The High Plains aquifer extends from 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 dataset consists of three sets of water-level measurements. The first set are the supplemental water-level measurements for 547 wells screened in the High Plains aquifer, not located in New Mexico, measured in predevelopment and at least once for 2015 through 2018, but not for 2019. These supplemental measurements were used to calculate historical water-level change values for predevelopment to 2015 to 2018 and approximate water-level change values from predevelopment to 2019 to substantiate the map of water-level changes, predevelopment (about 1950) to 2019 (figure 1 in https://doi.org/10.3133/sir20235143). The water-level measurements used to calculate historical water-level changes from predevelopment are (1) 218 wells measured in predevelopment and in 2018, but not measured in 2019, which are used to calculate water-level change, predevelopment to 2018, (2) 152 wells measured in predevelopment and in 2017, but not measured in 2018 or 2019, which are used to calculate water-level change, predevelopment to 2017, (3) 117 wells measured in predevelopment and in 2016, but not measured in 2017, 2018, or 2019, which are used to calculate water-level change, predevelopment to 2016, and (4) 60 wells measured in predevelopment and in 2015, but not measured in 2016, 2017, 2018, or 2019, which are used to calculate water-level change, predevelopment to 2015. The second and third sets of water-level measurements were used to approximate water-level change, predevelopment to 2019, but did not have predevelopment water-level measurements. The second set included 292 wells, which were located in areas where water level declines from predevelopment to 1980 were 50 feet or more (Luckey and others, 1981; Cederstrand and Becker, 1999) and were measured in 1980 and in 2019, but not measured in the predevelopment period. For these wells, approximate water-level changes, predevelopment to 2019, were calculated as the starting value of the polygon range (for example 50 ft for the polygon of declines from 50 to 75 ft) from the map of water-level change, predevelopment to 1980, plus measured water-level change from 1980 to 2019. The third set of water-level measurements used to calculate approximate water-level changes were from 1,213 wells that were measured on or before 6/15/1978 (termed post-development) and in 2019, but not in the predevelopment period. For these wells, approximate water-level changes, predevelopment to 2019, were calculated as the water level, 2019, minus water level, post-development.