Evaluating Drought Vulnerability of Small Community Surface Water Supply Systems in the Midwest

This report presents approaches and data availability for evaluating the drought vulnerability of small community water supply systems in the Midwest that obtain water from surface water bodies, such as rivers, streams, natural lakes, and man-made reservoirs. A description is provided of the various types of surface water sources from which 320 small community systems in the Midwest, each serving 10,000 or fewer people, obtain their water. The small community surface water system most commonly obtains its supply from one or two small impounding reservoirs. However, a substantial number of communities instead obtain their water from either direct river withdrawals or off-channel storage of water withdrawn from streams and rivers. Sixty of these 320 small community surface water systems were interviewed to gather information on the availability of data to determine the drought vulnerability of these systems. Although hydrologic and physical data exist for evaluating many of these systems, relatively few of the interviewed system managers could provide such pertinent information. A summary of selected hydrologic data is provided that can be used to determine the relative severity of major historical drought periods for various portions of the Midwest. Focus is given to historical droughts and available data for the southern portion of the Midwest where most surface water supply systems are located, comprising parts of Kansas, Missouri, Iowa, Illinois, Indiana, and Ohio. Geographic differences in drought severity are described, as is the influence of the physical characteristics of a water supply on the “critical” drought duration that a community must consider. Basic water budget analyses of water supplies and data needs are presented. Reservoir capacity measurements and estimates of inflow are the most critical data in reliable assessment of water supply adequacy. Depending on data availability, estimation of inflows may be straightforward to highly uncertain. For water supply systems that withdraw directly from a stream or river, the existence of long-term stream gage data on that river is particularly crucial to evaluate supply adequacy, and such data for larger streams and rivers are often available. With impounding reservoirs, which are typically located on smaller streams, data for that stream may often not exist; however, data from a “surrogate” gage that is considered to be hydrologically similar are often sufficient to estimate water supply yield. Systems that use off-channel reservoirs often withdraw water from smaller streams that do not have data for accurate depiction of their yield, and these systems also appear to be the most vulnerable to severe drought conditions. Case studies are presented to provide examples of yield calculations and innovative approaches that selected small communities have undertaken for addressing drought vulnerability. The role of demand management (drought response and water conservation) in evaluating drought vulnerability is also presented. If hydrologic data and basic physical data such as storage capacity are lacking, it may be difficult for either system managers or experienced professionals to estimate a community system’s yield and potential drought impacts, particularly for off-channel reservoir and low channel dam systems. However, managers should attempt to understand the type of drought period likely to test the adequacy of the available supply and can begin recording basic system observations, such as daily withdrawal records and reservoir drawdown, in a readily-accessible form that will be useful for future evaluations.published or submitted for publicationis peer reviewe