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Introduction to Vernal Pools The Vernal Pool Grassland When rain falls on a vernal pool grassland, some water sinks into the ground and the rest flows slowly over the land as runoff. This runoff flows to streams or into depressions (low places) in the grassland. The water cannot move deeper into the ground in a vernal pool grassland because hardpan blocks its path. Hardpan is a layer of clay or minerals that water cannot pass through easily. The hardpan can be a few inches to a few feet below the ground surface. Under the grassland, the hardpan acts like the bottom of a bathtub holding up the water. As winter rains continue, rain and runoff saturate (fill with water) the soil above the hardpan. The water perches (sits) on the hardpan. In the upland (the higher, drier areas of vernal pool grasslands) we cannot see this water because the water table is below the soil surface. The only place we can see the perched water is in the depressions we call vernal pools. The only way for vernal pools to empty is by very slow movement of water through the ground or by evaporation. This can take days, weeks or months depending on the amount of rainfall, the air temperature and the size of the pool. While some vernal pools are bigger than a playground, many are no larger than a classroom. No two pools are exactly alike. Throughout the uplands are scattered large humps of soil called mima mounds. Nobody knows exactly how mima mounds and vernal pools formed because it happened long ago. It is likely that earthquakes, volcanoes and floods all helped shape the present land surface over the last half a million (500,000) years. Since human beings did not arrive in California until about 12,000 years ago, vernal pools have been a part of our landscape far longer than people.
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