Aims

1
A method for estimation of regional
meteorological and hydrological drought
characteristics
Hege Hisdal and Lena Tallaksen

• Introduction
• Aims
• Drought definition and data
• Method and Results
• Conclusions

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Introduction What is a drought?
Drought is a relative term
Depends on the area of interest
Depends on the climate of the region under study
It is characterised by a deviation from normal
conditions
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Introduction What is a drought?

• Contrary to floods a drought
• develops slowly in time and space
• cannot be forecasted based on a preceding
  precipitation deficit
• often needs to be described by several variables

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Introduction Motivation for the study
Traditionally drought analyses based on
precipitation and streamflow data use annual data
We need models for shorter time steps as a month,
a week or a day
We need information about the regional drought
characteristics
Droughts are regional in nature
Panu Sharma (2002) HSJ The spatial coverage
of drought duration, severity and/or intensity is
of significant importance in planning measures
towards mitigating impacts of drought
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Aims

• To test and develop a method to describe regional
  drought characteristics
• To compare meteorological and hydrological
  drought characteristics

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Drought definition
Great Britain Meteorological Office, 1951
drought is a period of more than some particular
number of days with precipitation less than some
specified small amount
Beran and Rodier (1985) drought is a decrease
of water availability in a particular period over
a particular area
The final report of the U.S. National Drought
Policy Commision, 2000 drought is a persistent
and abnormal moisture deficiency having adverse
impacts on vegetation, animals or people

• A quantitative definition
• to do quantitative analysis
• to be used for streamflow and precipitation
• to be used in a regional context

The truncation level method
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Drought event definitionAt-site, streamflow
The threshold level method

• Decisions
• time resolution
• season
• threshold level
• drought characteristic

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Data
Jutland
Zealand
Funen

• Period 1961-1990, monthly values
• Precipitation Frich et al., 1997-
  http//www.dmi.dk/fu/
• Streamflow European Water Archive (EWA)

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Drought event definitionAt-site, precipitation
The threshold level method

• Decisions
• time resolution month
• season no season
• threshold level 80 percentile
• drought characteristic duration and severity

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Method

• Divide Denmark into 21 x 21 grid-cells of 0.220
  x 0.150 (14 x 17 km)

• Simulate long time series of monthly
  precipitation and streamflow in each grid cell
• Select the drought events in each simulated time
  series - PDS
• Derive the empirical probability distribution
  functions of the area covered by a drought, the
  drought deficit volume and duration
• Produce Severity-Area-Frequency curves

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MethodSimulation of time series, Example
precipitation
1 Subtract the mean value
2 Perform the EOF expansion
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MethodSimulation of time series, Example
precipitation

• 3 Use kriging to interpolate the mean value and
  the three first weight coefficients

1. W.C. Interpolated Standard
error
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MethodSimulation of time series, Example
precipitation

• 4 Remove the seasonal variation by subtracting
  the monthly mean and dividing by the monthly
  standard deviation
• 5 Find the distribution of the temporal functions
• 6 Generate temporal functions (Monte Carlo
  simulations) corresponding to 1000 years of data
• 7 Generate time series in all grid-cells

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MethodSelect drought events
Estimated deficit at point ui at the time t
Total area with deficit
Total areal deficit (mm)
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ResultsRegional drought characteristics
Streamflow
Precipitation
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ResultsSeverity-Area-Frequency curves
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Conclusions I

• The EOF-method combined with Kriging and Monte
  Carlo simulations is shown to be a valuable
  method for simulating precipitation and
  streamflow over a region.
• Drought severity-area-frequency curves can easily
  be obtained from the simulated series.
• Applications to other regions might be valuable
  for further evaluation and refinements of the
  procedure proposed.

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Conclusions II

• Meteorological drought in Denmark occur more
  often and generally cover larger areas than
  hydrological droughts. The duration of
  meteorological droughts is less than for
  hydrological drought. However, there is no major
  difference in total drought deficit volumes.
• The large differences between hydrological and
  meteorological droughts suggest that it is
  preferable to apply streamflow data in water
  resources planning and management.