Drought in Travis County

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Drought in Travis County

• Chris Shaw
• CE394K.2
• Spring 2007

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Outline of Presentation

• Introduction/Objective
• Drought
• Indices
• Methods
• Results
• Summary

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Introduction

• Water is important
• Transportation
• Agriculture
• Domestic Use
• Commercial and Industrial Use
• Recreation
• Drought negatively impacts these uses
• Tools needed to predict and classify drought

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Objective

• I approached this project as an opportunity to
• Learn more about drought
• Learn more about tools available to classify and
  forecast drought.
• As an exercise in determining drought conditions
  for a local area, in this case Travis County.

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What is Drought

• The immediate cause of drought is the predominant
  sinking motion of air (subsidence) that results
  in compressional warming or high pressure, which
  inhibits cloud formation and results in lower
  relative humidity and less precipitation.

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Definitions

• Conceptual vs. Operational
• Conceptual definitions, help people understand
  the concept of drought.
• Example Drought is a protracted period of
  deficient precipitation resulting in extensive
  damage to crops, resulting in loss of yield.
• Operational definitions help people identify the
  beginning, end, and degree of severity of a
  drought.

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Meteorological/Agricultural

• Meteorological-usually an expression of
  precipitations departure from normal over time.
• Agricultural-Links various characteristics of
  meteorological or hydrological drought to
  agricultural impacts.
• precipitation shortages
• differences between actual and potential
  evapotranspiration
• soil water deficits,
• reduced ground water or reservoir levels.

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Hydrological

• Hydrological drought refers to deficiencies in
  surface and subsurface water supplies. It is
  measured as streamflow and as lake, reservoir,
  and groundwater levels. There is a time lag
  between lack of rain and less water in streams,
  rivers, lakes, and reservoirs, so hydrological
  measurements are not the earliest indicators of
  drought.
• Although climate is a primary contributor to
  hydrological drought, other factors such as
  changes in land use (deforestation), land
  degradation, and dam construction also
  contribute.

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Socioeconomic

• Socioeconomic- associates the supply and demand
  of some economic good with elements of
  meteorological, hydrological, and agricultural
  drought.
• occurs when the demand for an economic good
  exceeds supply as a result of a weather-related
  shortfall in water supply.
• occurs when physical water shortage starts to
  affect people, individually and collectively.

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How is drought measured and represented?

• No single operational definition of drought works
  in all circumstances, and this is a big part of
  why policy makers, resource planners, and others
  have more trouble recognizing and planning for
  drought than they do for other natural disasters.
  In fact, most drought planners now rely on
  mathematic indices to decide when to start
  implementing water conservation or drought
  response measures.

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Drought models or indices

• Percent of Normal
• Standardized Precipitation Index (SPI)
• Surface Water Supply Index (SWSI)
• Reclamation Drought Index (RDI)
• Deciles
• Crop Moisture Index (CMI)
• Palmer Drought Severity Index (PDSI)

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Indices

• Percent of Normal - a simple calculation suited
  to the needs of TV weathercasters and general
  audiences.
• SPI - The SPI is an index based on the
  probability of precipitation for any time scale.
• SWSI - designed to complement the Palmer in the
  state of Colorado
• RDI - calculated at the river basin level

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Indices

• Deciles - Groups monthly precipitation into
  deciles, used in Australia
• CMI Palmer derivative, reflects short term
  moisture supply across major crop-producing
  regions, not intended to assess long-term
  droughts
• PDSI - Soil moisture algorithm calibrated for
  relatively homogeneous regions. U.S. government
  agencies and states rely on the Palmer.
• Chose PDSI
• 30 years data required

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PDSI Calculation

• Inputs Temperature, Precipitation, Normal
  Temperatures, Latitude, and Available Water
  Holding Capacity (AWC) of the soil.
• The temperature values are the average daily
  temperature for each time period (month/week).
• Precipitation the total amount received over each
  time period.
• Normal temperatures are long-term average
  temperature for each period.
• Latitude used to approximate the amount of
  sunlight the location receives, which is part of
  Thornthwaite's calculation of PET.

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PDSI Calculation

• For each period, the following values must be
  calculated
• Potential Evapotranspiration
• Potential Recharge
• Potential Runoff
• Potential Loss
• Actual Evapotranspiration
• Recharge
• Runoff
• Loss

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PDSI Calculation

• Calculate the moisture departure for each period
• The moisture anomaly is calculated
• To calibrate the PDSI, values of the duration
  factors and the climate characteristic must be
  determined
• To determine the value of the duration factors p
  and q, the linear relationship between the length
  of extreme dry spells and the value of the
  accumulated Z-index over those spells is
  determined using the least-squares method.

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PDSI Calculation

• The PDSI is calculated for each period using the
  moisture anomaly that was approximated. Then each
  value of the Z-index is weighted according to
  where the 2nd and 98th percentiles of the PDSI
  fall compared with the expected -4.00 and 4.00.
• The PDSI values are calculated iteratively using
  the Z-index and the duration factors. Each of the
  intermediate indices X1, X2, and X3 are
  calculated as necessary for each period in order.
  The probability of the current spell ending is
  also calculated.

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Study Area Considerations

• County chosen over HUC or watershed/basin
• Location of measurement sites and length of
  records required for some data, most notably
  precipitation and soil moisture, limited the site
  data available.
• Site location is relatively central to the county
  extents. Site moved to Camp Mabry in early part
  of this decade.

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Site Location
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PDSI Calculator

• Fortunately I discovered a site that would do the
  calculation for me.
• http//nadss.unl.edu/PDSIReport/index.jsp
• SPI calculator available as well, but does not
  appear to work at this time.
• Shortcomings outputs, limited sites

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Results
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Travis County, 1990-1997
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Travis County 1998-2006
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Summary

• An abundance of indices available
• Need to match the model to the job
• As with most climate models there is a fair
  amount of uncertainty
• Increasing availability of products like I used
• Need more sites to support these kinds of
  efforts.

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Questions?