A streamflow drought event, defined by applying the threshold level approach on streamflow time series, is composed of three parameters: duration, severity and magnitude. This study reveals statistical characteristics on streamflow drought event parameters and detects spatial and temporal trends in the streamflow drought in terms of frequency, duration and severity in Nebraska. The studies are conducted on three time periods : 1970-2001 (60 stations), 1950-2001 (43 stations), and 1932-2001 (9 stations). The statistical tests performed on the drought event parameters include correlation between event parameters tests, Hurst coefficients and lag-one coefficients, and trend-free pre-whitening Mann-Kendall (TFPW-MK) tests. The analysis shows that there is no uniform trend on the streamflow drought in the whole state. However, some trends are evident for specific regions. Specifically, it is most likely that droughts in the Republican watershed have become more intense; whereas the drought has become slightly alleviated in the Missouri and nearby watersheds.

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Determination of streambed hydraulic conductivity is of great importance in the analysis of stream-aquifer interactions and stream ecosystems. In this paper, in situ falling-head standpipe permeameter tests were conducted to determine streambed vertical hydraulic conductivity (K v ) of two connected layers of sediments at 60 test locations from eight sites in the Elkhorn River, Nebraska. Our results show that the K v values for the upper layer of sediments are generally larger than those in their respective lower layer of sediments. The individual K v value for the upper layer of sediments from all test locations of the eight sites range from 2.7 to 104.9 m/d, and the average value is 26.6 m/d. The individual K v value ranges from 0.4 to 73.4 m/d and the average value is 16.1 m/d for the lower layer of sediments. We believe that hyporheic processes can enhance larger streambed K v in the upper layer of sediments. Inflow and outflow through hyporheic zone produce more porous and permeable sediments and further result in an increasing of streambed K v . Moreover, we observe that bioturbation activities can destroy the clogging layer of the exposed sediments and create pore spaces in the sediments, which also further increases streambed K v . streambed vertical hydraulic conductivity, permeameter test, depth of streambed sediment, hyporheic processes, bioturbation Citation: Song J X, Chen X H, Cheng C, et al. Variability of streambed vertical hydraulic conductivity with depth along the Elkhorn River, Nebraska, USA. Chinese Sci Bull, 2010, 55: 992−999,

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