Gravity Monitoring of Canal Infiltration: Results from Dutch Flats, Nebraska

Gravity monitoring provides a cost-effective means of monitoring subsurface mass changes of engineering interest. In July to October 2003, we conducted a gravity monitoring study of canal infiltration in the Dutch Flats irrigated farming area of western Nebraska, USA. An unlined east-west canal on the north edge of the monitoring network leaks southward into the existing shallow aquifer. Due to irrigation requirements, infiltration from the canal started in early spring, before the first measurement campaign, and continued until August. Sixteen gravity stations were co-located on monitoring wells near the canal, with an additional two far-field reference stations. Absolute gravity measurements by the National Geodetic Survey on the reference stations in July and September show no significant change at either station. Relative to the July baseline, the August campaign shows gravity increases of ~20 μGal at the northern-most stations, with smaller changes at other stations. September and October surveys show gravity increases of ~20 μGal more southward, and gravity values decreasing to near zero at the canal. Gravity changes are limited to [-54,+30] μGal during the experiment, suggesting water loss from the shallow aquifer as the dominant process during the gravity monitoring experiment.