Design of a sediment data-collection program in Kansas as affected by time trends

Abstract

Data-collection programs need to be reexamined periodically in order to insure their usefulness, efficiency, and applicability. The possibility of time trends in sediment concentration, in particular, makes the examination with new statistical techniques desirable. After adjusting sediment concentrations for their relation to streamflow rates and by using a seasonal adaptation of Kendall's nonparametric statistical test, time trends of flow-adjusted concentrations were detected for 11 of the 38 sediment records tested that were not affected by large reservoirs. Ten of the 11 trends were toward smaller concentrations; only 1 was toward larger concentrations. Of the apparent trends that were not statistically significant (0.05 level) using data available, nearly all were toward smaller concentrations. Because the reason for the lack of statistical significance of an apparent trend may be inadequacy of data rather than absence of trend and because of the prevalence of apparent trends in one direction, the assumption was made that a time trend may be present at any station. This assumption can significantly affect the design of a sediment data-collection program. Sudden decreases (step trends) in flow-adjusted sediment concentrations were found at all stations that were short distances downstream from large reservoirs and that had adequate data for a seasonal adaptation of Wilcoxon's nonparametric statistical test. Examination of sediment records in the 1984 data-collection program of the Kansas Water Office indicated 12 stations that can be discontinued temporarily because data are now adequate. Data collection could be resumed in 1992 when new data may be needed because of possible time trends. New data are needed at eight previously operated stations where existing data may be inadequate or misleading because of time trends. Operational changes may be needed at some stations, such as hiring contract observers or installing automatic pumping samplers. Implementing the changes in the program can provide a substantial increase in the quantity of useful information on stream sediment for the same funding as the 1984 level.