Drought in Travis County Chris Shaw CE394K.2 Spring 2007

shaw.ppt Outline of Presentation Introduction/Objective Drought Indices Methods Results Summary

shaw.ppt 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

shaw.ppt 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.

shaw.ppt 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.

shaw.ppt 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.

shaw.ppt Meteorological/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.

shaw.ppt 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.

shaw.ppt 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.

shaw.ppt 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.

shaw.ppt 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)

shaw.ppt 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

shaw.ppt 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

shaw.ppt 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.

shaw.ppt PDSI 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

shaw.ppt 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.

shaw.ppt 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 2 nd and 98 th percentiles of the PDSI fall compared with the expected and 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.

shaw.ppt 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.

shaw.ppt Site Location

shaw.ppt PDSI Calculator Fortunately I discovered a site that would do the calculation for me. SPI calculator available as well, but does not appear to work at this time. Shortcomings – outputs, limited sites

shaw.ppt Results

Travis County,

shaw.ppt Travis County

shaw.ppt 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.

shaw.ppt Questions?