Water Balance – 05 Conclusions and Discussion

Water balance for Crater Lake, Oregon by Manuel Nathenson

 Conclusions and Discussion

Finding the best values for the water balance for Crater Lake is far more illusive than one would expect from the great quantity of data available. With only precipitation data from Park Headquarters and data on lake levels, one cannot do better than than show that the coefficient B (the ratio of lake-level change to the amount of precipitation above the long-term average) lies between about 1.2 and 1.5. The addition of precipitation data from the North Rim gage yields a better result because of the improved ability to model water level variations. The formal limits of one standard error are that B lies between 1.26 and 1.39 with a mean value of 1.325. The long-term average precipitation needed to keep lake level constant is well determined at 169 ± 2 cm. Phillips (1968) estimated that his water balance was accurate to 5% over the long term and 10% in a single year. Based on the statistical analysis in this paper, his estimate is correct. Averaging precipitation from Park Headquarters with adjusted values from the North Rim gage results in significantly improved model of lake level variation from precipitation data.

The water balance based on daily data indicates that precipitation on the lake is about 10% higher (186 cm) than that measured at Park Headquarters (using B a 1.-1 fromTable 5). For B = 1.325, B -B a – 0.225 or about 17% of the water supply is from inflow from the crater walls. Given that the crater rim is 22% of the lake’s drainage basin and assuming that maybe 113 of the snow evaporates during melting on the rim, average precipitation on the rim would be about 10 % higher (205 cm) than that on the lake and about 21% higher than that at Park Headquarters. Given that precipitation at the gage on the North Rim (121 cm) is only 72% of that at Park Headquarters, the precipitation on the south rim at higher elevations must be significantly greater than that at the lower elevation of Park Headquarters.