Application of near surface geophysical methods to image water table response in an Alpine Meadow, Northern California.
Abstract:Meadows are recognized for their value to the ecological, hydrologic, and aesthetic functions of a watershed. As natural water retention sinks, meadows attenuate floods, improve water quality and support herbaceous vegetation that stabilize streambanks and promote high biodiversity. Alpine meadows are especially vital, serving as freshwater sources and distributing to lower lying provinces through ground and surface water interaction. These complexes are highly vulnerable to drought conditions, altered seasonal precipitation patterns, and mismanaged land use.
One such location, Van Norden meadow located in the Donner Summit area west of Lake Tahoe, is one of the largest sub-alpine meadows in the Sierra Nevada mountain range of Northern California. Van Norden meadow offers a natural hydrologic laboratory. Ownership transfer of the area from a local land trust to the Forestry Service requires restoration toward natural meadow conditions, and involves notching the dam in 2016 to reduce currently impounded water volumes from 250 to less than 50 acre-feet. To monitor the effects of notching the dam on the upstream meadow conditions, better understanding of the surface and groundwater hydrology both pre-and post-base level alteration is required. Comprehensive understanding of groundwater flux that supports meadow reaches relies on knowledge of their often complex stratigraphic and structural subsurface framework. In recent years hydrogeophysics has emphasized the combination of near surface geophysical techniques, collaborated with well and borehole measures, to qualitatively define these parameters. Building on a preliminary GPR investigation conducted in 2014, in which 44 270 MHz transect lines were collected, we returned to Van Norden meadow in late summer 2015 to collect lower frequency GPR (50 and 100 MHz) and electrical resistivity profiles to better define the groundwater table, sedimentary, and structural features of the meadow.