SPATIAL VARIABILITY AND ESTIMATION OF GROUNDWATER FLUX FROM LOCAL DISCHARGE MEASUREMENTS

We applied a recently developed, point-scale automated seepage meter (ASM) in streambeds in the Nebraska Sand Hills, USA in five dense arrays with areas ranging from 13.5 m² – 28.0 m² (23 – 39 points), to investigate the small-scale spatial variability of groundwater seepage flux (specific discharge, q). Streambed vertical hydraulic conductivity (K) was also measured. Based on high-resolution contour plots and anisotropic semi-variograms, results demonstrated average correlation scales of q and K in the stream width direction to be 1.57 m and 1.08 m, respectively, and 1.97 m and 1.61 m, respectively, in the stream length direction. The results also indicated that just one row of points (with five to seven points in each row) will have a 43% - 63% probability of estimating the groundwater flux within the 90% confidence intervals for streambed areas up to 28.0 m².

To investigate the ability of the seepage meter to produce accurate mean q at larger scales, ASMs were deployed in four stream reaches (170 – 890 m), arranged in three to six transects per reach across the channel. Each transect consisted of three to eight points evenly spaced across the stream width. In each reach, the mean q value from the ASMs generally agreed within measurement error with a q value based on stream discharge measurements from chemical tracer dilution and an acoustic Doppler velocimeter. The results demonstrate the viability of a modest number of ASM measurements (0.03 – 0.12 points per meter stream length) to determine the overall groundwater flux to the study stream and can guide sampling campaigns for groundwater studies.