Lakewatch
An interpretive discussion of the past five runoff years
by Greg Reis
When does evaporation exceed
inflow? Somebody asked me that question yesterday, and
the answer is that it depends—it happens at a
different time every year, and sometimes more than once.
The charts shown here, however, are a better answer. 
To fully appreciate the charts, a few things must be explained. A runoff year is April 1 of the named year to March 31 of the following year, starting and ending when the snowpack in the mountains is usually at a maximum. When runoff plus direct precipitation into the lake exceeds evaporation, the lake rises, and when runoff plus precipitation is lower than evaporation, the lake falls.
The chart below shows a different line for each of the last 5 runoff years. If we start on the lowest line—at the beginning of the 1994 runoff year—Mono Lake’s April 1 level was 6375.5 feet above sea level. Runoff and evaporation were both relatively low at this time of year, but they were equal—the line is flat. 1994 was a dry year, so snowmelt runoff came early and didn’t last long, and Mono Lake’s level peaked around June 1, the second time in the runoff year that evaporation and runoff were equal. After June 1, evaporation exceeded runoff, and the lake level dropped until November 1, when evaporation decreased to a point where it equaled runoff for the third time.
1995 was an extremely wet year, and between January and September Mono Lake rose 3.4 vertical feet. Mono Lake rose roughly 2 feet in each of the following years, finally bringing us to April 1, 1999, at a lake level of 6384.8. Note that within these rises, however, there is a similar pattern that all years share: a flat line or slow rise in early spring; a relatively fast rise in late spring and early summer; a decreasing or flat line in late summer and fall; and a fast rise in winter (a result of low evaporation and high precipitation). This seasonal pattern is explained by a cyclical evaporation curve interacting with the cyclical runoff curve as shown in the chart above.
During the years of large DWP water diversions, runoff into the lake was much less and there was a much longer period when evaporation exceeded runoff. As a result the lake reached its annual high stand in early spring and declined 2 or more feet by the end of the year.
What will the chart at the left look like in the 1999-2000 runoff year? DWP forecasts that this past winter’s snowpack combined with median precipitation in the upcoming months will result in 94% of average runoff in the 1999-2000 runoff year. The predicted runoff and precipitation will cause the lake to peak near the end of July around 6385.5 and end up at an April 1, 2000 level of 6384.8. That is the same level as it was this past April 1—which would be the first time there is not a net rise since the 1992 runoff year.
The phenomenal 12 foot lake rise we have witnessed since January 1993 will slow down even when we have average or wetter than average years because the greater lake surface area at higher lake levels means the total volume of evaporation is greater and requires more runoff to maintain the same level.
Greg Reis is the Committee’s Website and Canoe Coordinator. He may soon speak German.
Return to Summer 1999 Newsletter
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