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Evapotranspiration and groundwater patterns in montane meadows of the Sierra Nevada, CA


Meadows comprise less than 1 percent of the land area of the Sierra Nevada, however, they play an important role, often linking the forested hillslope to the stream, in hydrologic processes of Sierra Nevada catchments. This study investigates evapotranspiration and groundwater patterns in montane meadows in the Sierra Nevada, CA. The dissertation is divided into three sections. 1) The first section uses direct eddy covariance measurements and Applied Neural Network models in order to determine annual evapotranspiration (ET) from 2008 to 2014; annual ET rates in Long Meadow ranged from 321 to 392 mm and was higher in warm, dry years. Based on literature estimates of ET increases of 25-50%, due to meadow restoration, the overall increase in catchment ET, due to restoration efforts, would only amount to 0.03-0.05%. 2) The second section uses in situ measurements of groundwater table fluctuations and evapotranspiration in order to constrain soil specific yield. This study found that the variability of soil specific yield generally decreased with increasing depth to water; variability was also minimized when the surface slope was between 3-7 degrees. A method is proposed using these parameters for selecting monitoring well locations to estimated soil specific yield and, subsequently, ET for relatively low cost and impact. This method was demonstrated at 5 locations in two additional meadow systems. Average daily ET from July 1-31, 2013 ranged from 2.7 to 6.3 mm d-1 in these two meadows, compared to an average daily ET rate measured in Long Meadow of 4.6 mm d-1. 3) The third section investigates groundwater sources and patterns in a montane meadow and how the meadow links up-gradient hydrologic sources to the down-gradient stream. This was accomplished using a combination of physical measurements—including hydrostatic head, stream flow, snow depth, and soil moisture content, and analysis of water chemistry—including major ions, alkalinity, and stable and radioactive isotopes of water. Results show that meadow groundwater is comprised of two sources, a shallow subsurface flow with groundwater gradient signals dependent on snowpack, and a deeper subsurface flow that persists through the growing season—even after several years of drought. Individually and in combination, the studies in this dissertation significantly add to the knowledge of meadow hydrology in the Sierra Nevada. Both the first and third chapters found meadow systems still being supplied with groundwater, even after several years of the current drought in California. This suggests that meadow resilience in the face of drought is tied to significant groundwater storage capacity within a meadow catchment. Systems where deeper groundwater sources can be identified through analysis of stream chemistry or where significant groundwater storage capacity can be identified through geophysics or physiographic analysis should be prioritized for restoration efforts.

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