Hydrology – 24 SURFACE RUNOFF AND GROUNDWATER INFLOW

Hydrology of Crater, East and Davis Lakes, Oregon by Kenneth N. Phillips
 EAST LAKE

WATER SUPPLY TO THE LAKE

SURFACE RUNOFF AND GROUNDWATER INFLOW

As noted above, surface inflow is ephemeral and of negligible volume, and some -of the ground-water flow in the caldera may not reach either lake. The maximum amounts of combined surface- and groundwater flow that could reach the lakes in an average year can be approximated by deducting the estimated annual evapotranspiration loss (12 in.) from the total precipitation (35 in.) on the tributary areas surrounding the lakes. The upper limit of the average annual runoff is 7,350 acre-feet for East Lake basin and is 9.750 acre-feet for Paulina Lake basin; the sum is 17,100 acre-feet.

Thus, the average potential supply to the two lakes is about 23,800 acre-feet per year (6,650 acre-ft from direct precipitation and 17,100 acre-ft from runoff). Evaporation of 28 inches from the surface of the two lakes annually would remove 5,300 acre-feet and would leave about 18,500 acre-feet to be expected as annual outflow.

The estimated average flow of 20 cfs at the outlet of Paulina Lake (table 8) is equivalent to a runoff of 14,500 acre-feet, or 4,000 acre feet less than the expected volume. The estimated annual excess of precipitation over evaporation on the surface of Paulina Lake and the ‘adjacent and topographically tributary area of 5,080 acres is 10,500 acre-feet. To obtain the estimated runoff of 14,500 acre-feet from the entire caldera, only about 4,000 acre-feet per year is needed from East Lake basin. That is only half the average annual runoff that the East Lake basin is believed to generate. Therefore, it is probable either that a large part of the water in East Lake basin never reaches Paulina Lake or that Paulina Lake basin loses some water by percolation out of the basin.

However, Paulina Lake itself probably does not lose much water by seepage. If it did, the stage in a series of dry years would drop below its outlet; there is no record of such an occurrence.

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