INSAR AND PSINSAR REVEAL CHANGING AQUIFER SYSTEM RESPONSE TO PUMPING AND ARTIFICIAL RECHARGE IN LAS VEGAS VALLEY, NEVADA

We first examined the spatial pattern and magnitude of subsidence induced by heavy groundwater pumping in Las Vegas using InSAR in 1999, at which time the most active part of the aquifer system was the Northwest subsidence bowl which was compacting at a maximum rate of 2-3 cm/yr. Conventional InSAR and permanent scatterer InSAR (PSInSAR) studies we have conducted through 2005 now reveal that the rate of subsidence throughout the valley has been significantly reduced, and in some cases reversed, since the late 1990's. We have used an enhanced ERS and Envisat data set to examine time-series between 1992-2005 which show that the Northwest bowl remains the only significant zone of subsidence, and that some portions of the valley are exhibiting elastic recovery. We used the PSInSAR methodology to examine 50 ERS-1 and -2 scenes and 19 Envisat scenes and to identify more than 15 million PS targets in Las Vegas permitting construction of detailed time series, examination of non-linear trends, seasonal fluctuations, and deceleration rates. A comparison of 1992-2000 ERS data with 2002-2005 Envisat data indicates that average rates have decreased in the Northwest bowl from >20 mm/yr to <5-8 mm/yr. The PS time-series data show that the average 1992-2000 deceleration rate was on the order of ~1 mm/yr2. Since 2000 the area exhibiting uplift has enlarged, and the maximum uplift rate is much as 8-10 mm/yr. The PSInSAR rates and patterns are in good agreement with conventional InSAR results which show similar subsidence rates and delineate the same areas of uplift. A comparison of PS rates with downhole extensometer data and continuous GPS data provide additional independent validation of the PS methodology. Time-series rates on PS targets located on the extensometer and the GPS sites show comparable subsidence rates of ~5 mm/yr between 1995-1999 and 2002-2005, respectively. The deceleration of subsidence rates and the elastic aquifer recovery is attributable to the artificial recharge program initiated by the Las Vegas Valley Water District where up to 28,000 acre-feet (34hm3)/yr have been recharged to the aquifer system resulting in as much as 30 m of water-level rise.