pgs). Available online by the NOAA National Climatic Data Center at http://www.ncdc.noaa.gov/temp-and-precip/drought/docs/palmer.pdf for the Office of Climatology of the U.S. Weather Bureau.

The Palmer Drought Index is based on a supply-and-demand model of soil moisture. Supply is comparatively straightforward to calculate, but demand is more complicated as it depends on many factors, not just temperature and the amount of moisture in the soil but also hard-to-calibrate factors including evapotranspiration and recharge rates. Palmer tried to overcome these difficulties by developing an algorithm that approximated them based on the most readily available data, precipitation and temperature.

The index has proven most effective in determining long-term drought, a matter of several months, but it is not as good with conditions over a matter of weeks. It uses a 0 as normal, and drought is shown in terms of negative numbers; for example, negative 2 is moderate drought, negative 3 is severe drought, and negative 4 or less is extreme drought. Palmer's algorithm also is used to describe wet spells, using corresponding positive numbers; for example, 2 is unusually moist, 3 is very moist, and 4 or more is extremely moist. Palmer also developed a formula for standardizing drought calculations for each individual location based on the variability of precipitation and temperature at that location. The Palmer index can therefore be applied to any site for which sufficient precipitation and temperature data is available.

Critics have argued that the utility of the Palmer index is weakened by the arbitrary nature of Palmer's algorithms, including the technique used for standardization and arbitrary designation of drought severity classes and internal temporal memory.{{Cite journal |last1=Ma |first1=Mingwei |last2=Ren |first2=Liliang |last3=Yuan |first3=Fei |last4=Jiang |first4=Shanhu |last5=Liu |first5=Yi |last6=Kong |first6=Hao |last7=Gong |first7=Luyan |date=2014-11-15 |title=A new standardized Palmer drought index for hydro-meteorological use: A NEW STANDARDIZED PALMER DROUGHT INDEX |url=https://onlinelibrary.wiley.com/doi/10.1002/hyp.10063 |journal=Hydrological Processes |language=en |volume=28 |issue=23 |pages=5645–5661 |doi=10.1002/hyp.10063|url-access=subscription }} The Palmer index's inability to account for snow and frozen ground also is cited as a weakness.{{cite journal |last1=Alley |first1=W. M. |title=The Palmer Drought Severity Index: Limitations and Assumptions |journal=Journal of Applied Meteorology and Climatology |date=1984 |volume=23 |issue=7 |pages=1100–1109 |doi=10.1175/1520-0450(1984)023<1100:TPDSIL>2.0.CO;2 |doi-access=free |bibcode=1984JApMe..23.1100A }}

The Palmer index is widely used operationally, with Palmer maps published weekly by the United States Government's National Oceanic and Atmospheric Administration. It also has been used by climatologists to standardize global long-term drought analysis. Global Palmer data sets have been developed based on instrumental records beginning in the 19th century.Dai, Aiguo et al.: A Global Dataset of Palmer Drought Severity Index for 1870–2002: Relationship with Soil Moisture and Effects of Surface Warming, Journal of Hydrometeorology, Vol. 5, No. 6, pp. 1117–30, December 2004 In addition, dendrochronology has been used to generate estimated Palmer index values for North America for the past 2000 years, allowing analysis of long term drought trends.Cook, E.R. et al.: Long-Term Changes in the Western United States, Science, Vol. 306, No. 5698, pp. 1015–18, November 5, 2004 It has also been used as a means of explaining the Late Bronze Age collapse.

In the US, regional Palmer maps are featured on the cable channel Weatherscan.