Measuring & Modeling Groundwater Fluctuations in Two Montane Meadows in Southern California

Abstract:

Mountain meadows play a critical role in the hydrology of California’s watersheds by preventing flooding, improving water quality, and delivering moisture downstream. However, meadows are also highly sensitive to changes in water availability, making drought a particularly potent threat. Here, we present fluctuations in groundwater levels in two montane meadows (Bluff Meadow & Lodgepole Meadow) located in the San Bernardino National Forest of southern California. A piezometer network of 24 wells was installed across this 30 acre region to record weekly water table fluctuations during the JJA dry season (precipitation = 15.67 millimeters/month) between 2015 and 2019. These data were spatially interpolated using ArcGIS and then used to create and calibrate a hydrologic model for these meadows. By integrating precipitation, relative humidity, solar irradiance, and temperature with water table measurements, we calculate evapotranspiration and model a simple water balance for this region. A ratio model is combined with the water balance model to effectively predict weekly (and monthly) dry season water table depths in both time and space across this diversely vegetated landscape. Multiple precipitation scenarios (model input) are run through the model to produce predictions of water availability (model output) for these meadows. Our results show a reduction of 50% of precipitation (relative to the 1981-2010 period) produces a decline of the local water table beyond vegetation rooting depths (> 2 meters) for more than 8 consecutive months of the year. With precipitation projections forecasting below-normal conditions by the year 2100, understanding how limited rainfall affects the fate of these meadow ecosystems is paramount.