干旱区生态环境知识资源中心

Groundwater pumping in the Bicycle Basin in the northern Mojave Desert, one of the major sources of water supply at Fort Irwin National Training Center (NTC), California, has increased since the 1990s. The increased pumping has resulted in as much as 25 m of water-level decline between 1990 and 2009 and as much as 270 mm of subsidence occurred during 1993-2006. Land-surface deformation, consisting of an earth fissure and sink-like depressions, occurred in 2005 on the Bicycle Lake (dry) playa in the southern part of the basin. Earth fissures are a major concern to the NTC because aircraft runways constructed on the playa are used for transporting troops and supplies. Giant desiccation cracks or macropolygons also have been observed on the playa indicating that soil desiccation may play a role in the fissure formation. A monitoring system was developed to evaluate the driving mechanisms of fissuring and to provide an on-going assessment of the ground-failure hazard. Land-surface deformation and movement across the fissure is being monitored using: (I) repeated geodetic levelling surveys, (2) electronic distance measurement (EDM) surveys, (3) high-resolution ground-based, tripod-mounted Light Detection and Ranging surveys (T-LiDAR) for measurement of three-dimensional (3-D) changes across the fissure and adjacent areas, and (4) tape extensometer (tapex) measurements for horizontal changes across the fissure. The desiccation mechanism is being assessed using heat-dissipation sensors (HDPs) coupled with laboratory-scale experiments of soil desiccation. Preliminary results from repeat EDM surveys were inconclusive but repeat tapex measurements show that the fissure appears to be opening at a rate of similar to 1-2 mm/year which may be in response to similar to 4 m measured water-level decline during 2008-2009. Combining automated measurements from tiltmeters and HDPs may help provide a tool for real-time monitoring of fissure hazards.