Mother Nature is becoming increasingly generous to the Golden State. While precipitation ran behind historic averages in October and November, a wet December has almost closed the gap (see Northern Sierra 8 Station Precipitation chart). Based on the storms in the first of half of January, actual precipitation may soon be running ahead of historic averages.
The actual precipitation for October-December provides useful information regarding the actual precipitation for October-April of the water year (see Cumulative Northern Sierra 8 Station Index (October-April). Actual precipitation October through December explains a statistically significant one-third of the annual variation in actual precipitation October through April. Given actual precipitation through December, the model predicts that the expected precipitation through April will be 45 inches, or 98 percent of the historic average.
As discussed last month, there are three factors driving final SWP Allocations: amount of water in storage at Lake Oroville at the beginning of the water year (which is known), actual precipitation in October-April, and a declining trend in SWP allocations of 3 percentage points per year.
Models, of course, have underlying variability in their predictions (see the Probability Distribution of Forecasted Final 2016 SWP Allocations chart). The expected Final 2016 SWP Allocation is now 22%. The probability of a zero allocation is only 5%. The median prediction is 22%. There is now only a 20% chance that the final allocation will be below the current 10% allocation. On the flip side, there is about a 80% chance that the final allocation will be above the current 10% allocation.
There are two wild cards to keep one’s eye on as we wait for the Department of Water Resources determination of the Final SWP Allocation for 2016.
First, there is the fickleness of Mother Nature. What will precipitation be in the coming months? January storms portend a continuing favorable factor towards increased SWP allocations.
Second, there are man-made actions. What will be the impact of environmental constraints on project operations? Does the model’s trend decline of 3 percentage points per year for the Final SWP Allocation capture reduced allocations from regulatory constraints?
In the end, there is much uncertainty surrounding the outcome. JOW will update the predictions monthly for actual precipitation through April.
Written by Rodney T. Smith