Detection of changes in streamflow and floods resulting from climate fluctuations and land use-drainage changes

Abstract

Detection of effects of changing climate on the hydrologic responses of rivers can be further complicated by changes in land use, drainage, and water use. To discern effects of human-caused changes in a basin and those due to precipitation over time, a comparison was made of annual mean flows and peakflows in Midwestern basins that experienced increases in annual precipitation and heavy rain events during 1940–1990. Two pairs of basins, one pair in a rural area and one pair in an urbanized area, were selected for in-pair comparisons, with one basin in each pair experiencing more land use and drainage changes during 1940–1990 than the other basin. All basins experienced significant upward trends in annual precipitation and annual mean flows. Human-produced changes affecting runoff in both rural basins accounted for about two-thirds of the fluctuations in the mean flows, and precipitation changes accounted for the other third. However, much of the change in peakflows in the rural basin undergoing sizable changes in drainage was due to these changes (85%) versus 75% in the rural basin without comparable shifts in drainage. The mean and peak flows of the two urban basins showed considerably more response to precipitation shifts than those of the two rural basins. The urbanized area doubled within one urban basin during 1940–1990, and these land use changes explained much more of the increase in mean flows and peakflows there than in the urban basin with less change in land use. By 1990 precipitation accounted for 69% of the upward trend in mean flows since 1941 in the heavily developed urban basin, as compared to 37% of the trend in the less settled urban basin. For purposes of assessing climate change, the precipitation changes over fifty years in all basins produced marked uptrends in basin streamflow, but the magnitude of the precipitation effect was masked by the land use and drainage changes. The results illustrate the need for careful analysis of natural basin characteristics (soils and basin shape), land use and drainage changes, and of various precipitation conditions if the influence of shifting precipitation on hydrologic conditions is to be detected, accurately measured, and correctly interpreted. For such studies the paired basin comparison techniques appears to be a valuable approach.