ESTIMATING MOUNTAIN BLOCK RECHARGE FOR THE LOWER OWENS VALLEY, CA UNDER MODERN AND LATE PLEISTOCENE BOUNDARY CONDITIONS

Owens Valley, in east-central California, lies in the rainshadow of the Sierra Nevada mountains. Prior to the early 20th century, this valley held a lake that covered 270 square kilometers. Diversion of surface water by the city of Los Angeles has resulted in complete desiccation; what remains today is a dry playa. Modern estimates of the quantity and movement of water in the underlying confined aquifer system (fed by infiltration at the mountain/valley margin) are based on an assumption of equilibrium with modern climate. Calculations of modern mountain block recharge, however, indicate that recharge rates may be inadequate to produce observed discharge. Isotopic dating of water from deep wells on the west and east sides of the valley near the dry playa yield dates of approximately 20,000 years before present. These lines of evidence indicate that Owens Lake playa may still be influenced by late Pleistocene recharge. We use ArcView GIS to calculate both modern and 18 ka (last glacial maximum) mountain block recharge based on representative precipitation information and the Maxey-Eakin method, and compare these values to observed discharge to determine whether the aquifer system is in disequilibrium with modern boundary conditions. This is an important consideration, as long-term remediation of dust hazard in Owens Valley is dependent on water resource estimates that have been based on current estimates of equilibrium and associated discharge values.