Hydrologic and Vegetative Modeling of Vernal Pools in the Sierra Nevada
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Vernal pools are ephemeral wetlands that exist in local geographic depressions with relatively impermeable substrates. Vernal pools are typically filled with water primarily from direct precipitation in the winter and spring months and are dried because of evaporation and seepage in the summer and fall months. The unusual hydrology of vernal pools has led to unique species compositions within the pool as few plant species can tolerate the hydrologic extremes found in vernal pools. Of the roughly 100 different species of plants often found in California vernal pools, 90% are native and 55% are endemic to California. Land use changes and climate change threaten vernal pools. Understanding the impacts of climate change to vernal pool hydrology and the plant community will be important for managing these sensitive ecosystems. Previous vernal pool modeling efforts have been limited to hydrologic quantification while qualitatively discussing the impacts to vegetation. Creating coupled hydrologic and vegetative models is critical to quantitatively understanding impacts to vernal pool vegetation.A mass balance hydrologic model was created that uses precipitation and temperature as climate inputs and generates a pool stage time-series as an output. Three vegetation models were created from an existing plant community classification system. The vegetation models use the pool stage time-series from the hydrologic model to estimate a vegetative community distribution within the pool. Bias-corrected data from three global climate models (GCM) were used as climate inputs for coupled models of vernal pool hydrology and plant community distribution. Climate data from the years 1991-2000 and 2091-2100 for the A2 and B1 emission scenarios from each GCM were used. Changes in plant communities were compared between the 1991-2000 and 2091-2100 time segments for each emission scenario.The hydrologic model results indicate that predicted average annual maximum depth in 2091-2100 did not significantly differ from current conditions under either the A2 or B1 emission scenario. Hydroperiod was predicted to significantly decrease under the A2 scenario, but not under the B1 scenario. Vegetative model results indicate that the pool area containing plant communities associated with vernal pool specialists decreased under the A2 and B1 emission scenarios with the decrease more pronounced under the A2 scenario. Overall, the study indicates that creating coupled hydrologic and vegetation models for vernal pools provide insight on potential impacts of hydrology and climate change on vernal pool plant communities, but modles of sensitive hydrologic systems like vernal pools require climate data with high spatial and temporal resolution.