The High Plains aquifer extends from south of about 32 degrees to almost 44 degrees north latitude and from about 96 degrees 30 minutes to 106 degrees west longitude. The aquifer underlies about 175,000 square miles in parts of Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming. This digital data set is the supplemental water-level measurements from 1,897 wells located in Colorado, Kansas, Nebraska, Oklahoma, South Dakota, or Texas and measured in various time periods, which were used to historical water-level change values for predevelopment to 2011 to 2014 and approximate water-level change values from predevelopment to 2015 to substantiate the map of water-level changes, predevelopment (about 1950) to 2015. The water-level measurements and the calculated historical water-level change values are (1) 219 wells measured in predevelopment and in 2014, but not measured in 2015, which are used to calculate water-level change, predevelopment to 2014, (2) 135 wells measured in predevelopment and in 2013, but not measured in 2014 or 2015, which are used to calculate water-level change, predevelopment to 2013, (3) 94 wells measured in predevelopment and in 2012, but not measured in 2013, 2014, or 2015, which are used to calculate water-level change, predevelopment to 2012, and (4) 57 wells measured in predevelopment and in 2011, but not measured in 2012, 2013, 2014, or 2015, which are used to calculate water-level change, predevelopment to 2011. One of two sets of water-level measurements used to calculate an approximate water-level-change values from predevelopment to 2015 are from 302 wells that are located in the areas where water level declines from predevelopment to 1980 (Luckey and others, 1981; Cederstrand and Becker, 1999) were 50 feet or more and were measured in 1980 and in 2015, but not measured in the predevelopment period. For these wells, approximate water-level change is calculate as the beginning contour interval from the map of water-level change, predevelopment to 1980 plus water-level change from 1980 to 2015. The second set of water-level measurements used to calculate approximate water-level change are from 1,090 wells that were measured on or before 6/15/1978 (termed post-development) and in 2015, but not in the predevelopment period. For these wells, approximate water-level change, predevelopment to 2015, is calculate as the water level, 2015, minus water level, post-development.