Drought in Travis CountyDrought in Travis County

Chris Shaw

CE394K.2

Spring 2007

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Outline of PresentationOutline of PresentationIntroduction/ObjectiveDroughtIndicesMethodsResultsSummary

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IntroductionIntroductionWater is important

– Transportation – Agriculture– Domestic Use– Commercial and Industrial Use– Recreation

Drought negatively impacts these usesTools needed to predict and classify drought

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ObjectiveObjectiveI 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 DroughtWhat is DroughtThe 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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DefinitionsDefinitions 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/AgriculturalMeteorological/Agricultural

Meteorological-usually an expression of precipitation’s 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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HydrologicalHydrological

• 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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SocioeconomicSocioeconomic

• 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 How is drought measured and represented?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 indicesDrought models or indicesPercent of NormalStandardized Precipitation Index (SPI)Surface Water Supply Index (SWSI)Reclamation Drought Index (RDI)DecilesCrop Moisture Index (CMI)Palmer Drought Severity Index (PDSI)

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IndicesIndices

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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IndicesIndices 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 CalculationPDSI 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 CalculationPDSI Calculation

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

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PDSI CalculationPDSI 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 CalculationPDSI 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 ConsiderationsStudy 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 LocationSite Location

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PDSI CalculatorPDSI CalculatorFortunately I discovered a site that would

do the calculation for me.http://nadss.unl.edu/PDSIReport/index.jspSPI calculator available as well, but does

not appear to work at this time.Shortcomings – outputs, limited sites

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ResultsResults

Station ID 410428 1990 -3.5 1996 -1.08 2002 1.9Station NameAustin Mueller Muni AP 1991 0.94 1997 0.52 2003 1.83Latitude 30.321 1992 2.89 1998 2.82 2004 0.13Longitude -97.76 1993 3.7 1999 1.35 2005 2.28Index Self Calibrated PDSI 1994 -1.89 2000 -1.74 2006 -2.48

1995 1.83 2001 1.39

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Travis County, 1990-1997Travis County, 1990-1997

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Travis County 1998-2006Travis County 1998-2006

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SummarySummaryAn abundance of indices availableNeed to match the model to the jobAs with most climate models there is a fair

amount of uncertaintyIncreasing availability of products like I

usedNeed more sites to support these kinds of

efforts.

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Questions?Questions?