Hydroperiod changes as clues to impacts on Cache River Riparian wetlands

Abstract

A hydrologic analysis of historic stream gage data collected on the Cache River at Patterson, Arkansas is presented as a basis for impact analysis of riverine wetlands. Subtle, long-term changes in hydroperiod that could collectively have major impacts on wetland functions are quantified. Harmonic analysis, time-scale analysis, and conventional methods of hydrologic analysis of gage data at decade intervals are employed. These various techniques indicate a steady decline in the magnitude and predictability of the base flow during low flow periods, beginning with the 1920s and becoming increasingly more pronounced into the 1980s. Complementary information suggests that hydroperiod alterations can be associated with increased ground-water pumping and associated land use changes in the Cache River basin. Hydrologic indices wer developed that range from intuitively simple but generally insensitive indices based on means, medians, ranges, and discharge-duration curves to indices that are based on methods sensitive to subtle changes in hydrologic patterns. One set of sensitive indices generated by harmonic analysis is relatively simple to obtain and useful to explore changes in the pattern of discharges or stages in wetlands. The timescale analysis can be employed to provide a relatively high resolution quantification of changes in hydrology that can be related to long-term changes in land- or water-use patterns. The hydrologic methods presented in these analyses are simple enough for routine application (when adequate data are available) but sufficiently sophisticated to identify subtle changes in hydroperiod associated with impacts and passage of time. Changes in hydroperiod identified by these methods may have the potential to explain changes in biotic communities or wetland structure as part of comprehensive wetland studies.