Hawaii Drought

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Hawaii Drought

• Pao-Shin Chu
• State Climatologist
• Department of Meteorology
• University of Hawaii

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• Drought is a chronic and troublesome problem in
  Hawaii, at one time or another affecting
  virtually every part of the state.  These events
  often reduce crop yields, diminish livestock
  herds, desiccate streams, irrigation ditches and
  reservoirs, deplete groundwater supplies, and
  lead to forest and brush fires.  
• Source Hawaii Drought Monitor

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• Drought can be characterized from the
  perspectives of meteorology, agriculture,
  hydrology, and socio-economic impacts. 
• The meteorological perspective would describe
  drought as a rainfall deficit compared with some
  normal or expected rainfall amount. 
• The agricultural perspective could describe
  drought by its impacts on the agricultural
  industry due to reduced rainfall and water supply
  (e.g., crop loss, herd culling, etc.).
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• Hydrological descriptions of drought may compare
  stream flows, ground water, and reservoir levels
  to normal conditions.
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• Drought can also be described from the
  socio-economic perspective by the direct and
  indirect impacts droughts have on society and the
  economy (e.g., increased unemployment due to
  failure of an industry because of drought). 

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Historical statewide drought events (Hawaii
Drought Monitor and R88)
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HRI time series (green bars) 27 stations from 3
islands(Hawaii, Oahu, Kauai) Normalized winter
(NDJFM) rainfallNino 3.4 SST anomalies (solid
curve), Diamond for El Niño
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Correlation coefficients between the seasonal SOI
and the seasonal Hawaiian rainfall
index.Quenouilles (1952) method was used to
account for the reduction of effective numbers of
degrees of freedom due to persistence. One and
two asterisks indicate significance at the 5 and
1 percent levels, respectively.
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Drought forecasts from simple regression models
for winter 1983

• Predicted rainfall Observed rainfall
• Determined from
• Spring 1982 -0.089 -1.10
• Summer 1982 -0.982 -1.10
• Autumn 1982 -0.894 -1.10

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Chu and Chen (2005) Interannual and interdecadal
rainfall variations in the Hawaiian Islands. J.
Climate, 18, 4796-4813.
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Winter rainfall difference (inches) of the (El
Niño/PDO) minus (La Niña/-PDO) composite. Dots
for rainfall stations (272).
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The nonparametric Mann-Whitney test for winter
rainfall difference of the (El Niño/PDO) minus
(La Niña/-PDO).
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Winter composite of SST (shading) and surface
wind (vectors). The unit for SST is C and for
wind vectors is m/s.
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Winter 200-mb wind vectors. Isotach interval is
10 m/s. Area with wind speed greater than 40 m/s
is shaded.
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Winter composite of east-west vertical
circulation. Longitude-height section of zonal
wind and negative pressure vertical velocity (U,
-?) is averaged over 15-25N. Shading is for
statistical significance. The Hawaiian Islands
are bordered approximately by two vertical lines.

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• To monitor meteorological droughts, the
  Standardized Precipitation Index (SPI) is used.
• The SPI relies on monthly precipitation as the
  only input. SPI values can be generated for
  multiple time series (e.g., 3-mo, 12-mo). This
  is useful because drought affects various sectors
  over a wide range of time scales.

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• Computation of the SPI involves fitting a gamma
  probability density to a set of precipitation
  totals for a station using the maximum likelihood
  approximations.
• The resulting gamma parameters are used to find
  the cumulative probability of a precipitation
  event for the given time scale.
• The cumulative probability is then transformed to
  the standard Gaussian random variable with mean
  zero and unit variance, which is the SPI.

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The SPI has been embraced by NDMC, WRCC, and the
NWS/Honolulu Forecast Office.
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• Drought classification based on SPI
• (NDMC)
• SPI values Designation
• 0.00 to -0.99 Mild Drought
• -1.00 to -1.49 Moderate Drought
• -1.50 to -1.99 Severe Drought
• -2.00 or less Extreme Drought

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UH Hawaii State Climate Office and SSRIFunded
through DLNRs Water Commission Office

• Drought Risk and Vulnerability Assessment and GIS
  Mapping Project

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• ? Another index, the Keetch/Byram Drought Index
  (KBDI), has been used for the southeast U.S. over
  30 years.

? The KBDI, which conceptually describes the soil
moisture deficit, is used to assess wildfire
potential as part of the revised 1988 U.S.
National Fire Danger Rating System.
? This index depends on daily rainfall amount,
daily maximum temperature, and the mean annual
rainfall.
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• The relationship between the KBDI and fire
  activity, as measured by total areas burned, in
  the Hawaiian Islands was explored (Dolling, Chu,
  Fujioka, 2005, Agricultural and Forest
  Meteorology).
• Total acres burned (TAB) data from DLNR. Records
  are short, only 20 yrs (1976-96).
• Because of the non-normally distributed nature of
  the TAB, a log transformation is applied to the
  TAB data. The correlation between KBDI and TAB
  is significant for Hawaii with a p-value well
  below the 5 level.

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A strong relationship between the KBDI and fire
activity is found for the islands of Maui and
Hawaii.
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Summary

• Reviewed historical drought events in Hawaii.
• Drought in Hawaii generally follows an El Niño
  event. Long-lead winter (spring) rainfall
  prediction is feasible.
• Drought is also affected by the PDO cycle.
  Rainfall difference between (El Niño/PDO) and
  (La Niña/-PDO).

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• SPI time series for HVNP since 1950.
• Drought risk and vulnerability assessment for
  Hawaii County.
• Keetch/Byram drought index (KBDI) Relation
  between KBDI and fire activity for Hawaii is
  evident.