January 26, 2018
Rainfall: In contrast to one year ago, when parts of California were starting to flood from a relentless series of atmospheric rivers, Water Year 2018 has seen only a few brief periods of wet weather, punctuating an otherwise persistently dry and warm start. A few scattered storms in November gave way to unseasonably clear, calm, and dry weather in December. There was only one day with measurable precipitation in December totaling 0.18 inches, making it the third driest December on record. For the season to date, total rainfall in the City measures 8.28 inches, just over half the long-term average amount of 16 inches for this time of year. Monthly and cumulative rainfall amounts are shown in Figures 1 and 2.
|Figure 1||Figure 2|
In the City’s watershed around Loch Lomond Reservoir, total rainfall measures 13.40 inches for the season to date, in contrast to the more than four feet of rain that was recorded this time last year.
The short term forecast has mostly sunny and dry weather through the end of January and into early February. Long-term, the National Weather Service Climate Prediction Center is showing equal chances of normal precipitation across California in its 3-month outlook for the period February through April 2018 that was generated January 18.
Stream Flow: Figure 3 shows the mean monthly stream flow in the San Lorenzo River for the season to date, along with the long-term average monthly values for comparison.The San Lorenzo River flowed strong all summer long due to heavy rains last year. Beginning in the month of October though, flows dropped off to levels slightly to well below average due to meager rainfall received thus far. It usually takes about a foot or more of rain in the watershed to saturate soils before significant winter runoff occurs. This year, with the long, dry stretch in December, that hydrologic process is still taking shape.
Reservoir Storage: The 2018 Water Year started October 1 with Loch Lomond Reservoir at about 93 percent of capacity. The combination of strong surface water flows this past summer and continuing low system demand resulted in plant operators not having to draw much water down from the lake during the dry season, keeping storage high. Currently, the reservoir is holding at 93.5 percent of capacity, with the lake level about three feet below the spillway elevation.
Water Year Classification: The Water Department uses a water year classification system to characterize the City’s overall annual water supply condition. Under this classification system, the water year beginning October 1 is designated as one of four types – Wet, Normal, Dry, or Critically Dry – depending on the total annual discharge of the San Lorenzo River, measured at the stream gage in Felton, and expressed in acre-feet.
As might be expected with such dry start to winter, cumulative discharge from the San Lorenzo River is running well below normal. The long-term average for cumulative discharge at this time of year is about 33,000 acre-feet. So far this year, the river has generated less than 9,000 acre feet of runoff, barely more than a quarter of normal (Figure 4). As a result, the Water Year remains provisionally classified as Critically Dry. In contrast, exactly one year ago, annual discharge from the San Lorenzo River had already exceeded 117,000 acre feet, and the year classification was about to roll over from Normal to Wet in what was to eventually become the wettest year on record in terms of cumulative annual discharge.
Initial Outlook for 2018: As with elsewhere in California, the City’s surface water storage is in good shape, but the relative lack of rainfall so far is starting to feel reminiscent of conditions experienced a few years ago. Where last year the outlook was clear; this year it is more uncertain.
On the positive side, reservoir storage is healthy and the chances of spilling this year are still pretty good. System demand remains restrained. Historically, it is not uncommon to see large storms and/or plentiful rainfall during the months of February and March.
The question is: will there be enough rain during the latter half of the wet season to reverse the trend of the first half? At this time, it is still too early to draw conclusions about what kind of year 2018 will be. Just as there has been extreme year-to-year variability over the last few years, the same variability in weather patterns often occurs on a shorter time scale within a single season.
The Water Department will continue to monitor water supply conditions, and will reevaluate the water supply outlook in early March. At that time, staff will have the information upon which to make a monthly projection of the City’s water supply availability and evaluate the adequacy of this supply to meet expected water demands within the City’s water service area for the rest of 2018. One key variable is the level of instream flow releases that the City will need to meet during the dry season for fish rearing, which is unknown at this time. Instream flows are organized into five categories or hydrologic conditions. Which category will apply this summer depends on cumulative runoff in the San Lorenzo River over the entire wet season, which won’t be established until later in winter or early spring.
With annual demand last year totaling only 2.66 billion gallons and the lake nearly full, it is hard to envision being in a position that would require the City to impose water restrictions to preserve reservoir storage in 2018. Still, the City relies on river and stream flows for the majority of its water supply and the yield of those sources is closely tied to annual rainfall. Should weather conditions continue to be as dry, or drier, than they have been so far this year, the Water Department stands ready to implement demand reduction measures as the situation requires.
The Initial Water Supply Outlook for 2018 will be presented at the Water Commission Meeting on February 5th, 2018