How the Main Pool Forms and Disappears

See Vernal pools of the Santa Rosa Plateau for an introduction to the Main Pool of the Santa Rosa Plateau.

The following diagram illustrates the flow of water in the Main Pool:

The cross-section in the above plot is west (l) - east (r) at the location of the outlet of the Main Pool. A cross-section in the north-south direction would show roughly equal height rims on the north and south as on the west. The rim on the east has been lowered significantly at the stream outlet due to stream erosion.

The darker uppermost black line is the ground surface. The thinner lower black line is the bottom of the soil layer, defined as that portion of the basalt that has been weathered and can hold significant groundwater. This may also include some of the upper almost-intact basalt if it is fractured enough to hold significant water.

The blue line is the top of the permanent, slowly-changing groundwater level. The groundwater extends at least to the bottom of the soil layer, and probably also saturates the basalt as well. I've drawn the groundwater level as rising a bit to the left, since it is likely that even the deeper basalt is fractured and hence contains a small amount of groundwater. However, any groundwater stored in the deeper part of the basalt is almost entirely irrelevant to the processes discussed here.

The above plot shows the groundwater level appropriate for when the pool is full; the groundwater level is lower when the pool is not full. When the pool first becomes dry, the groundwater level is 16 inches lower and just touches the deepest part of the pool bottom.

The direction of groundwater flow is indicated by the arrows. It flows into the pool from the surrounding area except at the outlet, where it flows out of the pool. In this case, the percolation dominantly occurs through the weathered basalt in the area of the stream outlet, as groundwater flow under the surface and sides of the stream outlet.

The most important process to filling the pool is surface water runoff, as shown by the black arrow above, which fills the pool by raising the groundwater level. There is also a contribution from temporary water flow in the soil level above the permanent groundwater level.

The maximum Main Pool Depth is 16 inches as measured as the Boardwalk, and only a few inches more at its lowest point, and is determined by the height of the outlet. The soil depth is significantly greater under the pool since water spends a lot more time chemically altering the basalt there. The water is not perched on top of the soil level, as it would be if the clay soil formed a "tight seal"; the entire soil level below the elevation of the top of the pool is saturated with water, and water flows through the clay soil.

It is now easy to see why evaporation is unimportant to the decline in the pool level. Consider what would happen if one inch of water was suddenly removed from the top of the pool. Groundwater would immediately begin to flow into the pool from the surrounding saturated soil and try to restore the previous pool depth.

As I have drawn it, the radius of saturated soil to the left is about twice the radius from the center of the pool, and the same radius applies to the north and south sides of the pool as well. A simple calculation, given in the next paragraph, shows that there is roughly 2.25 times as much water stored in groundwater as in the pool itself. Thus if one immediately removed one inch of water from the pool, the water level would soon rise from inflowing groundwater until the final decrease was only 0.3 inches. Since evaporation in winter is usually much less than one inch, the pool level could decrease by much less than 0.3 inches in winter due to evaporation. The observed decrease in pool level of one inch per week must thus be almost entirely due to percolation.

(Here is the calculation; mathematically-challenged people should skip this paragraph and take my word for it. Area is proportional to the radius squared, resulting in four times as much water in a pool with twice the radius. Thus there is three (four minus one) times as much water outside the pool itself. The double radius only applies on three of the four sides of the pool, so the final factor is three times three-fourths = 2.25. Removing one inch of water from the pool thus results in a net change in the groundwater level of 1.00 / 3.25 = 0.3 inches.)

How the water creates the pool in the first place has changed as the basalt was weathered. Initially, rainfall directly ran off the basalt surface to the bottom of these depressions, and any rainfall at all formed a pool (puddle at that time) with further runoff flowing out of the pool. As the basalt weathered to soil, the first rainfall no longer directly ran off, but was captured by the soil. Only when the soil becomes too saturated to hold more water can further rainfall run off to the bottom of the depression.

However, the first runoff still doesn't create a pool. The bottom of the depression contains much more soil than the higher portions of the depression, since the weathering rate is much higher there. The initial runoff goes to saturating that deeper soil and forming a groundwater layer beginning at the bottom of that soil. It is only when that groundwater layer is deep enough to reach the surface that a pool forms. See Amount of Rain Needed to Form the Main Vernal pool.

The scenario above is contrary to the previous suggestion that pool formation depends on the clay "swelling" and forming a "tight seal" when wet that is impenetrable to percolation (see, for example, Lathrop and Workman 1991). This suggestion is contradicted by the fact that the measured percolation rate is in fact one inch per week, not zero inches per week. Furthermore, the effect of groundwater level was seen clearly in the formation of the 2006 and 2008 pools. In 2006, a pool formed from less rainfall that would normally be needed, due to the high groundwater table after the 2005 heavy rainfall year. In 2008, the pool formation required more rainfall than would normally be needed, due to the low groundwater table after the 2007 drought.

Salinity data (Collie and Lathrop 1976) also strongly rule out evaporation as the cause of the disappearance of the water in the pool. If the pool primarily evaporated, the salinity should increase dramatically with time in direct proportion to the initial water volume divided by the current water volume until salt begins to precipitate out of solution. Instead, the salinity is nearly constant with time, as expected if the pool percolates away. In fact, Collie and Lathrop suggested percolation as a possible source to explain their observed results for salinity.

Also, if evaporation were important, obvious salt should be present on the surface of the bottom of the pool when it dries up. No salt deposits have been observed by me in over a decade of observations.

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Copyright © 2008 by Tom Chester.
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Updated 14 January 2008.