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Publication Abstract

Uncertainties and Data Needs in Evaluating the Adequacy of Community Surface Water Supply Systems in Illinois. Knapp, H. Vernon, 2007  Illinois State Water Survey, Champaign, IL,  ISWS CR 2007-08    Full Text Available

This report examines uncertainties in data inputs used in estimating drought vulnerability and yield of community surface water systems in Illinois. Not only are uncertainties in individual data components characterized, but also comparative influence of those data components in the overall computation of yield. The two most influential data components identified through this analysis are: 1) existing reservoir capacity for community systems that use reservoir storage and 2) streamflow characteristics during drought, both for determining availability of flow for direct withdrawals from those streams as well as determining the cumulative amount of water that flows into or can be pumped to reservoirs. Additional uncertainties from climate data (precipitation and evaporation) are considerably less influential in yield estimation.

Data needs for estimating surface water system yields are identified not only by the uncertainties in and influences of various data components, but also are prioritized by cost and timeline for obtaining data and the expected likelihood that additional data noticeably could affect conclusions regarding drought vulnerability of individual systems. Considering these factors, obtaining bathymetric surveys for water supply reservoirs that do not already have such data represents the most effective way to improve yield estimates for Illinois surface water supplies as a whole. Many reservoirs in Illinois never have had sedimentation or bathymetric surveys. Results of this study show that available capacity estimates for these reservoirs are apt to be biased toward overestimation and have an estimated standard error of 28 percent. The amount of bias and error is greater for smaller reservoirs. Reservoirs with sedimentation surveys that are more than 35 years old may be subject to the same level of bias and uncertainty as unsurveyed reservoirs.

Streamflow data for water supply analyses remains a long-term data need. Most community supply reservoirs (impounding and off-channel) are in relatively small rural watersheds of less than 50 square miles. In most of these cases, there never has been a streamgage in the watershed, and estimates of drought inflow used for yield analyses typically are based on historical streamgage records from nearby watersheds that are assumed to have hydrologically similar flow regimes. Because the number of small watershed gages in the Illinois streamgaging network has been reduced substantially over the past three decades, very few gages in operation are appropriate for regional analysis of small watersheds. As a result, if a severe drought occurred today, there would be few continuous flow records to document the type of drought flow conditions necessary for application in future water supply analyses. Establishment of streamgages in small watersheds near existing water supply reservoirs is necessary to maintain, much less improve, levels of uncertainty in flow data for future water supply analyses and planning.

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