Spatial Variability and Estimation of Groundwater Flux from Local Discharge Measurements

We applied a recently developed automated seepage meter (ASM) in streambeds in the Nebraska Sand Hills, USA in five dense arrays over areas 13.5 m2 – 28.0 m2 each (169 points total), to investigate the small-scale spatial variability of groundwater seepage flux (specific discharge, q). Streambed vertical hydraulic conductivity (K) was also measured. Results provided: (a) high-resolution contour plots of q and K, (b) anisotropic semi-variograms demonstrating greater correlation scales of q and K along the stream length than across the stream width, and (c) the number of rows of points (perpendicular to streamflow) needed to represent the groundwater flux of areas up to 28.0 m2.
To investigate the ability of the seepage meter to produce accurate mean q at larger scales, seepage meters 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 seepage meters was compared to a q value based on stream discharge measurements from chemical tracer dilution and an acoustic Doppler velocimeter. Reach-scale estimates of q from seepage meters and from stream discharge data generally agreed within measurement error. The results indicate the viability of a modest number of seepage meter measurements to determine the overall groundwater flux to the study stream and can guide sampling campaigns for groundwater studies.