Kansas Geological Survey, Open-file Report 2011-16
by
Geoffrey Bohling and Brownie Wilson
KGS Open File Report 2011-16
Oct. 2011
The High Plains aquifer is the primary source of water for the High Plains region of western and south-central Kansas. Some water is also withdrawn from underlying bedrock units, primarily Cretaceous strata, in this region. The Kansas Geological Survey (KGS) and the Kansas Department of Agriculture's Division of Water Resources (DWR) measure water levels in aquifers of the High Plains region on an annual basis in a network of over 1380 wells, in order to assist in the management of this vital resource. This report presents statistical and geostatistical analyses for the High Plains region in Kansas based on data from the 2011 water-level measurements and water-level changes for the one-year and five-year periods preceding the 2011 measurements. The majority of the 2011 measurements were obtained between January 3 and January 7, 2011, although measurement dates range from Dec. 20, 2010, to March 24, 2011.
Throughout this report we refer to water-level declines, with a positive decline meaning an increase in depth to water from the land surface (or decrease in water-table elevation) and a negative decline meaning a decrease in depth to water from the land surface (increase in water-table elevation). Water levels are measured in the winter so that the water table (or potentiometric surface) will have had a chance to recover from the more transient and localized effects of pumping for irrigation. The measurements are presumed to represent a new "static" water level, with the difference from the previous year's measurements representing the net loss or gain of saturated thickness over the preceding year. The difference in depth to water between the January 2010 and January 2011 measurements represents the water level decline for 2010.
Recent work carried out as part of the Kansas Geological Survey's High Plains Aquifer Calibration Monitoring Well Program ("index well program") has demonstrated that the January water level measurements may be far from static, fully recovered values (Stotler et al., 2011). Water level recovery from the previous pumping season can continue throughout the winter and often is still incomplete when the next season's pumping begins in the spring. Water levels can also show significant responses to atmospheric pressure variations that must be accounted for in order to obtain accurate estimates of annual differences. Furthermore, the index well program has made it clear that in some areas the High Plains aquifer can not be accurately represented as a single unconfined aquifer, a conceptualization that implicitly underlies the two-dimensional interpolation approach that has played a central role in the geostatistical analysis of the annual water level measurements for a number of years now. In fact, the accuracy of this conceptualization has been called into question in previous versions of this report (Bohling and Wilson, 2007; 2006), where we noted that some wells have exhibited large and consistent differences in water level from their neighboring wells for a number of years, most likely as a result of persistent vertical gradients between different units tapped by the wells in these areas.
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Kansas Geological Survey, Geohydrology
Placed online Dec. 15, 2011
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